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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.
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There are 4 Main Types of Preprocessor Directives: Macros. File Inclusion. Conditional Compilation. Other directives.
Use the #ifdef statement when you want to compile a section only if a specified expression has been defined with #define. Use #ifndef when you want to compile a section only if a specified expression has not been defined.
In the C Programming Language, the #endif directive is used to define the following directives: #if, #ifdef , and #ifndef . Whenever the #endif directive is encountered in a program, it determines if the preprocessing of #if, #ifdef, or #ifndef has been completed successfully.
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.
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