Should preprocessor instructions be on the beginning of a line?

问题

A while ago I have discovered an (rather ancient) C Compiler, which scanned macros this way (Pseudo code):

 if line.startswith("#include") or line.startswith("#define"):
     ...

.. Which kind of raised the question for me where macros should really be placed, at the beginning of a line, like so:

void stuff()
{
#if defined(WIN32) || defined(_WIN32)
    ...
#else
#if defined(__GNUC__)
    ...
#else
    ...
#endif
#endif
}

Or rather like so (as that's the way I do it, for improved readability):

void stuff()
{
    #if defined(WIN32) || defined(_WIN32)
    ...
    #else
    #   if defined(__GNUC__)
    ...
    #   else
    ...
    #   endif
    #endif
}

Is the way one indents the Preprocessor code standardized, that is, no matter how i indent it, it will always work the same way?

回答1:

Some old C compilers required that the #define (for example) be flush with the left margin:

#define FOO bar

Other C compilers required only that the # be at the left margin, so you could:

#    define FOO bar

Newer C compilers tend to accept the # after any leading whitespace:

    #define FOO bar

If you want compatibility with such older compilers, you should at least put your # in the first column. If compatibility doesn't matter, then it's up to you.

I would usually try not to embed #ifdef blocks inside functions, so the whole question of whether they should be indented mostly goes away.

回答2:

from gcc C preprocessor documentation:

Preprocessing directives are lines in your program that start with #'. Whitespace is allowed before and after the#'.

回答3:

No, they don't need to be at the beginning of the line, but they can only have blanks (spaces, tabs, ...) before them.

Usually they're put at the beginning of the line because they're not subjected to the scopes they're into, since they're preprocessed before actual C code.

回答4:

Doesn't matter. See it this way, if code was not idented and in 1 line it still should compile(only code, preprocessor/includes at some other things needs a seperate line).

Edit: seems to be that really old compiler are picky about this. Preprocessor should be at one line, just like other non-code things like includes

回答5:

I don't think the compiler "cares" if you have spaces before the preprocessed - it should be the same...

来源：https://stackoverflow.com/questions/4721978/should-preprocessor-instructions-be-on-the-beginning-of-a-line

标签

c++

c-preprocessor