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[except.ctor]
# 14 Exception handling [[except]](./#except)
## 14.3 Stack unwinding [except.ctor]
[1](#1)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exceptions.tex#L386)
As control passes from the point where an exception is thrown
to a handler,
objects are destroyed by a process,
specified in this subclause, called [*stack unwinding*](#def:stack_unwinding "14.3 Stack unwinding [except.ctor]")[.](#1.sentence-1)
[2](#2)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exceptions.tex#L393)
Each object with automatic storage duration is destroyed if it has been
constructed, but not yet destroyed,
since the try block was entered[.](#2.sentence-1)
If an exception is thrown during the destruction of temporaries or
local variables for a return statement ([[stmt.return]](stmt.return "8.8.4 The return statement")),
the destructor for the returned object (if any) is also invoked[.](#2.sentence-2)
The objects are destroyed in the reverse order of the completion
of their construction[.](#2.sentence-3)
[*Example [1](#example-1)*: struct A { };
struct Y { ~Y() noexcept(false) { throw 0; } };
A f() {try { A a;
Y y;
A b; return {}; // #1} catch (...) {}return {}; // #2}
At #1, the returned object of type A is constructed[.](#2.sentence-4)
Then, the local variable b is destroyed ([[stmt.jump]](stmt.jump "8.8 Jump statements"))[.](#2.sentence-5)
Next, the local variable y is destroyed,
causing stack unwinding,
resulting in the destruction of the returned object,
followed by the destruction of the local variable a[.](#2.sentence-6)
Finally, the returned object is constructed again at #2[.](#2.sentence-7)
— *end example*]
[3](#3)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exceptions.tex#L428)
If the initialization of an object
other than by delegating constructor
is terminated by an exception,
the destructor is invoked for
each of the object's subobjects
that were known to be initialized by the object's initialization and
whose initialization has completed ([[dcl.init]](dcl.init "9.5 Initializers"))[.](#3.sentence-1)
[*Note [1](#note-1)*:
If such an object has a reference member
that extends the lifetime of a temporary object,
this ends the lifetime of the reference member,
so the lifetime of the temporary object is effectively not extended[.](#3.sentence-2)
— *end note*]
A subobject is [*known to be initialized*](#def:known_to_be_initialized "14.3 Stack unwinding [except.ctor]") if it is not an anonymous union member and
its initialization is specified
- [(3.1)](#3.1)
in [[class.base.init]](class.base.init "11.9.3 Initializing bases and members") for initialization by constructor,
- [(3.2)](#3.2)
in [[class.copy.ctor]](class.copy.ctor "11.4.5.3 Copy/move constructors") for initialization by defaulted copy/move constructor,
- [(3.3)](#3.3)
in [[class.inhctor.init]](class.inhctor.init "11.9.4 Initialization by inherited constructor") for initialization by inherited constructor,
- [(3.4)](#3.4)
in [[dcl.init.aggr]](dcl.init.aggr "9.5.2 Aggregates") for aggregate initialization,
- [(3.5)](#3.5)
in [[expr.prim.lambda.capture]](expr.prim.lambda.capture "7.5.6.3 Captures") for the initialization of
the closure object when evaluating a [*lambda-expression*](expr.prim.lambda.general#nt:lambda-expression "7.5.6.1 General [expr.prim.lambda.general]"),
- [(3.6)](#3.6)
in [[dcl.init.general]](dcl.init.general "9.5.1 General") for
default-initialization, value-initialization, or direct-initialization
of an array[.](#3.sentence-3)
[*Note [2](#note-2)*:
This includes virtual base class subobjects
if the initialization
is for a complete object, and
can include variant members
that were nominated explicitly by
a [*mem-initializer*](class.base.init#nt:mem-initializer "11.9.3 Initializing bases and members [class.base.init]") or [*designated-initializer-clause*](dcl.init.general#nt:designated-initializer-clause "9.5.1 General [dcl.init.general]") or
that have a default member initializer[.](#3.sentence-4)
— *end note*]
If the destructor of an object is terminated by an exception,
each destructor invocation
that would be performed after executing the body of the destructor ([[class.dtor]](class.dtor "11.4.7 Destructors")) and
that has not yet begun execution
is performed[.](#3.sentence-5)
[*Note [3](#note-3)*:
This includes virtual base class subobjects if
the destructor was invoked for a complete object[.](#3.sentence-6)
— *end note*]
The subobjects are destroyed in the reverse order of the completion of
their construction[.](#3.sentence-7)
Such destruction is sequenced before entering a
handler of the [*function-try-block*](except.pre#nt:function-try-block "14.1 Preamble [except.pre]") of the constructor or destructor,
if any[.](#3.sentence-8)
[4](#4)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exceptions.tex#L480)
If the [*compound-statement*](stmt.block#nt:compound-statement "8.4 Compound statement or block [stmt.block]") of the [*function-body*](dcl.fct.def.general#nt:function-body "9.6.1 General [dcl.fct.def.general]") of a delegating constructor
for an object exits via
an exception, the object's destructor is invoked[.](#4.sentence-1)
Such destruction is sequenced before entering a handler of the[*function-try-block*](except.pre#nt:function-try-block "14.1 Preamble [except.pre]") of a delegating constructor for that object, if any[.](#4.sentence-2)
[5](#5)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exceptions.tex#L489)
[*Note [4](#note-4)*:
If the object was allocated by a [*new-expression*](expr.new#nt:new-expression "7.6.2.8 New [expr.new]") ([[expr.new]](expr.new "7.6.2.8 New")),
the matching [deallocation function](basic.stc.dynamic.deallocation "6.8.6.5.3 Deallocation functions [basic.stc.dynamic.deallocation]"),
if any, is called to free the storage occupied by the object[.](#5.sentence-1)
— *end note*]

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[except.handle]
# 14 Exception handling [[except]](./#except)
## 14.4 Handling an exception [except.handle]
[1](#1)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exceptions.tex#L500)
The[*exception-declaration*](except.pre#nt:exception-declaration "14.1 Preamble [except.pre]") in a[*handler*](except.pre#nt:handler "14.1 Preamble [except.pre]") describes the type(s) of exceptions that can cause
that[*handler*](except.pre#nt:handler "14.1 Preamble [except.pre]") to be entered[.](#1.sentence-1)
The[*exception-declaration*](except.pre#nt:exception-declaration "14.1 Preamble [except.pre]") shall not denote an incomplete type, an abstract class type, or an rvalue reference type[.](#1.sentence-2)
The[*exception-declaration*](except.pre#nt:exception-declaration "14.1 Preamble [except.pre]") shall not denote a pointer or reference to an
incomplete type, other than “pointer to cv void”[.](#1.sentence-3)
[2](#2)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exceptions.tex#L521)
A handler of type“array of T” orfunction type T is adjusted to be of type
“pointer to T”[.](#2.sentence-1)
[3](#3)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exceptions.tex#L530)
A[*handler*](except.pre#nt:handler "14.1 Preamble [except.pre]") is a match for
an exception object
of typeE if
- [(3.1)](#3.1)
The [*handler*](except.pre#nt:handler "14.1 Preamble [except.pre]") is of type cv T orcv T& andE and T are the same type (ignoring the top-level [*cv-qualifier*](dcl.decl.general#nt:cv-qualifier "9.3.1 General [dcl.decl.general]")*s*), or
