Header files and multiple source files

Once a program grows, split it into several files. Declarations, which say that a function exists, go in a header file such as budget.h. Definitions, the actual code, go in a source file such as budget.cpp.

Then main.cpp includes the header to use them. Each .cpp file is compiled on its own, then linking joins the results, so g++ main.cpp budget.cpp -o app builds both. Protect every header with #pragma once or include guards, so it is not pasted in twice.

Put function definitions in the .cpp file, not the header. Otherwise you get "multiple definition" linker errors when two files include it, unless the function is marked inline. Never write using namespace std; in a header, because it leaks into every file that includes it. CMake automates all of this in the projects stage.