std::rethrow_if_nested(3)

std::rethrow_if_nested(3) 💖

okay bestie, welcome to the C++ Standard Libary page for std::rethrow_if_nested — and yes, the manual name literally says “Libary”, I’m not fixing it, that’s canon 😭. This is lowkey the “open the nested exception like a little matryoshka doll” function, and we’re gonna get into it. No cap, every detail is staying.

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🌸 NAME 🌸

• std::rethrow_if_nested — std::rethrow_if_nested (self-titled main character energy, we love that for her)

✨ Synopsis ✨

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

So here’s the vibe check: 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 — it just does nothing, periodt.

Otherwise, performs this exact little move:

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) — literally nothing, bestie, not even a vibe

🦋 Notes 🦋

Unlike many related functions, this function is not intended to be called with a std::exception_ptr but rather an actual exception reference. So don’t hand it the pointer-wrapper situation — hand it the actual reference, iykyk.

🔥 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 🌈

This example Demonstrates construction and recursion through a nested exception object.

The original literally says // Run this code, so here’s the code bestie:

// 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 — a mixin type to capture and store current exceptions (C++11) (class)
• throw_with_nested — throws its argument with std::nested_exception mixed in (C++11) (function template)

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💋 footer tea, not copied but lovingly name-dropped: source/version http://cppreference.com, date 2024.06.10, title stamp std::rethrow_if_nested(3) — periodt, that’s a wrap bestie 🫶