Функции и Прототип функции

#include <iostream>
 
void foo() 
{
    std::cout << 42;
}
 
int main()
{
    foo();
}

#include <iostream>
 
int main()
{
    std::cout << 42;
}

#define WIN32_LEAN_AND_MEAN
#include <windows.h>
 
double getTime() {
    LARGE_INTEGER freq, val;
    QueryPerformanceFrequency(&freq);
    QueryPerformanceCounter(&val);
    return (double)val.QuadPart / (double)freq.QuadPart;
}
 
 
 
int main()
{
    double d0 = getTime();
 
    // function to measure
    
    double timeInMilliseconds = 1000 * (getTime() - d0);
    printf("%lf", timeInMilliseconds);
    getchar();
    return 0;
}

int _tmain(int argc, _TCHAR* argv[])
{
    using namespace std::chrono;
    while (1){
        high_resolution_clock::time_point t1 = high_resolution_clock::now();
 
//--------------------------            
        double df(100000000);
        while (--df) {
 
            if ((int)df % 2 == 0);
        }
//--------------------------
        
        high_resolution_clock::time_point t2 = high_resolution_clock::now();
 
        duration<double>time_span = duration_cast<duration<double>>(t2 - t1);
 
        std::cout << "It took me: " << time_span.count() << " seconds.";
        std::cout << std::endl;
    std::cout << " =========================== \n"; Sleep(500);
             }
    return 0;
}