std::rethrow_if_nested (3) - Linux Manuals

std::rethrow_if_nested: std::rethrow_if_nested

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& e) {
          print_exception(e, 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);
      }
  }

Output:

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