cppdraft_translate/cppdraft/except/pre.md

[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)