- [(3.2)](#3.2)
the [*handler*](except.pre#nt:handler "14.1 Preamble [except.pre]") is of type cv T orcv T& andT is an unambiguous public base class of E, or
- [(3.3)](#3.3)
the [*handler*](except.pre#nt:handler "14.1 Preamble [except.pre]") is of type cv T or const T& where T is a pointer or pointer-to-member type andE is a pointer or pointer-to-member type
that can be converted to T by one or more of
* [(3.3.1)](#3.3.1)
a standard [pointer conversion](conv.ptr "7.3.12 Pointer conversions [conv.ptr]") not involving conversions
to pointers to private or protected or ambiguous classes
* [(3.3.2)](#3.3.2)
a [function pointer conversion](conv.fctptr "7.3.14 Function pointer conversions [conv.fctptr]")
* [(3.3.3)](#3.3.3)
a [qualification conversion](conv.qual "7.3.6 Qualification conversions [conv.qual]"), or
- [(3.4)](#3.4)
the [*handler*](except.pre#nt:handler "14.1 Preamble [except.pre]") is of type cv T or const T& where T is a pointer or pointer-to-member type and E is std​::​nullptr_t[.](#3.sentence-1)
[*Note [1](#note-1)*:
A[*throw-expression*](expr.throw#nt:throw-expression "7.6.18 Throwing an exception [expr.throw]") whose operand is an integer literal with value zero does not match a handler of
pointer or pointer-to-member type[.](#3.sentence-2)
A handler of reference to array or function type
is never a match for any exception object ([[expr.throw]](expr.throw "7.6.18 Throwing an exception"))[.](#3.sentence-3)
— *end note*]
[*Example [1](#example-1)*: class Matherr { /* ... */ virtual void vf(); };class Overflow: public Matherr { /* ... */ };class Underflow: public Matherr { /* ... */ };class Zerodivide: public Matherr { /* ... */ };
void f() {try { g(); } catch (Overflow oo) {// ...} catch (Matherr mm) {// ...}}
Here, theOverflow handler will catch exceptions of typeOverflow and theMatherr handler will catch exceptions of typeMatherr and of all types publicly derived fromMatherr including exceptions of typeUnderflow andZerodivide[.](#3.sentence-4)
— *end example*]
[4](#4)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exceptions.tex#L611)
The handlers for a try block are tried in order of appearance[.](#4.sentence-1)
[*Note [2](#note-2)*:
This makes it possible to write handlers that can never be
executed, for example by placing a handler for a final derived class after
a handler for a corresponding unambiguous public base class[.](#4.sentence-2)
— *end note*]
[5](#5)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exceptions.tex#L619)
A... in a handler's[*exception-declaration*](except.pre#nt:exception-declaration "14.1 Preamble [except.pre]") specifies a match for any exception[.](#5.sentence-1)
If present, a... handler shall be the last handler for its try block[.](#5.sentence-2)
[6](#6)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exceptions.tex#L629)
If no match is found among the handlers for a try block,
the search for a matching
handler continues in a dynamically surrounding try block
of the same thread[.](#6.sentence-1)
[7](#7)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exceptions.tex#L635)
If the search for a handler
exits the function body of a function with a
non-throwing exception specification,
the function std​::​terminate ([[except.terminate]](except.terminate "14.6.2 The std​::​terminate function")) is invoked[.](#7.sentence-1)
[*Note [3](#note-3)*:
An implementation is not permitted to reject an expression merely because, when
executed, it throws or might
throw an exception from a function with a non-throwing exception specification[.](#7.sentence-2)
— *end note*]
[*Example [2](#example-2)*: extern void f(); // potentially-throwingvoid g() noexcept { f(); // valid, even if f throwsthrow 42; // valid, effectively a call to std​::​terminate}
The call tof is well-formed despite the possibility for it to throw an exception[.](#7.sentence-3)
— *end example*]
[8](#8)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exceptions.tex#L661)
If no matching handler is found,
the function std​::​terminate is invoked;
whether or not the stack is unwound before this invocation ofstd​::​terminate is implementation-defined ([[except.terminate]](except.terminate "14.6.2 The std​::​terminate function"))[.](#8.sentence-1)
[9](#9)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exceptions.tex#L669)
A handler is considered [*active*](#def:exception_handling,handler,active "14.4 Handling an exception [except.handle]") when
initialization is complete for the parameter (if any) of the catch clause[.](#9.sentence-1)
[*Note [4](#note-4)*:
The stack will have been unwound at that point[.](#9.sentence-2)
— *end note*]
Also, an implicit handler is considered active when
the function std​::​terminate is entered due to a throw[.](#9.sentence-3)
A handler is no longer considered active when the
catch clause exits[.](#9.sentence-4)
[10](#10)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exceptions.tex#L680)
The exception with the most recently activated handler that is
still active is called the[*currently handled exception*](#def:exception_handling,currently_handled_exception "14.4 Handling an exception [except.handle]")[.](#10.sentence-1)
[11](#11)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exceptions.tex#L686)
Referring to any non-static member or base class of an object
in the handler for a[*function-try-block*](except.pre#nt:function-try-block "14.1 Preamble [except.pre]") of a constructor or destructor for that object results in undefined behavior[.](#11.sentence-1)
[12](#12)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exceptions.tex#L692)
Exceptions thrown in destructors of objects with static storage duration or in
constructors of objects associated with non-block variables with static storage duration are not caught by a[*function-try-block*](except.pre#nt:function-try-block "14.1 Preamble [except.pre]") on
the [main function](basic.start.main "6.10.3.1 main function [basic.start.main]")[.](#12.sentence-1)
Exceptions thrown in destructors of objects with thread storage duration or in constructors of objects associated with non-block variables with thread storage duration are not caught by a[*function-try-block*](except.pre#nt:function-try-block "14.1 Preamble [except.pre]") on the initial function of the thread[.](#12.sentence-2)
[13](#13)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exceptions.tex#L702)
If a return statement ([[stmt.return]](stmt.return "8.8.4 The return statement")) appears in a handler of the[*function-try-block*](except.pre#nt:function-try-block "14.1 Preamble [except.pre]") of a
constructor, the program is ill-formed[.](#13.sentence-1)
[14](#14)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exceptions.tex#L708)
The currently handled exception
is rethrown if control reaches the end of a handler of the[*function-try-block*](except.pre#nt:function-try-block "14.1 Preamble [except.pre]") of a constructor or destructor[.](#14.sentence-1)
Otherwise, flowing off the end of
the [*compound-statement*](stmt.block#nt:compound-statement "8.4 Compound statement or block [stmt.block]") of a [*handler*](except.pre#nt:handler "14.1 Preamble [except.pre]") of a [*function-try-block*](except.pre#nt:function-try-block "14.1 Preamble [except.pre]") is equivalent to flowing off the end of
