问题
I recall a claim made by one of my professors in an introductory C course. He stated that the #define preprocessor command enables a programmer to create a constant for use in later code, and that the command was a part of the C language.
/* Is this truly C code? */
#define FOO 42
Since this was in an introductory programming class, I suspect that he was merely simplifying the relationship between the source file and the compiler, but nevertheless I wish to verify my understanding.
Are preprocessor statements completely independent from the C language (dependent on the specific compiler used) or are they explicitly described in the C99 standard? Out of curiosity, did K&R ever mention preprocessor macros?
回答1:
Yes, the standard describes the preprocessor. It's a standardized part of the C language.
Note that #include, which is essential for modularization of code, is a preprocessor directive.
In the publically-available draft of the C99 standard, the preprocessor is described in section 6.10.
回答2:
The preprocessor is indeed part of the C and C++ standard (chapter 16 in the C++ standard) and the standards describe how the preprocessor and the language interact (for example it is illegal to re-#define the C keywords).
However the C preprocessor can work with other languages than C for any kind of simple file preprocessing (I have seen it used with LaTeX files for example).
回答3:
Yes the preprocessor is part of the C language. Conceptually it is ran before the source is compiled.
Along with constant definitions, the preprocessor is used to implement two very important constructs:
#includewhich brings other files into the compilation unit.include guards; i.e. the pattern,
#if !defined(METAWORD) #define METAWORD 1 /* struct definition, function prototype */ #endif
Out of interest, these two usages have survived into C++, constant definition can be implemented in other (better?) ways.
来源：https://stackoverflow.com/questions/17149001/are-c-preprocessor-statements-a-part-of-the-c-language