std::shared_future::wait(3)

💅 std::shared_future::wait — C++ Standard Library, bestie edition 🎀

okay so you're on the std::shared_future::wait page, which lives in the C++ Standard Library manual. sit down, hydrate, and let's get into it 🫶

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✨ NAME ✨

std::shared_future::wait — literally just std::shared_future::wait, no notes, that's the whole name 💅

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🌸 Synopsis 🌸

void wait() const;  (since C++11)

Blocks until the result becomes available. After the call, valid() == true ✨

⚠️ The behavior is undefined if valid() == false before the call to this function. like. undefined undefined. don't do it 💀

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🦋 Parameters 🦋

(none) — she takes zero arguments, high maintenance queen 👑

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⭐ Return value ⭐

(none) — she gives you nothing back, and honestly? iconic.

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💜 Exceptions 💜

May throw implementation-defined exceptions. vague, mysterious, a little bit main character energy 🙈

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🌈 Notes 🌈

• implementations are encouraged to detect the case where valid() == false before the call, and to throw a std::future_error with an error condition of std::future_errc::no_state. so like, they should catch you being messy, but no cap, it's just "encouraged" 👀

🔥 Calling wait on the same std::shared_future from multiple threads is not safe. the intended use is that each thread that waits on the same shared state has its own copy of a std::shared_future. one future per thread, bestie. share the state, not the object 💅

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🍓 Example 🍓

// Run this code

#include <chrono>
#include <future>
#include <iostream>
#include <thread>

int fib(int n)
{
    if (n < 3)
        return 1;
    else
        return fib(n - 1) + fib(n - 2);
}

int main()
{
    std::shared_future<int> f1 = std::async(std::launch::async, []() { return fib(40); });
    std::shared_future<int> f2 = std::async(std::launch::async, []() { return fib(43); });

    std::cout << "waiting... " << std::flush;
    const auto start = std::chrono::system_clock::now();

    f1.wait();
    f2.wait();

    const auto diff = std::chrono::system_clock::now() - start;
    std::cout << std::chrono::duration<double>(diff).count() << " seconds\n";

    std::cout << "f1: " << f1.get() << '\n';
    std::cout << "f2: " << f2.get() << '\n';
}

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🧚 Possible output 🧚

waiting... 1.61803 seconds
f1: 102334155
f2: 433494437

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👑 See also 👑

• wait_for — waits for the result, returns if it is not available for the specified timeout duration (public member function)
• wait_until — waits for the result, returns if it is not available until specified time point has been reached (public member function)

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k bye bestie, go wait on your futures responsibly 🦋✨ — this lil page is from http://cppreference.com, dated 2024.06.10, and it's the std::shared_future::wait(3) manual page. mwah 💖🎀