17 Language support library [support]

17.9 Exception handling [support.exception]

17.9.1 General [support.exception.general]

The header defines several types and functions related to the handling of exceptions in a C++ program.

17.9.2 Header <exception> synopsis [exception.syn]

// all freestanding namespace std { class exception; class bad_exception; class nested_exception; using terminate_handler = void (*)(); terminate_handler get_terminate() noexcept; terminate_handler set_terminate(terminate_handler f) noexcept; [[noreturn]] void terminate() noexcept; int uncaught_exceptions() noexcept; using exception_ptr = unspecified; exception_ptr current_exception() noexcept; [[noreturn]] void rethrow_exception(exception_ptr p); template<class E> exception_ptr make_exception_ptr(E e) noexcept; template<class T> [[noreturn]] void throw_with_nested(T&& t); template<class E> void rethrow_if_nested(const E& e); }

17.9.3 Class exception [exception]

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namespace std { class exception { public: exception() noexcept; exception(const exception&) noexcept; exception& operator=(const exception&) noexcept; virtual ~exception(); virtual const char* what() const noexcept; }; }

The class exception defines the base class for the types of objects thrown as exceptions by C++ standard library components, and certain expressions, to report errors detected during program execution.

Except where explicitly specified otherwise, each standard library class T that derives from class exception has the following publicly accessible member functions, each of them having a non-throwing exception specification ([except.spec]):

(2.1)
default constructor (unless the class synopsis shows other constructors)
(2.2)
copy constructor
(2.3)
copy assignment operator

The copy constructor and the copy assignment operator meet the following postcondition: If two objects lhs and rhs both have dynamic type T and lhs is a copy of rhs, then strcmp(lhs.what(), rhs.what()) is equal to 0.

The what() member function of each such T satisfies the constraints specified for exception::what() (see below).

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exception(const exception& rhs) noexcept;
exception& operator=(const exception& rhs) noexcept;

Postconditions: If *this and rhs both have dynamic type exception then the value of the expression strcmp(what(), rhs.what()) shall equal 0.

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virtual ~exception();

Effects: Destroys an object of class exception.

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virtual const char* what() const noexcept;

Returns: An implementation-defined ntbs.

Remarks: The message may be a null-terminated multibyte string, suitable for conversion and display as a wstring ([string.classes], [locale.codecvt]).

The return value remains valid until the exception object from which it is obtained is destroyed or a non-const member function of the exception object is called.

17.9.4 Class bad_exception [bad.exception]

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namespace std { class bad_exception : public exception { public: // see [exception] for the specification of the special member functions const char* what() const noexcept override; }; }

The class bad_exception defines the type of the object referenced by the exception_ptr returned from a call to current_exception ([propagation]) when the currently active exception object fails to copy.

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const char* what() const noexcept override;

Returns: An implementation-defined ntbs.

17.9.5 Abnormal termination [exception.terminate]

17.9.5.1 Type terminate_handler [terminate.handler]

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using terminate_handler = void (*)();

The type of a handler function to be invoked by terminate when terminating exception processing.

Required behavior: A terminate_handler shall terminate execution of the program without returning to the caller.

Default behavior: The implementation's default terminate_handler calls abort().

17.9.5.2 set_terminate [set.terminate]

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terminate_handler set_terminate(terminate_handler f) noexcept;

Effects: Establishes the function designated by f as the current handler function for terminating exception processing.

Returns: The previous terminate_handler.

Remarks: It is unspecified whether a null pointer value designates the default terminate_handler.

17.9.5.3 get_terminate [get.terminate]

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terminate_handler get_terminate() noexcept;

Returns: The current terminate_handler.

[Note 1:

This can be a null pointer value.

— end note]

17.9.5.4 terminate [terminate]

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[[noreturn]] void terminate() noexcept;

Effects: Calls a terminate_handler function.

It is unspecified which terminate_handler function will be called if an exception is active during a call to set_terminate.

Otherwise calls the current terminate_handler function.

[Note 1:

A default terminate_handler is always considered a callable handler in this context.

— end note]

Remarks: Called by the implementation when exception handling must be abandoned for any of several reasons ([except.terminate]).

May also be called directly by the program.

17.9.6 uncaught_exceptions [uncaught.exceptions]

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int uncaught_exceptions() noexcept;

Returns: The number of uncaught exceptions .

Remarks: When uncaught_exceptions() > 0, throwing an exception can result in a call of the function std::terminate .

17.9.7 Exception propagation [propagation]

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using exception_ptr = unspecified;

The type exception_ptr can be used to refer to an exception object.

exception_ptr meets the requirements of Cpp17NullablePointer (Table 36).

Two non-null values of type exception_ptr are equivalent and compare equal if and only if they refer to the same exception.

The default constructor of exception_ptr produces the null value of the type.

exception_ptr shall not be implicitly convertible to any arithmetic, enumeration, or pointer type.

[Note 1:

An implementation can use a reference-counted smart pointer as exception_ptr.

— end note]

For purposes of determining the presence of a data race, operations on exception_ptr objects shall access and modify only the exception_ptr objects themselves and not the exceptions they refer to.

Use of rethrow_exception on exception_ptr objects that refer to the same exception object shall not introduce a data race.

[Note 2:

If rethrow_exception rethrows the same exception object (rather than a copy), concurrent access to that rethrown exception object can introduce a data race.

Changes in the number of exception_ptr objects that refer to a particular exception do not introduce a data race.

— end note]

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exception_ptr current_exception() noexcept;

Returns: An exception_ptr object that refers to the currently handled exception or a copy of the currently handled exception, or a null exception_ptr object if no exception is being handled.

The referenced object shall remain valid at least as long as there is an exception_ptr object that refers to it.

If the function needs to allocate memory and the attempt fails, it returns an exception_ptr object that refers to an instance of bad_alloc.

It is unspecified whether the return values of two successive calls to current_exception refer to the same exception object.

[Note 3:

That is, it is unspecified whether current_exception creates a new copy each time it is called.

— end note]

If the attempt to copy the current exception object throws an exception, the function returns an exception_ptr object that refers to the thrown exception or, if this is not possible, to an instance of bad_exception.

[Note 4:

The copy constructor of the thrown exception can also fail, so the implementation can substitute a bad_exception object to avoid infinite recursion.

— end note]

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[[noreturn]] void rethrow_exception(exception_ptr p);

Preconditions: p is not a null pointer.

Effects: Let u be the exception object to which p refers, or a copy of that exception object.

It is unspecified whether a copy is made, and memory for the copy is allocated in an unspecified way.

(10.1)
If allocating memory to form u fails, throws an instance of bad_alloc;
(10.2)
otherwise, if copying the exception to which p refers to form u throws an exception, throws that exception;
(10.3)
otherwise, throws u.

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template<class E> exception_ptr make_exception_ptr(E e) noexcept;

Effects: Creates an exception_ptr object that refers to a copy of e, as if: try { throw e; } catch(...) { return current_exception(); }

[Note 5:

This function is provided for convenience and efficiency reasons.

— end note]

17.9.8 nested_exception [except.nested]

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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();