17 Language support library [support]

17.9 Exception handling [support.exception]

17.9.6 Exception propagation [propagation]

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using exception_ptr = unspecified;
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The type exception_­ptr can be used to refer to an exception object.
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exception_­ptr meets the requirements of Cpp17NullablePointer (Table 33).
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Two non-null values of type exception_­ptr are equivalent and compare equal if and only if they refer to the same exception.
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The default constructor of exception_­ptr produces the null value of the type.
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exception_­ptr shall not be implicitly convertible to any arithmetic, enumeration, or pointer type.
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[ Note
:
An implementation might use a reference-counted smart pointer as exception_­ptr.
— end note
 ]
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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
 ]
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exception_ptr current_exception() noexcept;
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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
:
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
 ]
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[[noreturn]] void rethrow_exception(exception_ptr p);
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Preconditions: p is not a null pointer.
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Throws: The exception object to which p refers.
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template<class E> exception_ptr make_exception_ptr(E e) noexcept;
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Effects: Creates an exception_­ptr object that refers to a copy of e, as if: 
try {
  throw e;
} catch(...) {
  return current_exception();
}
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[ Note
:
This function is provided for convenience and efficiency reasons.
— end note
 ]