[support.exception] (original) (raw)

17 Language support library [support]

17.9 Exception handling [support.exception]

17.9.2 Header synopsis [exception.syn]

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;constexpr int uncaught_exceptions() noexcept;using exception_ptr = unspecified;constexpr exception_ptr current_exception() noexcept;[[noreturn]] constexpr void rethrow_exception(exception_ptr p);template<class E> constexpr exception_ptr make_exception_ptr(E e) noexcept;template<class T> [[noreturn]] constexpr void throw_with_nested(T&& t);template<class E> constexpr void rethrow_if_nested(const E& e);}

17.9.3 Class exception [exception]

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

The classexceptiondefines 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 exceptionhas the following publicly accessible member functions, each of them having a non-throwing exception specification ([except.spec]):

• default constructor (unless the class synopsis shows other constructors)
• copy constructor
• 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, thenstrcmp(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).

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

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

constexpr virtual ~exception();

Effects: Destroys an object of classexception.

constexpr virtual const char* what() const noexcept;

Returns: An implementation-defined ntbs, which during constant evaluation is encoded with the ordinary literal encoding ([lex.ccon]).

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

17.9.4 Class bad_exception [bad.exception]

namespace std { class bad_exception : public exception { public: constexpr const char* what() const noexcept override;};}

The classbad_exceptiondefines the type of the object referenced by the exception_ptrreturned from a call to current_exception ([propagation]) when the currently active exception object fails to copy.

constexpr 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]

using terminate_handler = void (*)();

The type of a handler functionto 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 callsabort().

17.9.5.2 set_terminate [set.terminate]

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 defaultterminate_handler.

17.9.5.3 get_terminate [get.terminate]

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]

[[noreturn]] void terminate() noexcept;

Effects: Calls a terminate_handler function.

It is unspecified whichterminate_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]

constexpr int uncaught_exceptions() noexcept;

Returns: The number of uncaught exceptions ([except.throw]) in the current thread.

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

17.9.7 Exception propagation [propagation]

using exception_ptr = _unspecified_;

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

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 onexception_ptr objects shall access and modify only theexception_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:

Ifrethrow_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_]

All member functions are marked constexpr.

constexpr exception_ptr current_exception() noexcept;

Returns: An exception_ptr object that refers to thecurrently handled exceptionor 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 anexception_ptr object that refers to it.

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

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

[Note 3:

That is, it is unspecified whether current_exceptioncreates 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_]

[[noreturn]] constexpr 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.

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

template<class E> constexpr 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]

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;[[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);}

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_]

constexpr nested_exception() noexcept;

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

[[noreturn]] constexpr 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.

constexpr exception_ptr nested_ptr() const noexcept;

Returns: The stored exception captured by this nested_exception object.

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

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_exceptionand constructed from std​::​forward<T>(t), otherwisestd​::​forward<T>(t).

template<class E> constexpr 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();