You can use the #ifdef and #ifndef preprocessor directives to check whether a symbol has
been defined (#ifdef) or whether it has not been defined (#ifndef).

Can you define which header file to include at compile time? Yes. This can be done by using

the #if, #else, and #endif preprocessor directives. For example, certain compilers use

different names for header files. One such case is between Borland C++, which uses the

header file alloc.h, and Microsoft C++, which uses the header file malloc.h. Both of these

headers serve the same purpose, and each contains roughly the same definitions. If,

however, you are writing a program that is to support Borland C++ and Microsoft C++, you

must define which header to include at compile time. The following example shows how this

can be done:

#ifdef _ _BORLANDC_ _

#include

#else

#include

#endif