Question about Template

[simendsjo]

On Sat, 23 Jun 2012 17:30:01 +0200, Michael <pongad at gmail.com> wrote:

> Hello!
>    I have a little question about templates. I am trying to write a  
> template "timer" that takes a function and times how long the function  
> takes to execute. Something like
>    auto time = timer!sort(array);
> is equivalent to
>    auto start = Clock.currStdTime();
>    sort(array);
>    auto time = Clock.currStdTime() - time;
>
> except of course that the variable "start" is not created in the former  
> case.
>
> The closest I came to the correct template is
> template(alias F) timer {
>    auto timer {
>      auto start = Clock.currStdTime();
>      F();
>      return Clock.currStdTime() - time;
>    }
> }
>
> The problem with the template is I cannot pass any argument into "F".  
> How should I fix this problem?
> Also, I am not entirely sure why I need to write "alias F" instead of  
> just "F", any explanation would be appreciated.
>
> Thank you guys,
> Michael


import std.datetime;
import std.traits;
import std.stdio;

struct TimerResult(T) {
     TickDuration duration;
     T result;
}

auto timer(alias fn, Args...)(Args args) {
     auto sw = StopWatch(AutoStart.yes);
     auto res = fn(args);
     return TimerResult!(typeof(res))(sw.peek, res);
}

T f(T)(T[] arr) {
     import core.thread;
     Thread.sleep(dur!"seconds"(1));
     return arr[0];
}

void main() {
     auto res = timer!f([1,2,3,4]);
     writeln("first elem: ", res.result);
     writeln("time used: ", res.duration.msecs);
}