What Is a Header Guard?
A header guard, also called an include guard, is a set of preprocessor directives that wrap a header so its contents are processed only the first time it is included. A unique macro is defined on first inclusion and checked on later ones to skip the body. This stops the same declarations from appearing twice in one translation unit.
Why it matters
Without a guard, a header pulled in through multiple include paths produces redefinition errors that break the build. Header guards are a basic safeguard for any nontrivial header.
Related guides
What Is a Preprocessor Directive?A preprocessor directive is an instruction handled before compilation that includes files, defines macros, or…
What Is a Translation Unit?A translation unit is the complete source seen by the compiler after preprocessing one source file, the unit…
What Is a Forward Declaration?A forward declaration tells the compiler a name exists and its type before its full definition, letting code…