Notes

Declaration: “this function exists, here’s its signature.” Definition: “here’s what it actually does.”

// Declaration (usually in a .h file) - a promise
float dotProduct(const vec3& a, const vec3& b);

// Definition (usually in a .cpp file) - the actual code
float dotProduct(const vec3& a, const vec3& b) {
    return a.x * b.x + a.y * b.y + a.z * b.z;
}

A function can be declared many times — every file that includes the header sees it — but must be defined exactly once across the whole program. That’s the One Definition Rule.

Why bother splitting them

Other .cpp files only need the declaration to call your function; they don’t need to see (or recompile against) the implementation. Change the body without touching the header, and only that one .cpp needs recompiling.

Related

Compilation process