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std::throw_with_nested

From cppreference.com
< cpp‎ | error
 
 
 
Defined in header <exception>
template< class T >
[[noreturn]] void throw_with_nested( T&& t );
(since C++11)
(constexpr since C++26)

If std::decay<T>::type 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::decay<T>::type, 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::decay<T>::type is CopyConstructible.

Contents

[edit] Parameters

t - the exception object to throw

[edit] Notes

Feature-test macro Value Std Feature
__cpp_lib_constexpr_exceptions 202411L (C++26) constexpr for exception types

[edit] Example

Demonstrates construction and recursion through a nested exception object.

#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 string 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

[edit] See also

a mixin type to capture and store current exceptions
(class) [edit]
throws the exception from a std::nested_exception
(function template) [edit]