the [*compound-statement*](stmt.block#nt:compound-statement "8.4 Compound statement or block [stmt.block]") of that function (see [[stmt.return]](stmt.return "8.8.4 The return statement"))[.](#14.sentence-2)
[15](#15)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exceptions.tex#L721)
The variable declared by the [*exception-declaration*](except.pre#nt:exception-declaration "14.1 Preamble [except.pre]"), of typecv T or cv T&, is initialized from the exception object,
of type E, as follows:
- [(15.1)](#15.1)
if T is a base class of E,
the variable is copy-initialized ([[dcl.init]](dcl.init "9.5 Initializers"))
from an lvalue of type T designating the corresponding base class subobject
of the exception object;
- [(15.2)](#15.2)
otherwise, the variable is copy-initialized ([[dcl.init]](dcl.init "9.5 Initializers"))
from an lvalue of type E designating the exception object[.](#15.sentence-1)
The lifetime of the variable ends
when the handler exits, after the
destruction of any objects with automatic storage duration initialized
within the handler[.](#15.sentence-2)
[16](#16)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exceptions.tex#L740)
When the handler declares an object,
any changes to that object will not affect the exception object[.](#16.sentence-1)
When the handler declares a reference to an object,
any changes to the referenced object are changes to the
exception object and will have effect should that object be rethrown[.](#16.sentence-2)

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[except.nested]
# 17 Language support library [[support]](./#support)
## 17.9 Exception handling [[support.exception]](support.exception#except.nested)
### 17.9.8 nested_exception [except.nested]
[🔗](#lib:nested_exception)
namespace std {class nested_exception {public:constexpr nested_exception() noexcept; constexpr nested_exception(const nested_exception&) noexcept = default; constexpr nested_exception& operator=(const nested_exception&) noexcept = default; constexpr virtual ~nested_exception() = default; // access functions[[noreturn]] constexpr void rethrow_nested() const; constexpr exception_ptr nested_ptr() const noexcept; }; template<class T> [[noreturn]] constexpr void throw_with_nested(T&& t); template<class E> constexpr void rethrow_if_nested(const E& e);}
[1](#1)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/support.tex#L4297)
The class nested_exception is designed for use as a mixin through
multiple inheritance[.](#1.sentence-1)
It captures the currently handled exception and stores it
for later use[.](#1.sentence-2)
[2](#2)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/support.tex#L4302)
[*Note [1](#note-1)*:
nested_exception has a virtual destructor to make it a
polymorphic class[.](#2.sentence-1)
Its presence can be tested for with dynamic_cast[.](#2.sentence-2)
— *end note*]
[🔗](#lib:nested_exception,constructor)
`constexpr nested_exception() noexcept;
`
[3](#3)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/support.tex#L4314)
*Effects*: The constructor calls current_exception() and stores the returned value[.](#3.sentence-1)
[🔗](#lib:rethrow_nested,nested_exception)
`[[noreturn]] constexpr void rethrow_nested() const;
`
[4](#4)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/support.tex#L4325)
*Effects*: If nested_ptr() returns a null pointer, the function calls the function std​::​terminate[.](#4.sentence-1)
Otherwise, it throws the stored exception captured by *this[.](#4.sentence-2)
[🔗](#lib:nested_ptr,nested_exception)
`constexpr exception_ptr nested_ptr() const noexcept;
`
[5](#5)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/support.tex#L4337)
*Returns*: The stored exception captured by this nested_exception object[.](#5.sentence-1)
[🔗](#lib:throw_with_nested,nested_exception)
`template<class T> [[noreturn]] constexpr void throw_with_nested(T&& t);
`
[6](#6)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/support.tex#L4348)
Let U be decay_t<T>[.](#6.sentence-1)
[7](#7)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/support.tex#L4351)
*Preconditions*: U meets the [*Cpp17CopyConstructible*](utility.arg.requirements#:Cpp17CopyConstructible "16.4.4.2 Template argument requirements [utility.arg.requirements]") requirements[.](#7.sentence-1)
[8](#8)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/support.tex#L4355)
*Throws*: If is_class_v<U> && !is_final_v<U> && !is_base_of_v<nested_exception, U> is true,
an exception of unspecified type that is publicly derived from bothU and nested_exception and constructed from std​::​forward<T>(t), otherwisestd​::​forward<T>(t)[.](#8.sentence-1)
[🔗](#lib:rethrow_if_nested,nested_exception)
`template<class E> constexpr void rethrow_if_nested(const E& e);
`
[9](#9)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/support.tex#L4371)
*Effects*: If E is not a polymorphic class type, or
if nested_exception is an inaccessible or ambiguous base class of E,
there is no effect[.](#9.sentence-1)
Otherwise, performs:if (auto p = dynamic_cast<const nested_exception*>(addressof(e))) p->rethrow_nested();

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[except.pre]
# 14 Exception handling [[except]](./#except)
## 14.1 Preamble [except.pre]
[1](#1)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exceptions.tex#L12)
Exception handling provides a way of transferring control and information
from a point in the execution of a thread to an exception handler
associated with a point previously passed by the execution[.](#1.sentence-1)
A handler will be invoked only by throwing an exception
in code executed in the handler's try block
or in functions called from the handler's try block[.](#1.sentence-2)
[try-block:](#nt:try-block "14.1 Preamble [except.pre]")
try [*compound-statement*](stmt.block#nt:compound-statement "8.4 Compound statement or block [stmt.block]") [*handler-seq*](#nt:handler-seq "14.1 Preamble [except.pre]")
[function-try-block:](#nt:function-try-block "14.1 Preamble [except.pre]")
try [*ctor-initializer*](class.base.init#nt:ctor-initializer "11.9.3 Initializing bases and members [class.base.init]")opt [*compound-statement*](stmt.block#nt:compound-statement "8.4 Compound statement or block [stmt.block]") [*handler-seq*](#nt:handler-seq "14.1 Preamble [except.pre]")
[handler-seq:](#nt:handler-seq "14.1 Preamble [except.pre]")
[*handler*](#nt:handler "14.1 Preamble [except.pre]") [*handler-seq*](#nt:handler-seq "14.1 Preamble [except.pre]")opt
[handler:](#nt:handler "14.1 Preamble [except.pre]")
catch ( [*exception-declaration*](#nt:exception-declaration "14.1 Preamble [except.pre]") ) [*compound-statement*](stmt.block#nt:compound-statement "8.4 Compound statement or block [stmt.block]")
[exception-declaration:](#nt:exception-declaration "14.1 Preamble [except.pre]")
[*attribute-specifier-seq*](dcl.attr.grammar#nt:attribute-specifier-seq "9.13.1 Attribute syntax and semantics [dcl.attr.grammar]")opt [*type-specifier-seq*](dcl.type.general#nt:type-specifier-seq "9.2.9.1 General [dcl.type.general]") [*declarator*](dcl.decl.general#nt:declarator "9.3.1 General [dcl.decl.general]")
[*attribute-specifier-seq*](dcl.attr.grammar#nt:attribute-specifier-seq "9.13.1 Attribute syntax and semantics [dcl.attr.grammar]")opt [*type-specifier-seq*](dcl.type.general#nt:type-specifier-seq "9.2.9.1 General [dcl.type.general]") [*abstract-declarator*](dcl.name#nt:abstract-declarator "9.3.2 Type names [dcl.name]")opt
...
