Is there a way to create a preprocessor macro for a function?

Question

Is it possible to create a C++ preprocessor macro based on a function result?

For example, I'd like to save the screen height dynamically in a preprocessor macro definition:

#define SCREEN_HEIGHT   GetSystemMetrics(SM_CYVIRTUALSCREEN)

Then I want to use the result to set values based on the screen height:

#if SCREEN_HEIGHT < 1200
    #define TOP_COORD     200
    #define BOTTOM_COORD  500
    #define LEFT_COORD    0
    #define RIGHT_COORD   1280
#else
    #define TOP_COORD     1100
    #define BOTTOM_COORD  1400
    #define LEFT_COORD    0
    #define RIGHT_COORD   1280
#endif

This doesn't work, as SCREEN_HEIGHT doesn't seem to be getting defined properly.

Is there a better way to accomplish this? Is this even possible? I want to be able to get this screen height info in the header file if possible, as this is part of a large chunk of legacy code.

score 5 · Accepted Answer · answered Mar 23 '12 at 16:58

No. Macros are fully evaluated at compile time, conceptually at least by a pre-processor, before the actual code is even compiled, and the expressions in #if, etc. must be preprocessor expressions. The height of the screen cannot be known until the program runs.

score 2 · Answer 2 · answered Mar 23 '12 at 16:58

2

No, it can't be done. How do you expect to make a compilation-time desition, which depends on a property of the screen in which the application will be run? You can't know that on compilation time.

answered Mar 23 '12 at 16:58

mfontanini

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Péter Török · Answer 3 · 2012-03-23T17:11:42.257

2

Real functions are usually getting called only runtime. Even function-like things such as sizeof (and generalized constant expressions - thanks to @Pubby for noting) are evaluated by the compiler after the preprocessing stage. By this time all preprocessor stuff has been replaced in the code - the compiler has no idea about things like #if. Therefore what you want is not possible using macros.

~~With templates, however, it is possible.~~ On second thought, not in this case, because the actual screen height can not be known until runtime.

You could define a struct to contain the coordinate values as constants, and store all of these in a map or other form of collection, then look it up dynamically at runtime, when you know the actual screen height.

edited Mar 23 '12 at 17:11

answered Mar 23 '12 at 16:59

Péter Török

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Functions can be called at compile time. – Pubby Mar 23 '12 at 17:01
@Pubby, thanks, I didn't know that. Added a link to a corresponding article. – Péter Török Mar 23 '12 at 17:06

score 0 · Answer 4 · answered Mar 23 '12 at 17:02

If GetSystemMetrics would be a macro, then you could do this. If GetSystemMetrics would be constexpr, then you could use traits.

But since GetSystemMetrics is a normal function, you have to work with normal C++.

struct system_metrics_ {
  int top, bottom, left, right;

  system_metrics_()
  {
    if (GetSystemMetrics(SM_CYVIRTUALSCREEN) < 1200) { /* first case */ }
    else { /* second case */ }
  }
};

// define this method outside the header
const system_metrics_& system_metrics() { static system_metrics_ sm; return sm; }

// legacy code
#define TOP_COORD     (system_metrics().top)
#define BOTTOM_COORD  (system_metrics().bottom)
#define LEFT_COORD    (system_metrics().left)
#define RIGHT_COORD   (system_metrics().right)

Is there a way to create a preprocessor macro for a function?

4 Answers4

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