std::throw_with_nested(3)

🌸 NAME 🌸

Hey bestie! Welcome to the std::throw_with_nested(3) page from the C++ Standard Libary manual — yes, that's the manual name, spelling as-is, no cap. This is the whole vibe for std::throw_with_nested, and the official name line is literally std::throw_with_nested - std::throw_with_nested. Iconic. 💅

std::throw_with_nested - std::throw_with_nested

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✨ Synopsis ✨

Defined in header <exception> — here's the signature, character-for-character:

template< class T >                            (since C++11)
[[noreturn]] void throw_with_nested( T&& t );

Okay so here's the tea, fr fr. 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, then this 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. That's some big nested-exception energy, periodt. 🌈

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

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

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🎀 Parameters 🎀

• t — the exception object to throw.

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🦋 Return value 🦋

(none) — literally none, bestie. It's [[noreturn]], so it is not coming back to report one. 😭

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🌸 Example 🌸

It demonstrates construction and recursion through a nested exception object — the whole point, bestie. The example starts with // Run this code, and here's the whole flex:

// 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

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👑 See also 👑

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

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Source/version: http://cppreference.com · date: 2024.06.10 · page: std::throw_with_nested(3). Bye bestie, go slay those nested exceptions! 💅✨🦋