The optional [*attribute-specifier-seq*](dcl.attr.grammar#nt:attribute-specifier-seq "9.13.1 Attribute syntax and semantics [dcl.attr.grammar]") in an [*exception-declaration*](#nt:exception-declaration "14.1 Preamble [except.pre]") appertains to the parameter of the catch clause ([[except.handle]](except.handle "14.4 Handling an exception"))[.](#1.sentence-3)
[2](#2)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exceptions.tex#L56)
A [*try-block*](#nt:try-block "14.1 Preamble [except.pre]") is a [*statement*](stmt.pre#nt:statement "8.1 Preamble [stmt.pre]") ([[stmt.pre]](stmt.pre "8.1 Preamble"))[.](#2.sentence-1)
[*Note [1](#note-1)*:
Within this Clause
“try block” is taken to mean both [*try-block*](#nt:try-block "14.1 Preamble [except.pre]") and[*function-try-block*](#nt:function-try-block "14.1 Preamble [except.pre]")[.](#2.sentence-2)
— *end note*]
[3](#3)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exceptions.tex#L68)
The [*compound-statement*](stmt.block#nt:compound-statement "8.4 Compound statement or block [stmt.block]") of a try block or of a handler is a
control-flow-limited statement ([[stmt.label]](stmt.label "8.2 Label"))[.](#3.sentence-1)
[*Example [1](#example-1)*: void f() {goto l1; // errorgoto l2; // errortry {goto l1; // OKgoto l2; // error l1: ; } catch (...) { l2: ; goto l1; // errorgoto l2; // OK}} — *end example*]
Agoto,break,return,
orcontinue statement can be used to transfer control out of
a try block or handler[.](#3.sentence-2)
When this happens, each variable declared in the try block
will be destroyed in the context that
directly contains its declaration[.](#3.sentence-3)
[*Example [2](#example-2)*: lab: try { T1 t1; try { T2 t2; if ([*condition*](stmt.pre#nt:condition "8.1 Preamble [stmt.pre]"))goto lab; } catch(...) { /* handler 2 */ }} catch(...) { /* handler 1 */ }
Here, executinggoto lab; will destroy firstt2,
thent1,
assuming the[*condition*](stmt.pre#nt:condition "8.1 Preamble [stmt.pre]") does not declare a variable[.](#3.sentence-4)
Any exception thrown while destroyingt2 will result in executinghandler 2;
any exception thrown while destroyingt1 will result in executinghandler 1[.](#3.sentence-5)
— *end example*]
[4](#4)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exceptions.tex#L144)
A[*function-try-block*](#nt:function-try-block "14.1 Preamble [except.pre]") associates a[*handler-seq*](#nt:handler-seq "14.1 Preamble [except.pre]") with the[*ctor-initializer*](class.base.init#nt:ctor-initializer "11.9.3 Initializing bases and members [class.base.init]"),
if present, and the[*compound-statement*](stmt.block#nt:compound-statement "8.4 Compound statement or block [stmt.block]")[.](#4.sentence-1)
An exception
thrown during the execution of the[*compound-statement*](stmt.block#nt:compound-statement "8.4 Compound statement or block [stmt.block]") or, for constructors and destructors, during the initialization or
destruction, respectively, of the class's subobjects,
transfers control to a handler in a[*function-try-block*](#nt:function-try-block "14.1 Preamble [except.pre]") in the same way as an exception thrown during the execution of a[*try-block*](#nt:try-block "14.1 Preamble [except.pre]") transfers control to other handlers[.](#4.sentence-2)
[*Example [3](#example-3)*: int f(int);class C {int i; double d;public: C(int, double);};
C::C(int ii, double id)try : i(f(ii)), d(id) {// constructor statements} catch (...) {// handles exceptions thrown from the ctor-initializer and from the constructor statements} — *end example*]
[5](#5)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exceptions.tex#L184)
In this Clause, “before” and “after” refer to the[“sequenced before”](intro.execution#def:sequenced_before "6.10.1 Sequential execution [intro.execution]") relation[.](#5.sentence-1)

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[except.spec]
# 14 Exception handling [[except]](./#except)
## 14.5 Exception specifications [except.spec]
[1](#1)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exceptions.tex#L752)
The predicate indicating whether a function cannot exit via an exception
is called the [*exception specification*](#def:exception_specification "14.5 Exception specifications [except.spec]") of the function[.](#1.sentence-1)
If the predicate is false,
the function has a[*potentially-throwing exception specification*](#def:potentially-throwing,exception_specification "14.5 Exception specifications [except.spec]"),
otherwise it has a[*non-throwing exception specification*](#def:non-throwing_exception_specification "14.5 Exception specifications [except.spec]")[.](#1.sentence-2)
The exception specification is either defined implicitly,
or defined explicitly
by using a [*noexcept-specifier*](#nt:noexcept-specifier "14.5 Exception specifications [except.spec]") as a suffix of a [function declarator](dcl.fct "9.3.4.6 Functions [dcl.fct]")[.](#1.sentence-3)
[noexcept-specifier:](#nt:noexcept-specifier "14.5 Exception specifications [except.spec]")
noexcept ( [*constant-expression*](expr.const#nt:constant-expression "7.7 Constant expressions [expr.const]") )
noexcept
[2](#2)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exceptions.tex#L774)
In a [*noexcept-specifier*](#nt:noexcept-specifier "14.5 Exception specifications [except.spec]"), the [*constant-expression*](expr.const#nt:constant-expression "7.7 Constant expressions [expr.const]"),
if supplied, shall be a contextually converted constant expression
of type bool ([[expr.const]](expr.const "7.7 Constant expressions"));
that constant expression is the exception specification of
the function type in which the [*noexcept-specifier*](#nt:noexcept-specifier "14.5 Exception specifications [except.spec]") appears[.](#2.sentence-1)
A ( token that follows noexcept is part of the[*noexcept-specifier*](#nt:noexcept-specifier "14.5 Exception specifications [except.spec]") and does not commence an
initializer ([[dcl.init]](dcl.init "9.5 Initializers"))[.](#2.sentence-2)
The [*noexcept-specifier*](#nt:noexcept-specifier "14.5 Exception specifications [except.spec]") noexcept without a [*constant-expression*](expr.const#nt:constant-expression "7.7 Constant expressions [expr.const]") is
equivalent to the [*noexcept-specifier*](#nt:noexcept-specifier "14.5 Exception specifications [except.spec]")noexcept(true)[.](#2.sentence-3)
[*Example [1](#example-1)*: void f() noexcept(sizeof(char[2])); // error: narrowing conversion of value 2 to type boolvoid g() noexcept(sizeof(char)); // OK, conversion of value 1 to type bool is non-narrowing — *end example*]
[3](#3)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exceptions.tex#L796)
If a declaration of a function
does not have a [*noexcept-specifier*](#nt:noexcept-specifier "14.5 Exception specifications [except.spec]"),
the declaration has a potentially throwing exception specification
unless it is a destructor or a deallocation function
or is defaulted on its first declaration,
in which cases the exception specification
is as specified below
and no other declaration for that function
shall have a [*noexcept-specifier*](#nt:noexcept-specifier "14.5 Exception specifications [except.spec]")[.](#3.sentence-1)
In an [explicit instantiation](temp.explicit "13.9.3 Explicit instantiation [temp.explicit]") a [*noexcept-specifier*](#nt:noexcept-specifier "14.5 Exception specifications [except.spec]") may be specified,
but is not required[.](#3.sentence-2)
If a [*noexcept-specifier*](#nt:noexcept-specifier "14.5 Exception specifications [except.spec]") is specified
in an explicit instantiation,
the exception specification shall be the same as
the exception specification of all other declarations of that function[.](#3.sentence-3)
A diagnostic is required only if the
exception specifications are not the same
within a single translation unit[.](#3.sentence-4)
[4](#4)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exceptions.tex#L817)
If a virtual function has a
non-throwing exception specification,
all declarations, including the definition, of any function
that overrides that virtual function in any derived class
shall have a non-throwing
exception specification,
unless the overriding function is defined as deleted[.](#4.sentence-1)
