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Help settle the debate that's going on in the comments at this question about bool and 1:
Can a standards-conforming C++ preprocessor allow one to use #define to redefine a language keyword? If so, must a standards-conforming C++ preprocessor allow this?
If a C++ program redefines a language keyword, can that program itself be standards conforming?
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The C preprocessor is a macro processor that is used automatically by the C compiler to transform your program before actual compilation. It is called a macro processor because it allows you to define macros, which are brief abbreviations for longer constructs.
Preprocessing directives are lines in your program that start with # . The # is followed by an identifier that is the directive name. For example, #define is the directive that defines a macro. Whitespace is also allowed before and after the # .
There are 4 Main Types of Preprocessor Directives:Macros. File Inclusion. Conditional Compilation. Other directives.
It is a pre-process of execution of a program using c/c++ language. To initialize a process of preprocessor commands, it's mandated to define with a hash symbol (#). It can preferably be the non-blank character, and for better readability, a preprocessor directive should start in the first column.
In C++, the closest thing to forbidding #defineing a keyword is §17.4.3.1.1/2, which only disallows it in a translation unit that includes a standard library header:
A translation unit that includes a header shall not contain any macros that define names declared or defined in that header. Nor shall such a translation unit define macros for names lexically identical to keywords.
The second sentence of that paragraph has been changed in C++0x to outright forbid #defineing a keyword (C++0x FCD §17.6.3.3.1):
A translation unit shall not #define or #undef names lexically identical to keywords.
Edit: As pointed out by Ken Bloom in comments to his answer, the rules have not changed in C++0x; the text has just been rearranged to confuse people like me. :-)
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Working from the 2005-10-19 C++ working draft (since I don't have a standard handy):
Section 16.3 defines the grammar for #define to be #define identifier replacement-list-newline (object-like macros) or one of several constructions beginning with #define identifier lparen (function-like macros). identifiers are defined in section 2.10 to be identifier-nondigit | identifier identifier-nondigit | identifier digit. Section 2.11 indicates that a certain list of identifiers are unconditionally treated as keywords in phase 7 of compilation (section 2.1), and I conclude that they are therefore not treated specially in phase 4, which is preprocessor expansion. Thus, it appears that the standard requires the preprocessor to allow you to redefine language keywords (listed in Section 2.11).
However, the preprocessor has a keyword of its own, namely defined, as well as a list of predefined macros (Section 16.8). Section 16.8 states that the behavior is undefined if you redefine these, but does not prohibit the preprocessor from recognizing these as macro names.
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