#define w/ function

Environment:

Microsoft Visual Studio 2008
Version 9.0.21022.8 RTM
Microsoft .NET Framework
Version 3.5 SP1
Installed Edition: Professional

Here is what I have:

Code:

---

#define SPELL_COST(spell, ch)  \
                switch((spell)) {        \
                        case SPELLID_CLW:        \
                                PLVL((ch));        \
                                break;        \
                }        \
        }

int a = SPELL_COST(SPELLID_CLW, ch);

---

I receive these errors upon compilation:

Quote:

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c:\project\spell_verb_cure_light_wound.cpp(65) : error C2059: syntax error : 'switch'
c:\project\spell_verb_cure_light_wound.cpp(65) : error C2143: syntax error : missing ';' before '{'
c:\project\spell_verb_cure_light_wound.cpp(65) : error C2046: illegal case
c:\project\spell_verb_cure_light_wound.cpp(65) : error C2043: illegal break
c:\project\spell_verb_cure_light_wound.cpp(159) : error C2059: syntax error : '}'
c:\project\spell_verb_cure_light_wound.cpp(159) : error C2143: syntax error : missing ';' before '}'
c:\project\spell_verb_cure_light_wound.cpp(159) : error C2059: syntax error : '}'

---

What am I doing wrong?

I didnt even know you could that... anyway, I got a similar thing working (or so it "appears" to work). The main differences were

Code:

---

#define SPELL_COST(spell, ch)  \
                switch((spell)) {        \
                        case SPELLID_CLW:        \
                                PLVL((ch));        \
                                break;        \
                }        \
        }

int a = SPELL_COST(SPELLID_CLW, ch);

---

The first red line is a close bracket without a matching open bracket--either add the opening bracket or remove this hanging close bracket (which will work because theres only one statement, I believe). The second red line needs to be inside of a function.

Yes, then extra hanging bracket was a mistake. I now got rid of that.

This: int a = SPELL_COST(SPELLID_CLW, ch);

is called from a different function on a .cpp file. It's not in the same .h file as the define.


I'm still getting the errors.

#define is a simple text substitution; whatever you have in the "body" of your macro is put in place of the macro name. So your code would look something like (removing the extraneous brace):

Code:

---

int a = switch((SPELLID_CLW)) { case SPELLID_CLW: PLVL((ch)); break; }

---

While this sort of thing is valid in some languages, like Ruby, it's not in C. The result of a switch statement is not something you can simply assign to a variable. I would recommend a function instead of a macro, but if you have to use a macro, the conditional operator ( ?: ) might be useful.

If assigning a switch to a variable works on your compiler (as nadroj implies it does for him), that's a compiler extension. I tried it with 5 compilers, and it worked with precisely none, so it doesn't seem to be a common extension if it is one at all.

Quote:

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Originally Posted by cas

If assigning a switch to a variable works on your compiler (as nadroj implies it does for him)

---

Quote:

---

Originally Posted by nadroj

I got a similar thing working

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Sorry if it was misleading, but by "similar thing" I just meant I was able to modify his #define and use it--not assign its "return" value to something. But what you say seems right, a "switch" doesnt really return anything, and using "?" is definitely a better choice, if what is required is an "if"-"else" type of thing.

Quote:

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Originally Posted by cas

I would recommend a function instead of a macro

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Or even an inline function right? I dont use either of these constructs often at all--what is the difference between an inline function and a "function-like" macro?

I simply made a small function rather than relying on a #define macro. I had something specific in mind, but for the main purpose the small function will do just fine.

Thank you all for your input and advice.

Quote:

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Or even an inline function right? I dont use either of these constructs often at all--what is the difference between an inline function and a "function-like" macro?

---

Well, one potential problem is that inline was not standard until C99. This isn't necessarily a large problem because many compilers have provided it as an extension for a long time.

There are some things a macro can do that a function can't. Some people like to set pointers to NULL after freeing them, and so a wrapper macro can be used:

Code:

---

#define xfree(ptr) do { free(ptr); ptr = NULL; } while(0)

---

You can't define a function to do this, because of type issues. You'd need a function for each pointer type, and moreover, it'd have to take a pointer to pointer as the argument so it could actually set the original pointer to NULL.

For debugging, macros are useful because if you use __FILE__ and __LINE__, they'll be referring to the proper place. As a quick and dirty exapmle:

Code:

---

#include <stdio.h>
#include <stdarg.h>

#define DBG(fmt, ...) fprintf(stderr, "%s:%d: " fmt, __FILE__, __LINE__, __VA_ARGS__)

static inline void dbg(const char *fmt, ...)
{
  va_list ap;

  va_start(ap, fmt);
  fprintf(stderr, "%s:%d: ", __FILE__, __LINE__);
  vfprintf(stderr, fmt, ap);

  va_end(ap);
}

int main(void)
{
  DBG("Error %d\n", 1);
  dbg("Error %d\n", 1);

  return 0;
}

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As a C programmer, I've had macros pounded into my head for years. But I've been coming around to inline functions and am using them a lot more now. If it can be done with an inline function, I use an inline function. I reserve macros for what I believe is their rightful place: only when there is no other option. I know they have their uses, but not as many as I used to use them for.

Functions look a whole lot nicer than multiline macros, anyway.

Without running that code, is the difference that DBG will print the correct line ( 2nd line of "main" function), whereas dbg will not? Of course (I imagine) they both print the correct file. I imagine your trying to show that theres some difference/limitation, but wasnt positive what it was. Actually, Ill give it a run now...

And I agree that any (non-trivial) task would look better as a function over a macro.

EDIT: Neat! Also a cool way to debug (of course opposed to using a debugger). I usually just throw print statements in and see what line is printed, etc. Of course they both achieve the same thing...but your suggestion prevents me from having to think of n different words to type, for the different "debug" statements (because it creates unique "words").