博文《C++多线程教程（二）衡量线程执行时间》讲述了一种衡量线程执行时间的方法。

通过measure函数对func函数的执行时间进行测量：

#include<iostream>
#include<thread>
using namespace std;

using namespace std::chrono;                //增加引用空间
template<class T>
void measure(T&& func) {
	auto beg_t = system_clock::now();       //开始时间
	func();								    //执行函数
	auto end_t = system_clock::now();       //结束时间
	duration<double> diff = end_t - beg_t;
	printf("performTest total time: ");
	cout << diff.count()<<endl;
}
void func() {
	cout << "This is func thread " << endl;
	int s = 0;
	for (int i = 0; i < 5; i++)
		s += i;
}

int main() {
	measure(func);
}

对于绝大部分情况，这种方式是足够了，但是缺陷也很明显：measure函数不支持带有参数的函数，而且无法打印函数的返回值。

所以本文对此方法进行优化，通过万能引用 + 变长参数模板 + std::future 实现支持对 携带任意参数和返回值的函数进行测量。优化后代码如下：

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

using namespace std;

template<class T, class... Args>
auto measure(T&& func, Args&&... args)->std::future<typename std::result_of<T(Args...)>::type>
{
	using return_type = typename std::result_of<T(Args...)>::type;
	auto task = std::make_shared<std::packaged_task<return_type()>>
		(std::bind(std::forward<T>(func), std::forward<Args>(args)...));
	std::future<return_type> res = task->get_future();
	auto begin = std::chrono::high_resolution_clock::now();
	(*task)();
	auto end = std::chrono::high_resolution_clock::now();
	auto elapsed = std::chrono::duration_cast<std::chrono::nanoseconds>(end - begin);
	printf("执行时间: % .3f seconds.\n", elapsed.count() * 1e-9);
	return res;
}

unsigned int func(unsigned int nloopTime) {
	cout << "This is func thread " << endl;
	unsigned int s = 0;
	for (unsigned int i = 0; i < nloopTime; i++)
		s += i;
	return s;
}

int main() {
	future<unsigned int> res1 = measure(func, 9000000);
	cout << "func函数返回值：" << res1.get() << endl;
	
	printf("-----------------------------\n");
	future<unsigned int> res2 = measure([](unsigned int nloopTime) {
		cout << "This is func thread " << endl;
		unsigned int s = 0;
		for (unsigned int i = 0; i < nloopTime; i++)
			s += i;
		return s;
		}, 9000000);
	cout << "代码块返回值：" << res2.get() << endl;
}

执行效果如下：