Boolean in ifdef: is “#ifdef A && B” the same as “#if defined(A) && defined(B)”?

问题

In C++, is this:

#ifdef A && B

the same as:

#if defined(A) && defined(B)

I was thinking it wasn't, but I haven't been able to find a difference with my compiler (VS2005).

回答1:

They are not the same. The first one doesn't work (I tested in gcc 4.4.1). Error message was:

test.cc:1:15: warning: extra tokens at end of #ifdef directive

If you want to check if multiple things are defined, use the second one.

回答2:

Conditional Compilation

You can use the defined operator in the #if directive to use expressions that evaluate to 0 or 1 within a preprocessor line. This saves you from using nested preprocessing directives. The parentheses around the identifier are optional. For example:
#if defined (MAX) && ! defined (MIN)  
Without using the defined operator, you would have to include the following two directives to perform the above example:
#ifdef max 
#ifndef min

回答3:

The following results are the same:

#define A
#define B
#if(defined A && defined B)
printf("define test");
#endif

#ifdef A
#ifdef B
printf("define test");
#endif
#endif

回答4:

For those that might be looking for example (UNIX/g++) that is a little different from the OP, this may help:

#if(defined A && defined B && defined C)
    const string foo = "xyz";
#else
#if(defined A && defined B)
    const string foo = "xy";
#else
#if(defined A && defined C)
    const string foo = "xz";
#else
#ifdef A
    const string foo = "x";
#endif
#endif
#endif
#endif

回答5:

As of VS2015 none of the above works. The correct directive is:

#if (MAX && !MIN)

see more here

来源：https://stackoverflow.com/questions/1312132/boolean-in-ifdef-is-ifdef-a-b-the-same-as-if-defineda-definedb

标签

c++

c-preprocessor

conditional-compilation