When repeated define variables, functions, classes, structures will be not only in the compilation of the source, when the target program links also may occur
c / c ++ compiler is a basic unit of .c or .cpp file, compile respective basic units are independent, conditional compilation # ifndef only guarantee the like (either alone or .c .cpp file) in the file header a base unit compiled not be repeated, but can not guarantee the same two or more headers in the base unit will not be repeated compiled
Header kinds of statements only as far as possible, do not have a definition.