Header Guard

Here is an example of header guard:

// A.h
#ifndef A_H
#define A_H

// Declaration of a function defined in A.cpp
void functionInA();

#endif // A_H

The #ifndef, #define, #endif are the header guard. It prevents the content in the header file being declared multiple times during the C++ build process.

Note that header guard would not guard across mutiple translation units. The code inside the guard will be preprocessed and compiled in each translation unit.