mirror of
https://github.com/NixOS/nix
synced 2025-06-25 10:41:16 +02:00
96eeb6f4ff
This is good practice to avoid pessimisations. Left comments for the reasoning why ctors should be noexcept. There are some tricky cases where we intentionally want throwing move ctors/assignments. But those cases should really be reviewed, since some of those can be replaced with more idiomatic copy/move-and-swap.
51 lines
1.2 KiB
C++
51 lines
1.2 KiB
C++
#pragma once
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///@file
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#include <future>
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#include <functional>
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namespace nix {
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/**
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* A callback is a wrapper around a lambda that accepts a valid of
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* type T or an exception. (We abuse std::future<T> to pass the value or
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* exception.)
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*/
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template<typename T>
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class Callback
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{
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std::function<void(std::future<T>)> fun;
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std::atomic_flag done = ATOMIC_FLAG_INIT;
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public:
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Callback(std::function<void(std::future<T>)> fun) : fun(fun) { }
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// NOTE: std::function is noexcept move-constructible since C++20.
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Callback(Callback && callback) noexcept(std::is_nothrow_move_constructible_v<decltype(fun)>)
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: fun(std::move(callback.fun))
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{
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auto prev = callback.done.test_and_set();
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if (prev) done.test_and_set();
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}
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void operator()(T && t) noexcept
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{
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auto prev = done.test_and_set();
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assert(!prev);
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std::promise<T> promise;
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promise.set_value(std::move(t));
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fun(promise.get_future());
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}
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void rethrow(const std::exception_ptr & exc = std::current_exception()) noexcept
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{
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auto prev = done.test_and_set();
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assert(!prev);
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std::promise<T> promise;
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promise.set_exception(exc);
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fun(promise.get_future());
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}
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};
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}
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