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std::rethrow_if_nested

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

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

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.

#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