17 Language support library [support]

17.9 Exception handling [support.exception]

17.9.8 nested_exception [except.nested]

namespace std { class nested_exception { public: nested_exception() noexcept; nested_exception(const nested_exception&) noexcept = default; nested_exception& operator=(const nested_exception&) noexcept = default; virtual ~nested_exception() = default; // access functions [[noreturn]] void rethrow_nested() const; exception_ptr nested_ptr() const noexcept; }; template<class T> [[noreturn]] void throw_with_nested(T&& t); template<class E> void rethrow_if_nested(const E& e); }

The class nested_exception is designed for use as a mixin through multiple inheritance.

It captures the currently handled exception and stores it for later use.

[Note 1:

nested_exception has a virtual destructor to make it a polymorphic class.

Its presence can be tested for with dynamic_cast.

— end note]

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nested_exception() noexcept;

Effects: The constructor calls current_exception() and stores the returned value.

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[[noreturn]] void rethrow_nested() const;

Effects: If nested_ptr() returns a null pointer, the function calls the function std::terminate.

Otherwise, it throws the stored exception captured by *this.

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exception_ptr nested_ptr() const noexcept;

Returns: The stored exception captured by this nested_exception object.

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template<class T> [[noreturn]] void throw_with_nested(T&& t);

Let U be decay_t<T>.

Preconditions: U meets the Cpp17CopyConstructible requirements.

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 both U and nested_exception and constructed from std::forward<T>(t), otherwise std::forward<T>(t).

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template<class E> void rethrow_if_nested(const E& e);

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.

Otherwise, performs: if (auto p = dynamic_cast<const nested_exception*>(addressof(e))) p->rethrow_nested();