std::throw_with_nested

Defined in header <exception>

template< class T >
[[noreturn]] void throw_with_nested( T&& t );

(since C++11)

If std::remove_reference_t<T> is a non-final non-union class type that is neither std::nested_exception nor derived from std::nested_exception, throws an exception of an unspecified type that is publicly derived from both std::nested_exception and from std::remove_reference_t<T>, and constructed from std::forward<T>(t). The default constructor of the nested_exception base class calls std::current_exception, capturing the currently handled exception object, if any, in a std::exception_ptr.

Otherwise, throws std::forward<T>(t).

Requires that std::remove_reference<T>::type is CopyConstructible.

Parameters

the exception object to throw

Return value

(none).

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& 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

nested_exception (C++11)	a mixin type to capture and store current exceptions (class)
rethrow_if_nested (C++11)	throws the exception from a `std::nested_exception` (function template)

std::throw_with_nested

Parameters

Return value

Example

See also