[*Example [2](#example-2)*: struct B {virtual void f() noexcept; virtual void g(); virtual void h() noexcept = delete;};
struct D: B {void f(); // errorvoid g() noexcept; // OKvoid h() = delete; // OK};
The declaration ofD​::​f is ill-formed because it
has a potentially-throwing exception specification,
whereasB​::​f has a non-throwing exception specification[.](#4.sentence-2)
— *end example*]
[5](#5)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exceptions.tex#L850)
An expression E is[*potentially-throwing*](#def:potentially-throwing,expression "14.5 Exception specifications [except.spec]") if
- [(5.1)](#5.1)
E is a [function call](expr.call "7.6.1.3 Function call [expr.call]") whose [*postfix-expression*](expr.post.general#nt:postfix-expression "7.6.1.1 General [expr.post.general]") has a function type,
or a pointer-to-function type,
with a potentially-throwing exception specification,
or
- [(5.2)](#5.2)
E implicitly invokes a function
(such as an overloaded operator,
an allocation function in a [*new-expression*](expr.new#nt:new-expression "7.6.2.8 New [expr.new]"),
a constructor for a function argument,
or a destructor)
that has a potentially-throwing exception specification,
or
- [(5.3)](#5.3)
E is a [*throw-expression*](expr.throw#nt:throw-expression "7.6.18 Throwing an exception [expr.throw]") ([[expr.throw]](expr.throw "7.6.18 Throwing an exception")),
or
- [(5.4)](#5.4)
E is a dynamic_cast expression that casts to a reference type and
requires a runtime check ([[expr.dynamic.cast]](expr.dynamic.cast "7.6.1.7 Dynamic cast")),
or
- [(5.5)](#5.5)
E is a typeid expression applied to a
(possibly parenthesized) built-in unary * operator
applied to a pointer to a
polymorphic class type ([[expr.typeid]](expr.typeid "7.6.1.8 Type identification")),
or
- [(5.6)](#5.6)
any of the [immediate subexpressions](intro.execution#def:immediate_subexpression "6.10.1 Sequential execution [intro.execution]") of E is potentially-throwing[.](#5.sentence-1)
[6](#6)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exceptions.tex#L887)
An implicitly-declared constructor for a class X,
or a constructor without a [*noexcept-specifier*](#nt:noexcept-specifier "14.5 Exception specifications [except.spec]") that is defaulted on its first declaration,
has a potentially-throwing exception specification
if and only if
any of the following constructs is potentially-throwing:
- [(6.1)](#6.1)
the invocation of a constructor selected by overload resolution
in the implicit definition of the constructor
for class X to initialize a potentially constructed subobject, or
- [(6.2)](#6.2)
a subexpression of such an initialization,
such as a default argument expression, or,
- [(6.3)](#6.3)
for a default constructor, a default member initializer[.](#6.sentence-1)
[*Note [1](#note-1)*:
Even though destructors for fully-constructed subobjects
are invoked when an exception is thrown
during the execution of a constructor ([[except.ctor]](except.ctor "14.3 Stack unwinding")),
their exception specifications do not contribute
to the exception specification of the constructor,
because an exception thrown from such a destructor
would call the function std​::​terminate rather than escape the constructor ([[except.throw]](except.throw "14.2 Throwing an exception"), [[except.terminate]](except.terminate "14.6.2 The std​::​terminate function"))[.](#6.sentence-2)
— *end note*]
[7](#7)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exceptions.tex#L917)
The exception specification for an implicitly-declared destructor,
or a destructor without a [*noexcept-specifier*](#nt:noexcept-specifier "14.5 Exception specifications [except.spec]"),
is potentially-throwing if and only if
any of the destructors
for any of its potentially constructed subobjects
has a potentially-throwing exception specification or
the destructor is virtual and the destructor of any virtual base class
has a potentially-throwing exception specification[.](#7.sentence-1)
[8](#8)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exceptions.tex#L927)
The exception specification for an implicitly-declared assignment operator,
or an assignment-operator without a [*noexcept-specifier*](#nt:noexcept-specifier "14.5 Exception specifications [except.spec]") that is defaulted on its first declaration,
is potentially-throwing if and only if
the invocation of any assignment operator
in the implicit definition is potentially-throwing[.](#8.sentence-1)
[9](#9)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exceptions.tex#L935)
A [deallocation function](basic.stc.dynamic.deallocation "6.8.6.5.3 Deallocation functions [basic.stc.dynamic.deallocation]") with no explicit [*noexcept-specifier*](#nt:noexcept-specifier "14.5 Exception specifications [except.spec]") has a non-throwing exception specification[.](#9.sentence-1)
[10](#10)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exceptions.tex#L940)
The exception specification for a comparison operator function ([[over.binary]](over.binary "12.4.3 Binary operators"))
without a [*noexcept-specifier*](#nt:noexcept-specifier "14.5 Exception specifications [except.spec]") that is defaulted on its first declaration
is potentially-throwing if and only if
any expression
in the implicit definition is potentially-throwing[.](#10.sentence-1)
[11](#11)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exceptions.tex#L948)
[*Example [3](#example-3)*: struct A { A(int = (A(5), 0)) noexcept;
A(const A&) noexcept;
A(A&&) noexcept; ~A();};struct B { B() noexcept;
B(const B&) = default; // implicit exception specification is noexcept(true) B(B&&, int = (throw 42, 0)) noexcept; ~B() noexcept(false);};int n = 7;struct D : public A, public B {int * p = new int[n]; // D​::​D() potentially-throwing, as the new operator may throw bad_alloc or bad_array_new_length// D​::​D(const D&) non-throwing// D​::​D(D&&) potentially-throwing, as the default argument for B's constructor may throw// D​::​~D() potentially-throwing};
Furthermore, ifA​::​~A() were virtual,
the program would be ill-formed since a function that overrides a virtual
function from a base class
shall not have a potentially-throwing exception specification
if the base class function has a non-throwing exception specification[.](#11.sentence-1)
— *end example*]
[12](#12)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exceptions.tex#L981)
An exception specification is considered to be [*needed*](#def:needed,exception_specification "14.5 Exception specifications [except.spec]") when:
- [(12.1)](#12.1)
in an expression, the function is selected by
overload resolution ([[over.match]](over.match "12.2 Overload resolution"), [[over.over]](over.over "12.3 Address of an overload set"));
- [(12.2)](#12.2)
the function is odr-used ([[basic.def.odr]](basic.def.odr#term.odr.use "6.3 One-definition rule"));
- [(12.3)](#12.3)
the exception specification is compared to that of another
declaration (e.g., an explicit specialization or an overriding virtual
function);
- [(12.4)](#12.4)
the function is defined; or
- [(12.5)](#12.5)
the exception specification is needed for a defaulted
function that calls the function[.](#12.sentence-1)
[*Note [2](#note-2)*:
A defaulted declaration does not require the
exception specification of a base member function to be evaluated
until the implicit exception specification of the derived
function is needed, but an explicit [*noexcept-specifier*](#nt:noexcept-specifier "14.5 Exception specifications [except.spec]") needs
the implicit exception specification to compare against[.](#12.5.sentence-2)
— *end note*]
The exception specification of a defaulted
function is evaluated as described above only when needed; similarly, the[*noexcept-specifier*](#nt:noexcept-specifier "14.5 Exception specifications [except.spec]") of a specialization
of a templated function
is instantiated only when needed[.](#12.sentence-2)

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[except.special]
# 14 Exception handling [[except]](./#except)
## 14.6 Special functions [except.special]
### [14.6.1](#general) General [[except.special.general]](except.special.general)
[1](#general-1)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exceptions.tex#L1017)
