std::rethrow_if_nested

 

C++

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Exception handling exception uncaught_exceptionuncaught_exceptions (until C++20*)  (C++17) exception_ptr (C++11) make_exception_ptr (C++11) current_exception (C++11) rethrow_exception (C++11) nested_exception (C++11) throw_with_nested (C++11) rethrow_if_nested (C++11) Exception handling failures terminate terminate_handler get_terminate (C++11) set_terminate bad_exception unexpected (until C++17*) unexpected_handler (until C++17*) get_unexpected (until C++17*) set_unexpected (until C++17*) Error numbers Error codes errno Assertions assert

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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

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 <exception>
#include <fstream>
#include <iostream>
#include <stdexcept>
#include <string>
 
// 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 (C++11)	a mixin type to capture and store current exceptions (class)
throw_with_nested (C++11)	throws its argument with std::nested_exception mixed in (function template)