ifndef

C++ does not compile when a variable is declared more than once. In fact, it turns out C++ files won't compile when the same variable, function, class, etc. is declared more than once.

The solution is to use # ifndef statements, which allow you to implement a technique called inclusion guards.

The ifndef statement stands for "if not defined". When you wrap your header files with #ifndef statements, the compiler will only include a header file if the file has not yet been defined. In the current main.cpp example, the "engine.h" file would be included first. Then the compiler includes "car.h". But "car.h" will try to include "engine.h" again; however, the inclusion guard in the "engine.h" file will ensure that "engine.h" does not get included again.

Here is what the "engine.h" file looks like with an ifndef statement

#ifndef ENGINE_H
#define ENGINE_H

#include <string>

class Engine
{
    private:
        std::string enginesize;

    public:

        Engine ();

        Engine (std::string);

        void setSize(std::string);

        std::string getSize();

};

#endif /* ENGINE_H */

Using all caps with the _H is a naming convention. It is also customary to put a comment after the #endif statement with the filename.

You would want to wrap all of your header files with #ifndef statements. That way other programs do not have to keep track of what files have already been included when they want to use your code.

Here are the results of including #ifndef statements in the engine and car header files. If you click on "Test Run", you will see that the code now compiles.