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::addressof(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_exception*>::value)
             > { };
       template <class T>
       void rethrow_if_nested_impl(const T& e, std::true_type)
       {
           if(auto nep = dynamic_cast<const std::nested_exception*>(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_dynamic_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 exceptions
   (C++11) (class)
   throw_with_nested throws its argument with std::nested_exception mixed in
   (C++11) (function template)