The function std​::​terminate ([[except.terminate]](#except.terminate "14.6.2 The std​::​terminate function"))
is used by the exception
handling mechanism for coping with errors related to the exception handling
mechanism itself[.](#general-1.sentence-1)
The function std​::​uncaught_exceptions ([[uncaught.exceptions]](uncaught.exceptions "17.9.6 uncaught_­exceptions"))
reports how many exceptions are uncaught in the current thread[.](#general-1.sentence-2)
The function std​::​current_exception ([[propagation]](propagation "17.9.7 Exception propagation")) and the classstd​::​nested_exception ([[except.nested]](except.nested "17.9.8 nested_­exception")) can be used by a program to
capture the currently handled exception[.](#general-1.sentence-3)
### [14.6.2](#except.terminate) The std​::​terminate function [[except.terminate]](except.terminate)
[1](#except.terminate-1)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exceptions.tex#L1030)
Some errors in a program cannot be recovered from, such as when an exception
is not handled or a std​::​thread object is destroyed while its thread
function is still executing[.](#except.terminate-1.sentence-1)
In such cases,
the function std​::​terminate ([[exception.terminate]](exception.terminate "17.9.5 Abnormal termination")) is invoked[.](#except.terminate-1.sentence-2)
[*Note [1](#except.terminate-note-1)*:
These situations are:
- [(1.1)](#except.terminate-1.1)
when the exception handling mechanism, after completing
the initialization of the exception object
but before
activation of a handler for the exception ([[except.throw]](except.throw "14.2 Throwing an exception")),
calls a function that exits
via an exception, or
- [(1.2)](#except.terminate-1.2)
when the exception handling mechanism cannot find a handler for a thrown exception ([[except.handle]](except.handle "14.4 Handling an exception")), or
- [(1.3)](#except.terminate-1.3)
when the search for a [handler](except.handle "14.4 Handling an exception [except.handle]") exits the function body of a function
with a non-throwing [exception specification](except.spec "14.5 Exception specifications [except.spec]"),
including when a contract-violation handler
invoked from an evaluation of
a function contract assertion ([[basic.contract.eval]](basic.contract.eval "6.11.2 Evaluation")) associated with the function
exits via an exception,
or
- [(1.4)](#except.terminate-1.4)
when the destruction of an object during [stack unwinding](except.ctor#def:stack_unwinding "14.3 Stack unwinding [except.ctor]") terminates by throwing an exception, or
- [(1.5)](#except.terminate-1.5)
when initialization of a non-block
variable with static or thread storage duration ([[basic.start.dynamic]](basic.start.dynamic "6.10.3.3 Dynamic initialization of non-block variables"))
exits via an exception, or
- [(1.6)](#except.terminate-1.6)
when destruction of an object with static or thread storage duration exits
via an exception ([[basic.start.term]](basic.start.term "6.10.3.4 Termination")), or
- [(1.7)](#except.terminate-1.7)
when execution of a function registered withstd​::​atexit or std​::​at_quick_exit exits via an exception ([[support.start.term]](support.start.term "17.5 Startup and termination")), or
- [(1.8)](#except.terminate-1.8)
when a[*throw-expression*](expr.throw#nt:throw-expression "7.6.18 Throwing an exception [expr.throw]") ([[expr.throw]](expr.throw "7.6.18 Throwing an exception"))
with no operand attempts to rethrow an exception and no exception is being
handled ([[except.throw]](except.throw "14.2 Throwing an exception")), or
- [(1.9)](#except.terminate-1.9)
when the function std​::​nested_exception​::​rethrow_nested is called for an object
that has captured no exception ([[except.nested]](except.nested "17.9.8 nested_­exception")), or
- [(1.10)](#except.terminate-1.10)
when execution of the initial function of a thread exits via
an exception ([[thread.thread.constr]](thread.thread.constr "32.4.3.3 Constructors")), or
- [(1.11)](#except.terminate-1.11)
for a parallel algorithm whose ExecutionPolicy specifies such
behavior ([[execpol.seq]](execpol.seq "26.3.6.3 Sequenced execution policy"), [[execpol.par]](execpol.par "26.3.6.4 Parallel execution policy"), [[execpol.parunseq]](execpol.parunseq "26.3.6.5 Parallel and unsequenced execution policy")),
when execution of an element access function ([[algorithms.parallel.defns]](algorithms.parallel.defns "26.3.1 Preamble"))
of the parallel algorithm exits via an exception ([[algorithms.parallel.exceptions]](algorithms.parallel.exceptions "26.3.4 Parallel algorithm exceptions")), or
- [(1.12)](#except.terminate-1.12)
when the destructor or the move assignment operator is invoked on an object
of type std​::​thread that refers to
a joinable thread ([[thread.thread.destr]](thread.thread.destr "32.4.3.4 Destructor"), [[thread.thread.assign]](thread.thread.assign "32.4.3.5 Assignment")), or
- [(1.13)](#except.terminate-1.13)
when a call to a wait(), wait_until(), or wait_for() function on a condition variable ([[thread.condition.condvar]](thread.condition.condvar "32.7.4 Class condition_­variable"), [[thread.condition.condvarany]](thread.condition.condvarany "32.7.5 Class condition_­variable_­any"))
fails to meet a postcondition, or
- [(1.14)](#except.terminate-1.14)
when a callback invocation exits via an exception
when requesting stop on
a std​::​stop_source or
a std​::​inplace_stop_source ([[stopsource.mem]](stopsource.mem "32.3.5.3 Member functions"), [[stopsource.inplace.mem]](stopsource.inplace.mem "32.3.9.3 Member functions")),
or in the constructor ofstd​::​stop_callback orstd​::​inplace_stop_callback ([[stopcallback.cons]](stopcallback.cons "32.3.6.2 Constructors and destructor"), [[stopcallback.inplace.cons]](stopcallback.inplace.cons "32.3.10.2 Constructors and destructor"))
when a callback invocation exits via an exception, or
- [(1.15)](#except.terminate-1.15)
when a run_loop object is destroyed
that is still in the running state ([[exec.run.loop]](exec.run.loop "33.12.1 execution​::​run_­loop")), or
- [(1.16)](#except.terminate-1.16)
when unhandled_stopped is called on
a with_awaitable_senders<T> object ([[exec.with.awaitable.senders]](exec.with.awaitable.senders "33.13.2 execution​::​with_­awaitable_­senders"))
whose continuation is not a handle to a coroutine
whose promise type has an unhandled_stopped member function, or
- [(1.17)](#except.terminate-1.17)
when an object scope of typestd​::​execution​::​simple_counting_scope orstd​::​execution​::​counting_scope is destroyed andscope.*state* is not equal to*joined*,*unused*, or*unused-and-closed* ([[exec.simple.counting.ctor]](exec.simple.counting.ctor "33.14.2.2.2 Constructor and Destructor")), or
- [(1.18)](#except.terminate-1.18)
when std​::​execution​::​get_parallel_scheduler is called andstd​::​execution​::​system_context_replaceability​::​query_parallel_scheduler_backend() returns a null pointer value ([[exec.par.scheduler]](exec.par.scheduler "33.15 Parallel scheduler")), or
- [(1.19)](#except.terminate-1.19)
when an exception is thrown from a coroutine std​::​execution​::​task ([[exec.task]](exec.task "33.13.6 execution​::​task"))
which doesn't support a std​::​execution​::​set_error_t(std​::​exception_ptr) completion[.](#except.terminate-1.sentence-3)
— *end note*]
[2](#except.terminate-2)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exceptions.tex#L1149)
In the situation where no matching handler is found, it isimplementation-defined
whether or not the stack is unwound
before std​::​terminate is invoked[.](#except.terminate-2.sentence-1)
In the situation where the search for a handler ([[except.handle]](except.handle "14.4 Handling an exception"))
exits the function body of a function
with a non-throwing exception specification ([[except.spec]](except.spec "14.5 Exception specifications")), it isimplementation-defined
whether the stack is unwound, unwound partially, or not unwound at all
before the function std​::​terminate is invoked[.](#except.terminate-2.sentence-2)
In all other situations, the stack shall not be unwound before
the function std​::​terminate is invoked[.](#except.terminate-2.sentence-3)
An implementation is not permitted to finish stack unwinding
prematurely based on a determination that the unwind process
