Including header files in C/C++ more than once [duplicate]

Yes, it’s useful when generating code with the preprocessor, or doing tricks like Boost.PP does.

For an example, see X Macros. The basic idea is that the file contains the body of the macro and you #define the arguments and then #include it. Here’s a contrived example:

macro.xpp

std::cout << MESSAGE;
#undef MESSAGE

file.cpp:

int main() {
# define MESSAGE "hello world"
# include "macro.xpp"
}

This also allows you to use #if and friends on the arguments, something that normal macros can’t do.