Can #if pre-processor directives be nested in C++?

问题

I have a question about Pre-processor directives in c++:

For example:

#ifndef QUESTION

//some code here

#ifndef QUESTION

//some code here

#endif

#endif

Can we use it in this way, and can the C++ compiler match the ifndef and endif in the right way?

回答1:

Yes, we can. The #endif statement matches to the previous #if #ifdef or #ifndef etc for which there hasn't been a corresponding #endif.

e.g.

#if  ----------|
#if  -----|    |
#endif ---|    |
#endif --------|

回答2:

Yes, you can nest #if/#endif blocks. Some C coding styles would tell you to write

#ifdef CONDITION1
# ifdef CONDITION2
# endif
#endif

using spaces to denote the level of nesting.

回答3:

In your code, the #ifndef QUESTION section will be discarded unless you #undef QUESTION.

Good luck!

来源：https://stackoverflow.com/questions/38963852/nested-else-handling-in-false-ifdef-in-c