For GCC this is highly optimized: "The preprocessor notices such header files, so that if the header file appears in a subsequent #include directive and FOO is

For GCC this is highly optimized: "The preprocessor notices such header files, so that if the header file appears in a subsequent #include directive and FOO is defined, then it is ignored and it doesn't preprocess or even re-open the file a second time. This is referred to as the multiple include optimization." http://gcc.gnu.org/onlinedocs/cppinternals/Guard-Macros.html http://gcc.gnu.org/onlinedocs/cppinternals/Guard-Macros.html