17 Language support library [support]

17.9 Exception handling [support.exception]

17.9.1 General [support.exception.general]

The header <exception> 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]

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]):
  • 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, 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).
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.
virtual ~exception();
Effects: Destroys an object of class exception.
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]

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.
const char* what() const noexcept override;
Returns: An implementation-defined ntbs.

17.9.5 Abnormal termination [exception.terminate] Type terminate_handler [terminate.handler]

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(). 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 default terminate_handler. 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] terminate [terminate]

[[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]

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]

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]
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]
[[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.
  • 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> 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: 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]
nested_exception() noexcept;
Effects: The constructor calls current_exception() and stores the returned value.
[[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.
exception_ptr nested_ptr() const noexcept;
Returns: The stored exception captured by this nested_exception object.
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).
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();