在C ++中使用条件定义宏

Question

I found some existing answer on SO, but I actually don't understand how does everything there works.

Basically, I want to define a macro in if/switch condition

if (condition)
{
#define SOME_MACRO
}

So, if some condition is true, there will be defined some new macro. But, as for now, it just defines macro anyway. I understand that it's not really how it should be done, but there's no normal explanation to that.

Answer 1

You can't mix macros and code like that. Macros are all processed before your code is compiled, it is just a convenience feature to save typing (and make code easier to read).

There is a macro syntax for conditions for example, you can do:

#if Condition
#define X
#endif

to conditionally define a macro.