C / C++ / MFC

6-Aug-03 8:12

Is it possible to use a macro on another macro?

For example (and purely for illustrative purposes), suppose I have the following:

#define YES "1"
#define NO "0"

#define StartYourEngines "1"

and in my code I would like to do something like this:

StartYourEngines = YES;

is that possible? Or how would I go about doing something like that (using macros)?

Thanks.

William

Fortes in fide et opere!

Larry Antram6-Aug-03 8:19

6-Aug-03 8:19

#define YES "1"
#define NO "0"

#define StartYourEngines YES

WREY6-Aug-03 9:02

6-Aug-03 9:02

Thanks for replying.

#define StartYourEngines YES

doesn't work. The compiler complains that "YES" is an undeclared identifier.

Actually, prior to posting the question, I had tried a few experiments and was unable to get a clean compile (understanding that a clean compile is not a guarantee for a correct execution of the program), which makes me think either it cannot be done, or it would require a clever way of getting around the compiler, and at the same time getting it to work correctly.

William

Fortes in fide et opere!

Larry Antram6-Aug-03 9:24

6-Aug-03 9:24

Compiles OK for me in VC6. Confused | :confused:

If you want to do conditional compilation using macros, then you would typically do something like this:

#define OPTION_A
//#define OPTION_B -- this is turned off since it is commented out
  
// then later in your code perhaps something like this
    
#ifdef OPTION_A
  // do option A here
#endif
  
#ifdef OPTION_B
  // do option B here
#endif

If you want to define a number of options, and create a default for one with any of them being dynamically available, you would do something like this:

#define OPTION_A 0
#define OPTION_B 1
  
#define DEFAULT_OPTION OPTION_A
  
int nOption = DEFAULT_OPTION;
  
// then later in your code perhaps something like this
  
switch( nOption )
{
    case OPTION_A:
    {
        // do option A here  
    }
    break;
  
    case OPTION_B:
    {
        // do option B here  
    }
    break;
}

WREY6-Aug-03 10:19

6-Aug-03 10:19

Thanks for replying.

You posted some very good samples here, and I'll see which one will work for my situation at hand.

Actually, I have a MACRO file that I '#include' in my application. In that MACRO file is where I have (and again for purely illustrative purposes):

#define YES "1"
#define NO "0"

#define StartYourEngines "YES" // using your suggestion
==============================

In the application, I have:

#include "MACROFILE.h"

etc.

if(StartYourEngines == YES)
{
...
StartYourEngines = NO;
}
==============================

That's what I was referring to about the compiler giving the error messages (about "YES" and "NO" being undeclared identifiers).

That's what I'm trying to accomplish without resorting to cluttering up the global namespace. Ordinarily, I would use a specialized class object to do what I'm after, but I'm suffering heavily from time pressure and I'm looking for the quickest and easiest way of getting the work done. Later, I could always go back and clean things up with a better solution, but for right now, I'm doing working lunches and other meals, and surviving on the least amount of sleep as possible.

Thanks again for your suggestions.

Smile | :)

William

Fortes in fide et opere!

Larry Antram6-Aug-03 10:40

6-Aug-03 10:40

If you expand your macros, the resulting code will look like this:

if( "1" == "1" )
{
   ...
   "1" = "0";
}

As you can see that's not valid code. If you want to change the value of StartYourEngines at runtime, then you'll need to use a variable something like this:

#define YES 1
#define NO 0
  
int StartYourEngines = YES;
  
if( StartYourEngines == YES )
{
   ...
   StartYourEngines = NO;
}

Expanding the macros from that example, the code will look like this:

int StartYourEngines = 1;
  
if( StartYourEngines == 1 )
{
   ...
   StartYourEngines = 0;
}

Larry Antram6-Aug-03 10:53

6-Aug-03 10:53

It looks like you might be wanting to run something once, if so you could do it like this:

class MyClass
{
protected:
   BOOL m_bFirstTime;
   
public:
   MyClass()
     : m_bFirstTime( TRUE )
   {
   }
   
   void MyFunction()
   {
      if( m_bFirstTime )
      {
          // do something here only the first time this function is called
   
          m_bFirstTime = FALSE;
      }
   }
};

Although, in real world apps it would be better to structure the class something like this:

class MyClass
{
protected:
   BOOL m_bInitialized;
  
public:
   MyClass()
     : m_bInitialized( FALSE )
   {
   }
  
   virtual ~MyClass()
   {
      UnInitialize();
   }
  
   BOOL Initialize()
   {
      BOOL bResult = FALSE;
      {
         // TODO: do whatever initialization is required, and if
         // it succeeds set the following values to indicate it
   
         m_bInitialized = TRUE;
         bResult = TRUE;
      }
      return bResult;
   }
  
   void UnInitialize()
   {
      if( m_bInitialized )
      {
          // TODO: clean up
  
          m_bInitialized = FALSE;
      }
   }
    
   BOOL MyFunction()
   {
       BOOL bResult = FALSE;
       {
          if( m_bInitialized )
          {
             // TODO: do something that required initialization
             // and set bResult to TRUE if it succeeded
   
             bResult = TRUE;
          }
       }
       return bResult;
   }
};

Or if you aren't using classes, and you want to be bad, you could do something like this:

void MyFunction()
{
   static v_bFirstTime = TRUE;
   
   if( v_bFirstTime )
   {
      // do something here only the first time this function is called
   
      v_bFirstTime = FALSE;
   }
}

Or this:

static g_bFirstTime = TRUE;
   
void MyFunction()
{
   if( g_bFirstTime )
   {
      // do something here only the first time this function is called
   
      g_bFirstTime = FALSE;
   }
}

WREY6-Aug-03 12:01