What can and can't the preprocessor do?

[HighCommander4]

I want the code

MY_MACRO(x)

to be replaced by the code

#ifdef my_class_##x
   #error My error message
#endif
class my_class_##x
{
     ...
};

However, this macro does not compile:

#define MY_MACRO(x) \
#ifdef my_class_##x \
    #error My error message \
#endif \
class my_class_##x; \
{ \
   ... \
};

Is there a way to accomplish something like this?

[Zaccheus]

I don't think you can have pre-processor instructions inside macros.

Even if you can, the statement #ifdef my_class_##x does not check whether the class my_class_##x has already been declared, it only checks whether the macro my_class_##x has already been defined.

Although I don't think it is a good idea to use macros in that way, I don't see why you can't just use the following:

#define MY_MACRO(x) \
class my_class_##x \
{ \
   ... \
};

If my_class_##x already exists then the compiler will complain anyway.

[Zaccheus]

However, this macro does not compile:

#define MY_MACRO(x) \
#ifdef my_class_##x \
    #error My error message \
#endif \
class my_class_##x; \
{ \
   ... \
};

I just noticed the ; after the x. That's a bug in itself.

[TheCPUWizard]

You need to do it the other way around (typing this off the top of my head...)

#if CONDITION
#define MYMACRO(X)  RealCode()
#else
#define MYMACRO(X)  #error "Contition Not Met"
#endif