Multiple preprocessor directives on one line in C+

Question

A hypothetical question: Is it possible to have a C++ program, which includes preprocessor directives, entirely on one line?

Such a line would look like this:

#define foo #ifdef foo #define bar #endif

What are the semantics of such a line?

Further, are there any combinations of directives which are impossible to construct on one line?

If this is compiler-specific then both VC++ and GCC answers are welcome.

Answer 1

Preprocessor directives are somewhat different than language statements, which are terminated by ; and use whitespace to delimit tokens. In the case of the preprocessor, the directive is terminated by a newline so it's impossible to do what you're attempting using the C++ language itself.

One way you could kind of simulate this is to put your desired lines into a separate header file and then #include it where you want. The separate header still has to have each directive on one line, but the point where you include it is just a single line, effectively doing what you asked.

Another way to accomplish something like that is to have a pre-C++ file that you use an external process to process into a C++ source file prior to compiling with your C++ compiler. This is probably rather more trouble than it's worth.

Answer 2

A preprocessing directive must be terminated by a newline, so this is actually a single preprocessing directive that defines an object-like macro, named foo, that expands to the following token sequence:

# ifdef foo # define bar # endif

Any later use of the name foo in the source (until it is #undefed) will expand to this, but after the macro is expanded, the resulting tokens are not evaluated as a preprocessing directive.

This is not compiler-specific; this behavior is defined by the C and C++ standards.