18 Language support library [language.support]

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

exception_ptr shall satisfy the requirements of NullablePointer ([nullablepointer.requirements]).

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: An implementation might 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: if rethrow_exception rethrows the same exception object (rather than a copy), concurrent access to that rethrown exception object may 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 ]

Returns: An exception_ptr object that refers to the currently handled exception ([except.handle]) 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: 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: The copy constructor of the thrown exception may also fail, so the implementation is allowed to substitute a bad_exception object to avoid infinite recursion. — end note ]

Requires: p shall not be a null pointer.

Throws: the exception object to which p refers.

Effects: Creates an exception_ptr object that refers to a copy of e, as if

try {
  throw e;
} catch(...) {
  return current_exception();
}

[ Note: This function is provided for convenience and efficiency reasons.  — end note ]