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

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.