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Showing content from https://en.cppreference.com/w/cpp/language/../named_req/../language/../error/current_exception.html below:

std::current_exception - cppreference.com

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.

This function can be called in a std::terminate_handler to retrieve the exception which has provoked the invocation of std::terminate.

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.

On the implementations that follow Itanium C++ ABI (GCC, Clang, etc), exceptions are allocated on the heap when thrown (except for std::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.

On Windows in managed CLR environments [1], the implementation will store a std::bad_exception when the current exception is a managed exception ([2]). Note that catch(...) catches also managed exceptions:

#include <exception>
#include <iostream>
#include <stdexcept>
#include <string>
 
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
    {
        [[maybe_unused]]
        char ch = std::string().at(1); // this generates a 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: __n (which is 1) >= this->size() (which is 0)'

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