about #define

Hi, all

I have a question about the following header file
I don't understand why #define is used
Does anyone know what's the purpose of it?

Thanks

Kai


// Kai.h

#ifndef KAI_CLASS1_H
#define KAI_CLASS1_H

namespace Kai
{
class class1 {
...
} ;
}

#endif

Code:

---

#ifndef HEADER_NAME_H
#define HEADER_NAME_H
// ...
#endif

---

Those are things that are added to header files to avoid re-definition of its contents. It is a convention now to add that every single time you create a header file.

Suppose you have a file A.cpp that includes a file B.h at the start of a program (or anytime). Then later on a file C.cpp also includes that file B.h.

What will happen in this case? "Compiler error! Redefinition of xxx & xxx & ..."

Those headers prevent that from happening.