will eventually cause an invocation of the functionstd​::​terminate[.](#except.terminate-2.sentence-4)

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[except.special.general]
# 14 Exception handling [[except]](./#except)
## 14.6 Special functions [[except.special]](except.special#general)
### 14.6.1 General [except.special.general]
[1](#1)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exceptions.tex#L1017)
The function std​::​terminate ([[except.terminate]](except.terminate "14.6.2 The std​::​terminate function"))
is used by the exception
handling mechanism for coping with errors related to the exception handling
mechanism itself[.](#1.sentence-1)
The function std​::​uncaught_exceptions ([[uncaught.exceptions]](uncaught.exceptions "17.9.6 uncaught_­exceptions"))
reports how many exceptions are uncaught in the current thread[.](#1.sentence-2)
The function std​::​current_exception ([[propagation]](propagation "17.9.7 Exception propagation")) and the classstd​::​nested_exception ([[except.nested]](except.nested "17.9.8 nested_­exception")) can be used by a program to
capture the currently handled exception[.](#1.sentence-3)

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[except.terminate]
# 14 Exception handling [[except]](./#except)
## 14.6 Special functions [[except.special]](except.special#except.terminate)
### 14.6.2 The std​::​terminate function [except.terminate]
[1](#1)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exceptions.tex#L1030)
Some errors in a program cannot be recovered from, such as when an exception
is not handled or a std​::​thread object is destroyed while its thread
function is still executing[.](#1.sentence-1)
In such cases,
the function std​::​terminate ([[exception.terminate]](exception.terminate "17.9.5 Abnormal termination")) is invoked[.](#1.sentence-2)
[*Note [1](#note-1)*:
These situations are:
- [(1.1)](#1.1)
when the exception handling mechanism, after completing
the initialization of the exception object
but before
activation of a handler for the exception ([[except.throw]](except.throw "14.2 Throwing an exception")),
calls a function that exits
via an exception, or
- [(1.2)](#1.2)
when the exception handling mechanism cannot find a handler for a thrown exception ([[except.handle]](except.handle "14.4 Handling an exception")), or
- [(1.3)](#1.3)
when the search for a [handler](except.handle "14.4 Handling an exception [except.handle]") exits the function body of a function
with a non-throwing [exception specification](except.spec "14.5 Exception specifications [except.spec]"),
including when a contract-violation handler
invoked from an evaluation of
a function contract assertion ([[basic.contract.eval]](basic.contract.eval "6.11.2 Evaluation")) associated with the function
exits via an exception,
or
- [(1.4)](#1.4)
when the destruction of an object during [stack unwinding](except.ctor#def:stack_unwinding "14.3 Stack unwinding [except.ctor]") terminates by throwing an exception, or
- [(1.5)](#1.5)
when initialization of a non-block
variable with static or thread storage duration ([[basic.start.dynamic]](basic.start.dynamic "6.10.3.3 Dynamic initialization of non-block variables"))
exits via an exception, or
- [(1.6)](#1.6)
when destruction of an object with static or thread storage duration exits
via an exception ([[basic.start.term]](basic.start.term "6.10.3.4 Termination")), or
- [(1.7)](#1.7)
when execution of a function registered withstd​::​atexit or std​::​at_quick_exit exits via an exception ([[support.start.term]](support.start.term "17.5 Startup and termination")), or
- [(1.8)](#1.8)
when a[*throw-expression*](expr.throw#nt:throw-expression "7.6.18 Throwing an exception [expr.throw]") ([[expr.throw]](expr.throw "7.6.18 Throwing an exception"))
with no operand attempts to rethrow an exception and no exception is being
handled ([[except.throw]](except.throw "14.2 Throwing an exception")), or
- [(1.9)](#1.9)
when the function std​::​nested_exception​::​rethrow_nested is called for an object
that has captured no exception ([[except.nested]](except.nested "17.9.8 nested_­exception")), or
- [(1.10)](#1.10)
when execution of the initial function of a thread exits via
an exception ([[thread.thread.constr]](thread.thread.constr "32.4.3.3 Constructors")), or
- [(1.11)](#1.11)
for a parallel algorithm whose ExecutionPolicy specifies such
behavior ([[execpol.seq]](execpol.seq "26.3.6.3 Sequenced execution policy"), [[execpol.par]](execpol.par "26.3.6.4 Parallel execution policy"), [[execpol.parunseq]](execpol.parunseq "26.3.6.5 Parallel and unsequenced execution policy")),
when execution of an element access function ([[algorithms.parallel.defns]](algorithms.parallel.defns "26.3.1 Preamble"))
of the parallel algorithm exits via an exception ([[algorithms.parallel.exceptions]](algorithms.parallel.exceptions "26.3.4 Parallel algorithm exceptions")), or
- [(1.12)](#1.12)
when the destructor or the move assignment operator is invoked on an object
of type std​::​thread that refers to
a joinable thread ([[thread.thread.destr]](thread.thread.destr "32.4.3.4 Destructor"), [[thread.thread.assign]](thread.thread.assign "32.4.3.5 Assignment")), or
- [(1.13)](#1.13)
when a call to a wait(), wait_until(), or wait_for() function on a condition variable ([[thread.condition.condvar]](thread.condition.condvar "32.7.4 Class condition_­variable"), [[thread.condition.condvarany]](thread.condition.condvarany "32.7.5 Class condition_­variable_­any"))
fails to meet a postcondition, or
- [(1.14)](#1.14)
when a callback invocation exits via an exception
when requesting stop on
a std​::​stop_source or
a std​::​inplace_stop_source ([[stopsource.mem]](stopsource.mem "32.3.5.3 Member functions"), [[stopsource.inplace.mem]](stopsource.inplace.mem "32.3.9.3 Member functions")),
or in the constructor ofstd​::​stop_callback orstd​::​inplace_stop_callback ([[stopcallback.cons]](stopcallback.cons "32.3.6.2 Constructors and destructor"), [[stopcallback.inplace.cons]](stopcallback.inplace.cons "32.3.10.2 Constructors and destructor"))
when a callback invocation exits via an exception, or
- [(1.15)](#1.15)
when a run_loop object is destroyed
that is still in the running state ([[exec.run.loop]](exec.run.loop "33.12.1 execution​::​run_­loop")), or
- [(1.16)](#1.16)
when unhandled_stopped is called on
a with_awaitable_senders<T> object ([[exec.with.awaitable.senders]](exec.with.awaitable.senders "33.13.2 execution​::​with_­awaitable_­senders"))
whose continuation is not a handle to a coroutine
whose promise type has an unhandled_stopped member function, or
- [(1.17)](#1.17)
when an object scope of typestd​::​execution​::​simple_counting_scope orstd​::​execution​::​counting_scope is destroyed andscope.*state* is not equal to*joined*,*unused*, or*unused-and-closed* ([[exec.simple.counting.ctor]](exec.simple.counting.ctor "33.14.2.2.2 Constructor and Destructor")), or
- [(1.18)](#1.18)
when std​::​execution​::​get_parallel_scheduler is called andstd​::​execution​::​system_context_replaceability​::​query_parallel_scheduler_backend() returns a null pointer value ([[exec.par.scheduler]](exec.par.scheduler "33.15 Parallel scheduler")), or
- [(1.19)](#1.19)
when an exception is thrown from a coroutine std​::​execution​::​task ([[exec.task]](exec.task "33.13.6 execution​::​task"))
which doesn't support a std​::​execution​::​set_error_t(std​::​exception_ptr) completion[.](#1.sentence-3)
— *end note*]
[2](#2)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exceptions.tex#L1149)
In the situation where no matching handler is found, it isimplementation-defined
whether or not the stack is unwound
before std​::​terminate is invoked[.](#2.sentence-1)
In the situation where the search for a handler ([[except.handle]](except.handle "14.4 Handling an exception"))
exits the function body of a function
with a non-throwing exception specification ([[except.spec]](except.spec "14.5 Exception specifications")), it isimplementation-defined
whether the stack is unwound, unwound partially, or not unwound at all
before the function std​::​terminate is invoked[.](#2.sentence-2)
In all other situations, the stack shall not be unwound before
the function std​::​terminate is invoked[.](#2.sentence-3)
