Boost Exception

exception_ptr

namespace
boost
    {
    typedef ---unspecified--- exception_ptr;
    }

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.

The referenced object remains valid at least as long as there is an exception_ptr object that refers to it.

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.

Thread safety:

The exception_ptr type is "as thread-safe as built-in types":

An exception_ptr instance can be "read" simultaneously by multiple threads
Different exception_ptr instances can be "written to" simultaneously by multiple threads, even when these instances refer to the same exception object

All other simultaneous accesses result in undefined behavior.

Nesting of exceptions:

An exception_ptr can be added as error_info to any boost::exception. This is a convenient way to nest exceptions. There is no limit on the depth of the nesting, however cyclic references result in undefined behavior.