The exception_ptr type can be used to refer to a copy of an exception object. It is Default Constructible, Copy Constructible, Assignable and Equality Comparable; exception_ptr's operations do not throw.
Two instances of 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. The null value is equivalent only to itself.
- It is legal for multiple threads to hold exception_ptr references to the same exception object.
- It is illegal for multiple threads to modify the same exception_ptr object concurrently.
- While calling current_exception makes a copy of the current exception object, it is still possible for the two copies to share internal state. Therefore, in general it is not safe to call rethrow_exception concurrently to throw the same exception object into multiple threads.