Std::rethrow_if_nested (3) Linux Manual Page

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

std::rethrow_if_nested (3) Linux Manual Page