C++ decorator measuring the execution time of a function

decorator.cpp

#include <functional>
#include <chrono>
#include <iostream>

template <class> struct ExeTime;

// Execution time decorator
template <class R, class... Args>
struct ExeTime<R(Args ...)> {
public:
    ExeTime(std::function<R(Args...)> func): f_(func) { } 

    R operator ()(Args ... args) {
        std::chrono::time_point<std::chrono::system_clock> start, end;
        std::chrono::duration<double> elasped;

        start = std::chrono::system_clock::now();
        R result = f_(args...);    
        end = std::chrono::system_clock::now();
        elasped = end - start;
        std::cout << elasped.count() << " seconds" << std::endl;

        return result;   
    }   

private:
    std::function<R(Args ...)> f_; 
};

template <class R, class... Args>
ExeTime<R(Args ...)> make_decorator(R (*f)(Args ...)) {
    return ExeTime<R(Args...)>(std::function<R(Args...)>(f));    
}

// Test function
int foo_loop(int n) {
    long long sum = 0;
    for (int i = 0; i < n; ++i) {
        sum += i;
    }   
    return sum;
}

int main() {
    auto et = make_decorator(foo_loop);    
    et(10000000);
    return 0;
}