C++ – How to pass non static member function into glutDisplayFunc

static functions are functions that are only visible to other functions in the same file (more precisely the same translation unit).

EDIT: For those who thought, that the author of the questions meant a 'class method': As the question is tagged C he means a plain old C function. For (C++/Java/...) class methods, static means that this method can be called on the class itself, no instance of that class necessary.

You have to define your static member outside the class definition and provide the initializer there.

First

// In a header file (if it is in a header file in your case)
class A {   
private:      
  static const string RECTANGLE;
};

and then

// In one of the implementation files
const string A::RECTANGLE = "rectangle";

The syntax you were originally trying to use (initializer inside class definition) is only allowed with integral and enum types.

Starting from C++17 you have another option, which is quite similar to your original declaration: inline variables

// In a header file (if it is in a header file in your case)
class A {   
private:      
  inline static const string RECTANGLE = "rectangle";
};

No additional definition is needed.

Starting from C++20 instead of const you can declare it constexpr in this variant. Explicit inline would no longer be necessary, since constexpr implies inline.