我们是否应在.h和.cpp文件中都包含宏（#define）

Question

If i use them in both the files. Going with the term "macro" in c++ it's just a replacement mechanism(doesn't know anything about c++ structure) that happens in pre-compilation stage.

Is this the reason that i should define macro(#define) in both the files or doesn't need to do that or am i missing anything.?

Answer 1

在.h文件中定义宏，然后仅在.cpp文件中包含该.h文件就足够了。

Answer 2

Assume a simple case first. You have the .cpp file using the .h file:

• Car.cpp
• Car.h

If you have a constant, say: #define KM_PER_MINUTE 3 . If this constant is used by any function inside the .h file, put this #define in the .h file:

Inside Car.h:
#ifndef CAR_H
#define CAR_H
... 
#define KM_PER_MINUTE 3
...
int calculate_total_KM(int minute) 
{
    return minute * KM_PER_MINUTE;
}
...
#endif 

On the other hand, if this constant is used only in the .cpp file, put this #define in the .cpp file:

Inside Car.cpp:
#define KM_PER_MINUTE 3
...
int calculate_total_KM(int minute) 
{
    return minute * KM_PER_MINUTE;
}
...

Answer 3

From the documentation :

The #define directives define the identifier as macro, that is instruct the compiler to replace all successive occurrences of identifier with replacement-list, which can be optionally additionally processed. If the identifier is already defined as any type of macro, the program is ill-formed unless the definitions are identical.

I guess it fully replies to your question.
It goes without saying that you can define a macro in a .h file, then include it wherever you need that macro.

Answer 4

If you put it in #ifndef then it doesn't matter, it will be included only once (the first occurrence). I you doesn't, then you'll get warning for already defined macro when the macro defined in the source and also the header with the macro included.

BTW: There is a third place, you can declare the macro in the makefile, or project settings.

Answer 5

'define macro(#define) in both the files' cause redefinition error or warning. It's not good. It may cause program compile error(good thing), or runtime error(terrible thing).