带有 co_await 的异步 HTTP 请求

Question

我有多个std::functions 在我的应用程序的主线程（而不是不同的线程）上调用（作为异步 HTTP 请求的结果），例如：

namespace model { struct Order{}; struct Trade{};}

std::function<void (std::string)> func1 = [](std::string http_answer)
{
    std::vector<model::Order> orders = ParseOrders(http_answer);
    std::cout << "Fetched " << orders.size() << " open/closed orders.");
}

std::function<void (std::string)> func2 = [](std::string http_answer)
{
    std::vector<model::Trade> trades = ParseTrades(http_answer);
    std::cout << "Fetched " << trades.size() << " trades.");
}

当func1 和func2 都解析了HTTP 答案时，如何调用process_result？

auto process_result = [](std::vector<model::Order> orders, std::vector<model::Trades> trades)
{
    std::cout << "Matching orders and trades.";
};

有co_await 或类似的解决方案吗？

Answer 1

您需要某种同步点。到目前为止还没有使用 co_await，所以这可能不是你想要的，但是在 c++17 中我会选择 std::promise / std::future，可能是这样的：

#include <iostream>
#include <functional>
#include <future>

std::promise<std::string> p1;
std::function<void (std::string)> func1 = [](std::string http_answer)
{
    // std::vector<model::Order> orders = ParseOrders(http_answer);
    // std::cout << "Fetched " << orders.size() << " open/closed orders.");
    p1.set_value(http_answer);
};

std::promise<std::string> p2;
std::function<void (std::string)> func2 = [](std::string http_answer)
{
    // std::vector<model::Trade> trades = ParseTrades(http_answer);
    // std::cout << "Fetched " << trades.size() << " trades.");
    p2.set_value(http_answer);
};

int main () {

    // whenever that happens...
    func1("foo");
    func2("bar");

    // synchronize on func1 and func2 finished
    auto answer1 = p1.get_future().get();
    auto answer2 = p2.get_future().get();

    auto process_result = [&](/* std::vector<model::Order> orders,  std::vector<model::Trades> trades */)
    {
        std::cout << "Matching orders and trades... " << answer1 << answer2;
    };

    process_result();

    return 0;
}

http://coliru.stacked-crooked.com/a/3c74f00125999fb6

https://en.cppreference.com/w/cpp/thread/future