Re: elapsed time 0 with std::cin

James Kanze <>
Sat, 24 Nov 2007 10:10:20 -0800 (PST)
On Nov 23, 8:46 pm, pekka <pe...@nospam.invalid> wrote:

I'm trying to measure user input time with my Timer class
object. It isn't as easy as I expected. When using std::cin
between timer start and stop, I get zero elapsed time. For
some unknown reason, the clock seems to stop ticking during
execution of std::cin.

Here's my code:

#include <ctime>
#include <iostream>
#include <string>

class Timer
        clock_t start_, nticks_;

        Timer() : nticks_(0) { start(); }
        ~Timer() {}

        void start() { start_ = clock(); }
        void stop() { nticks_ = clock() - start_; }
        double elapsed() const { return double(nticks_) / CLOCKS_PER_SEC;=



int main()
        std::string answer;
        Timer T;
// for (int n=0; n<100000000; ++n);
        std::cout << "? ";
        std::cin >> answer;
        std::cout << "time elapsed: " << T.elapsed() << "\n";

If I use the loop at the commented line, instead of std::cin,
the timer works as expected. I can't figure out what's wrong

According to the language standard, clock() is supposed to give
the systems best estimate of the CPU time used by the
application between successive calls to the function. If you're
program is waiting for keyboard input, it's not using the CPU,
so the value returned by clock() shouldn't increase.

If you want wall clock time, you should use time().

James Kanze (GABI Software)
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