An implementation is not permitted to finish stack unwinding
prematurely based on a determination that the unwind process
will eventually cause an invocation of the functionstd​::​terminate[.](#2.sentence-4)

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[except.throw]
# 14 Exception handling [[except]](./#except)
## 14.2 Throwing an exception [except.throw]
[1](#1)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exceptions.tex#L192)
Throwing an exception transfers control to a handler[.](#1.sentence-1)
[*Note [1](#note-1)*:
An exception can be thrown from one of the following contexts:[*throw-expression*](expr.throw#nt:throw-expression "7.6.18 Throwing an exception [expr.throw]")*s* ([[expr.throw]](expr.throw "7.6.18 Throwing an exception")),
allocation functions ([[basic.stc.dynamic.allocation]](basic.stc.dynamic.allocation "6.8.6.5.2 Allocation functions")),dynamic_cast ([[expr.dynamic.cast]](expr.dynamic.cast "7.6.1.7 Dynamic cast")),typeid ([[expr.typeid]](expr.typeid "7.6.1.8 Type identification")),[*new-expression*](expr.new#nt:new-expression "7.6.2.8 New [expr.new]")*s* ([[expr.new]](expr.new "7.6.2.8 New")), and standard library
functions ([[structure.specifications]](structure.specifications "16.3.2.4 Detailed specifications"))[.](#1.sentence-2)
— *end note*]
An object is passed and the type of that object determines which handlers
can catch it[.](#1.sentence-3)
[*Example [1](#example-1)*:
throw "Help!"; can be caught by a[*handler*](except.pre#nt:handler "14.1 Preamble [except.pre]") ofconstchar* type:try {// ...} catch(const char* p) {// handle character string exceptions here} andclass Overflow {public: Overflow(char,double,double);};
void f(double x) {throw Overflow('+',x,3.45e107);} can be caught by a handler for exceptions of typeOverflow:try { f(1.2);} catch(Overflow& oo) {// handle exceptions of type Overflow here}
— *end example*]
[2](#2)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exceptions.tex#L244)
When an exception is thrown, control is transferred to the nearest handler with
a matching type ([[except.handle]](except.handle "14.4 Handling an exception")); “nearest” means the handler
for which the[*compound-statement*](stmt.block#nt:compound-statement "8.4 Compound statement or block [stmt.block]") or[*ctor-initializer*](class.base.init#nt:ctor-initializer "11.9.3 Initializing bases and members [class.base.init]") following thetry keyword was most recently entered by the thread of control and not yet exited[.](#2.sentence-1)
[3](#3)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exceptions.tex#L257)
Throwing an exception
initializes an object with dynamic storage duration,
called the[*exception object*](#def:exception_handling,exception_object "14.2 Throwing an exception [except.throw]")[.](#3.sentence-1)
If the type of the exception object would be
an incomplete type ([[basic.types.general]](basic.types.general "6.9.1 General")),
an abstract class type ([[class.abstract]](class.abstract "11.7.4 Abstract classes")),
or a pointer to an incomplete type other thancv void ([[basic.compound]](basic.compound "6.9.4 Compound types")),
the program is ill-formed[.](#3.sentence-2)
[4](#4)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exceptions.tex#L269)
The memory for the exception object is
allocated in an unspecified way, except as noted in [[basic.stc.dynamic.allocation]](basic.stc.dynamic.allocation "6.8.6.5.2 Allocation functions")[.](#4.sentence-1)
If a handler exits by rethrowing, control is passed to another handler for
the same exception object[.](#4.sentence-2)
The points of potential destruction for the exception object are:
- [(4.1)](#4.1)
when an active handler for the exception exits by
any means other than
rethrowing,
immediately after the destruction of the object (if any)
declared in the [*exception-declaration*](except.pre#nt:exception-declaration "14.1 Preamble [except.pre]") in the handler;
- [(4.2)](#4.2)
when an object of type [std​::​exception_ptr](propagation#lib:exception_ptr "17.9.7 Exception propagation [propagation]") that refers to the exception object is destroyed,
before the destructor of std​::​exception_ptr returns[.](#4.sentence-3)
Among all points of potential destruction for the exception object,
there is an unspecified last one
where the exception object is destroyed[.](#4.sentence-4)
All other points [happen before](intro.races#def:happens_before "6.10.2.2 Data races [intro.races]") that last one[.](#4.sentence-5)
[*Note [2](#note-2)*:
No other thread synchronization is implied in exception handling[.](#4.sentence-6)
— *end note*]
The implementation may then
deallocate the memory for the exception object; any such deallocation
is done in an unspecified way[.](#4.sentence-7)
[*Note [3](#note-3)*:
A thrown exception does not
propagate to other threads unless caught, stored, and rethrown using
appropriate library functions; see [[propagation]](propagation "17.9.7 Exception propagation") and [[futures]](futures "32.10 Futures")[.](#4.sentence-8)
— *end note*]
[5](#5)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exceptions.tex#L308)
Let T denote the type of the exception object[.](#5.sentence-1)
Copy-initialization of an object of type T from
an lvalue of type const T in a context unrelated to T shall be well-formed[.](#5.sentence-2)
If T is a class type,
the selected constructor is odr-used ([[basic.def.odr]](basic.def.odr "6.3 One-definition rule")) and
the destructor of T is potentially invoked ([[class.dtor]](class.dtor "11.4.7 Destructors"))[.](#5.sentence-3)
[6](#6)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exceptions.tex#L319)
An exception is considered [*uncaught*](#def:uncaught_exception "14.2 Throwing an exception [except.throw]") after completing the initialization of the exception object
until completing the activation of a handler for the exception ([[except.handle]](except.handle "14.4 Handling an exception"))[.](#6.sentence-1)
[*Note [4](#note-4)*:
As a consequence, an exception is considered uncaught
during any stack unwinding resulting from it being thrown[.](#6.sentence-2)
— *end note*]
[7](#7)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exceptions.tex#L329)
If an exception is rethrown ([[expr.throw]](expr.throw "7.6.18 Throwing an exception"), [[propagation]](propagation "17.9.7 Exception propagation")),
it is considered uncaught from the point of rethrow
until the rethrown exception is caught[.](#7.sentence-1)
[*Note [5](#note-5)*:
The function std​::​uncaught_exceptions ([[uncaught.exceptions]](uncaught.exceptions "17.9.6 uncaught_­exceptions"))
returns the number of uncaught exceptions in the current thread[.](#7.sentence-2)
— *end note*]
[8](#8)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exceptions.tex#L339)
An exception is considered caught when a handler for that exception
becomes [active](except.handle#def:exception_handling,handler,active "14.4 Handling an exception [except.handle]")[.](#8.sentence-1)
[*Note [6](#note-6)*:
An exception can have active handlers and still be considered uncaught if
it is rethrown[.](#8.sentence-2)
— *end note*]
[9](#9)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exceptions.tex#L349)
If the exception handling mechanism
handling an uncaught exception
directly invokes a function that exits via an
exception, the function [std​::​terminate](except.terminate "14.6.2 The std​::​terminate function [except.terminate]") is invoked[.](#9.sentence-1)
[*Example [2](#example-2)*: struct C { C() { } C(const C&) {if (std::uncaught_exceptions()) {throw 0; // throw during copy to handler's [*exception-declaration*](except.pre#nt:exception-declaration "14.1 Preamble [except.pre]") object ([[except.handle]](except.handle "14.4 Handling an exception"))}}};
int main() {try {throw C(); // calls std​::​terminate if construction of the handler's// [*exception-declaration*](except.pre#nt:exception-declaration "14.1 Preamble [except.pre]") object is not elided ([[class.copy.elision]](class.copy.elision "11.9.6 Copy/move elision"))} catch(C) { }} — *end example*]
[*Note [7](#note-7)*:
If a destructor directly invoked by stack unwinding exits via an exception,std​::​terminate is invoked[.](#9.sentence-2)
— *end note*]