modify function during compiletime

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4.00/5 (1 vote)

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i got that simple problem to be solved:
i want a funktion that is called several times, but the first time it is called it should do something additional.

I know that aint work but you might get a better idea of what i want:

#define DOIT
#ifndef DONEITONCE
  #define DONEITONCE
  cout << "first time doing it" << endl;
#else
  cout << "did it allready" << endl;
#endif

Obviously that won't compile. The question is can i do something similar,
that the code is changed during compile time? I imagined something like this:

bool done = false;
template<bool b="">;
void doit() {
  doit<B>();
}
template<>
void doit<false>() {
  cout << "first time doing it" << endl;
  done = true;
}
template<>;
void doit<true>() {
  cout << "did it allready" << endl;
}
#define DOIT doit<done>();</done></true></false></bool>

but that throws a compilation error, too.
Anyway the function call should look like:

DOIT        // first call
DOIT        // second call

with an output like:

first time doing it
did it allready

I do not want to do this in an <code>if (b) xxx; else yyy;</code> manner
I really hope you can help me!
Tanks

Posted 10-Jun-11 1:40am

Wh0p

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Sergey Alexandrovich Kryukov 10-Jun-11 18:57pm

"In compile time" is wrong. It will be in run time, whatever you do.
--SA

5 solutions

Solution 2

Why don't you use, like us common mortal, a static variable? e.g.

C++

void doit()
{
  static bool already_done= false;

  if (already_done)
    cout << "Already done." << endl;
  else 
    cout << "First time." << endl;

  already_done = true;
}

Posted 10-Jun-11 1:48am

CPallini

Solution 4

Tank all of you for the fast response!
But I still would like the compiler to check for the first function call not the function itself, so the desired compiler behaviour would be like:

"this would mean a build directive, then you would need to create a variable or object with data binding to check for the number of compile times"

Thats more the thing I intended.

Though I'm not very familiar with that topic. Are there any good resources in the web? Using Visual Studio 2005 pro

Posted 10-Jun-11 3:18am

Wh0p

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S Houghtelin · Accepted Answer · 2011-06-10T02:08:00

You had me confused when you said at compile time, this would mean a build directive, then you would need to create a variable or object with data binding to check for the number of compile times.

If you meant to count number of times a function is accessed during run time, you have(at the time of writing) two answers in solutions 1 & 2

[Update]
From MSDN
http://msdn.microsoft.com/en-us/library/91621w01(VS.80).aspx[^]

From Technet
http://technet.microsoft.com/en-us/library/3sxhs2ty.aspx[^]

Sergey Alexandrovich Kryukov · Accepted Answer · 2011-06-10T12:56:00

Sorry that my answer is not about C++, but my idea is so unusual and universal so it could be interesting for you.

My solution is well implemented and tested; and I know for sure you can use this idea, because from in C++ you always can look at the stack and remember the calling point.

Please take a look at my article Wish You Were Here… Only Once[^].

—SA

Niklas L · Accepted Answer · 2011-06-10T01:45:00

Solution 1

If you're not worried about thread safety:

C++

void func()
{
    static bool first = true;

    if (first)
    {
        // Additional handling
        first = false;
    }

    // Normal execution
}

If you are worried about thread safety you will have to guard the flag used.

Posted 10-Jun-11 1:45am

Niklas L

Updated 10-Jun-11 1:46am

v2

Comments

Sergey Alexandrovich Kryukov 10-Jun-11 18:59pm

Exactly, my 5.

Please also take a look at my solution which I put mostly for fun. But for .NET this is absolutely universal and well-polished and tested solution, for robustness and performance, but also... funny.
Just take a look at my solution and article (have you seen it before?)...

--SA

Niklas L 11-Jun-11 4:18am

Yes I did read it before, and found it quite entertaining. A very innovative approach indeed.