In the context of the C or C++ programming languages, a library is called header-only if the full definitions of all macros, functions and classes comprising the library are visible to the compiler in a header file form. Header-only libraries do not need to be separately compiled, packaged and installed in order to be used. All that is required is to point the compiler at the location of the headers (the -I switch in gcc/g++), and then #include the header files into the application source. Another advantage is that the compiler's optimizer can do a much better job when all the library's source code is available.