c++ - 如何编写带有参数的函数，其中类型是用 'auto' 字推导的？

Question

我正在寻找一种干净的 c++11(最高 c++17)方法来编写一个函数，该函数只需将 fps 写入具有给定“开始”和“停止”时间(例如，给定间隔时间)的输出流.所以我有这段代码，例如:

#include <iostream>
int main(int argc, char** argv) {
    typedef std::chrono::high_resolution_clock time_t;
    while (1) {
        auto start = time_t::now();

        // here is the call of function that do something
        // in this example will be printing
        std::cout << "Hello world!"

        auto stop = time_t::now();
        fsec_t duration = stop - start;

        double seconds = duration.count();
        double fps = (1.0 / seconds);

        std::stringstream s;
        s << "FPS: " << fps;

        std::cout << s.str();
    }
}

我想做这样的事情:

#include <iostream>

std::ostream & printFPS(std::ostream &stream, auto start);

int main(int argc, char** argv) {

    while (1) {
        auto start = std::chrono::high_resolution_clock::now();

        // here is the call of function that do something
        // in this example will be printing
        std::cout << "Hello world!"

        printFPS(std::cout, start);
    }
}

std::ostream & printFPS(std::ostream &stream, auto start){

    auto stop = std::chrono::high_resolution_clock::now();
    std::chrono::duration<float> duration = stop - start;

    double seconds = duration.count();
    double fps = (1.0 / seconds);

    std::stringstream s;
    s << "FPS: " << fps;

    return stream << s.str();
}

GCC 提示我推断“开始”的类型是 std::chrono::time_point<std::chrono::_V2::system_clock, std::chrono::duration<long int, std::ratio<1, 1000000000> > > ，但我不想在这种类型的函数中编写(推导可能会改变(？)，而且它很长并且需要 typedef)，是否可以编写更优雅的函数，因为不允许自动输入参数？谢谢!

Answer 1

您可以使用decltype 来推断时间类型并将其用作参数的类型。

using time_type = decltype(std::chrono::high_resolution_clock::now());

std::ostream & printFPS(std::ostream &stream, time_type start);