c++ - 实现 parallel_for_each 函数

Question

我认为，由于 std::bind 无法推断返回类型的问题，我当前的尝试无法编译。实际的错误信息是

1>Source.cpp(24): error C2783: 'enable_if::value,std::_BindRx(_fastcall _Farg0::* )(_Ftypes...) volatile const,_Rx,_Farg0,_Ftypes...>,_Types...>>::type std::bind(Rx (_fastcall _Farg0::* const )(_Ftypes...) volatile const,_Types &&...)' : could not deduce template argument for '_Ret'

此外，我应该按值还是按引用将函数传递给 std::bind？ (通过 std::ref)。

template<class InputIt, class Function>
void parallel_for_each(InputIt first, const size_t elements, Function &function)
{
    unsigned int max_threads = std::max(1u, std::min(static_cast<unsigned int>(elements), std::thread::hardware_concurrency()));
    std::vector<std::thread> threads;
    threads.reserve(max_threads);
    size_t inc = elements / max_threads;
    size_t rem = elements % max_threads;
    std::cout << "inc = " << inc << '\n';
    auto last = first + elements;
    for (; first != last; first += rem > 0 ? inc + 1, --rem : inc)
    {
        std::cout << "rem = " << rem << '\n';
        std::cout << "first = " << *first << '\n';
        threads.emplace_back(std::bind(std::for_each, first, first + inc, function));
    }
    for (auto &t: threads) 
        t.join();
}

调用方式:

std::vector<int> numbers(678, 42);
parallel_for_each(begin(numbers), numbers.size(), [](int &x){ ++x; });
for (auto &n : numbers)
    assert(n == 43);
std::cout << "Assertion tests passed\n";

编辑:我将有问题的 for 循环替换为:

while (first != last)
{
    auto it = first;
    first += rem > 0 ? --rem, inc + 1 : inc;
    threads.emplace_back(std::bind(std::for_each<InputIt, Function>, it, first, function));
}

Answer 1

可以手动点，你要用什么模板

threads.emplace_back
(
   std::bind(std::for_each<InputIt, Function>, first, first + inc, function)
);

或者，您可以简单地使用 lambda，而不使用 std::bind。