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cppdraft/except/nested.md

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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();