std::current_exception(3)

NAME
std::current_exception - std::current_exception

Synopsis
   Defined in header <exception>
   std::exception_ptr current_exception() noexcept; (since C++11)
   If called during exception handling (typically, in a catch clause), captures the
   current exception object and creates an std::exception_ptr that holds either a copy
   or a reference to that exception object (depending on the implementation). The
   referenced object remains valid at least as long as there is an exception_ptr object
   that refers to it.
   If the implementation of this function requires a call to new and the call fails,
   the returned pointer will hold a reference to an instance of std::bad_alloc.
   If the implementation of this function requires copying the captured exception
   object and its copy constructor throws an exception, the returned pointer will hold
   a reference to the exception thrown. If the copy constructor of the thrown exception
   object also throws, the returned pointer may hold a reference to an instance of
   std::bad_exception to break the endless loop.
   If the function is called when no exception is being handled, an empty
   std::exception_ptr is returned.

Parameters
   (none)

Return value
   An instance of std::exception_ptr holding a reference to the exception object, or a
   copy of the exception object, or to an instance of std::bad_alloc or to an instance
   of std::bad_exception.

Notes
   On the implementations that follow Itanium C++ ABI (GCC, Clang, etc), exceptions are
   allocated on the heap when thrown (except for bad_alloc in some cases), and this
   function simply creates the smart pointer referencing the previously-allocated
   object, On MSVC, exceptions are allocated on stack when thrown, and this function
   performs the heap allocation and copies the exception object.

Example
// Run this code
 #include <iostream>
 #include <string>
 #include <exception>
 #include <stdexcept>
 void handle_eptr(std::exception_ptr eptr) // passing by value is ok
 {
     try {
         if (eptr) {
             std::rethrow_exception(eptr);
         }
     } catch(const std::exception& e) {
         std::cout << "Caught exception \"" << e.what() << "\"\n";
     }
 }
 int main()
 {
     std::exception_ptr eptr;
     try {
         std::string().at(1); // this generates an std::out_of_range
     } catch(...) {
         eptr = std::current_exception(); // capture
     }
     handle_eptr(eptr);
 } // destructor for std::out_of_range called here, when the eptr is destructed

Possible output:
 Caught exception "basic_string::at"

See also
   exception_ptr shared pointer type for handling exception objects
   (C++11) (typedef)
   rethrow_exception throws the exception from an std::exception_ptr
   (C++11) (function)
   make_exception_ptr creates an std::exception_ptr from an exception object
   (C++11) (function template)
   uncaught_exception
   uncaught_exceptions checks if exception handling is currently in progress
   (removed in C++20) (function)
   (C++17)