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std::rethrow_if_nested(3)     C++ Standard Libary    std::rethrow_if_nested(3)

NAME
       std::rethrow_if_nested -	std::rethrow_if_nested

Synopsis
	  Defined in header <exception>
	  template< class E >			 (since	C++11)
	  void rethrow_if_nested( const	E& e );

	  If E is not a	polymorphic class type,	or if std::nested_exception is
       an inaccessible
	  or ambiguous base class of E,	there is no effect.

	  Otherwise, performs

	if  (auto  p = dynamic_cast<const std::nested_exception*>(std::addres-
       sof(e)))
	    p->rethrow_nested();

Parameters
	  e - the exception object to rethrow

Return value
	  (none)

Notes
	  Unlike many related functions, this function is not intended	to  be
       called with a
	  std::exception_ptr but rather	an actual exception reference.

Possible implementation
	  namespace details {
	      template <class E>
	      struct can_dynamic_cast
		  : std::integral_constant<bool,
			std::is_polymorphic<E>::value &&
			(!std::is_base_of<std::nested_exception, E>::value ||
			  std::is_convertible<E*,	    std::nested_excep-
       tion*>::value)
		    > {	};

	      template <class T>
	      void rethrow_if_nested_impl(const	T& e, std::true_type)
	      {
		  if(auto   nep	  =   dynamic_cast<const    std::nested_excep-
       tion*>(std::addressof(e)))
		      nep->rethrow_nested();
	      }

	      template <class T>
	      void rethrow_if_nested_impl(const	T&, std::false_type) { }
	  }

	  template <class T>
	  void rethrow_if_nested(const T& t){
	      details::rethrow_if_nested_impl(t,	      details::can_dy-
       namic_cast<T>());
	  }

Example
	  Demonstrates construction and	recursion through a  nested  exception
       object.

       // Run this code

	#include <iostream>
	#include <stdexcept>
	#include <exception>
	#include <string>
	#include <fstream>

	//  prints the explanatory string of an	exception. If the exception is
       nested,
	// recurses to print the explanatory of	the exception it holds
	void print_exception(const std::exception& e, int level	=  0)
	{
	    std::cerr << std::string(level, ' ') << "exception:	" <<  e.what()
       << '\n';
	    try	{
		std::rethrow_if_nested(e);
	    } catch(const std::exception& nestedException) {
		print_exception(nestedException, level+1);
	    } catch(...) {}
	}

	//  sample function that catches an exception and wraps	it in a	nested
       exception
	void open_file(const std::string& s)
	{
	    try	{
		std::ifstream file(s);
		file.exceptions(std::ios_base::failbit);
	    } catch(...) {
		std::throw_with_nested(	std::runtime_error("Couldn't open "  +
       s) );
	    }
	}

	//  sample function that catches an exception and wraps	it in a	nested
       exception
	void run()
	{
	    try	{
		open_file("nonexistent.file");
	    } catch(...) {
		std::throw_with_nested(	std::runtime_error("run() failed") );
	    }
	}

	// runs	the sample function above and prints the caught	exception
	int main()
	{
	    try	{
		run();
	    } catch(const std::exception& e) {
		print_exception(e);
	    }
	}

Possible output:
	exception: run() failed
	 exception: Couldn't open nonexistent.file
	  exception: basic_ios::clear

See also
	  nested_exception  a mixin type to capture and	store  current	excep-
       tions
	  (C++11)	    (class)
	  throw_with_nested  throws  its  argument  with std::nested_exception
       mixed in
	  (C++11)	    (function template)

http://cppreference.com		  2022.07.31	     std::rethrow_if_nested(3)

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