Determining If Symbols Are Defined or Not
How can you check to see whether a symbol is defined?
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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
2007-02-26, 5714👍, 0💬
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