C++ Basic Class Layout

Question

Learning C++ and see the class laid out like this:

class CRectangle {
    int x, y;
  public:
    void set_values (int,int);
    int area () {return (x*y);}
};

void CRectangle::set_values (int a, int b) {
  x = a;
  y = b;
}

I know Java and methods(functions) in Java are written within the class. The class looks like a Java interface. I know I can write the class like this:

class CRectangle {
    int x, y;
  public:
    void set_values (int a, int b) {
      x = a;
      y = b;
    };
    int area () {return (x*y);}
};

But is there a difference or standard?

Answer 1

There's a difference. When you write the definition of the function within the class definition (case 2), then the function is considered to have been declared inline. This is standard C++.

Usage, is to declare the member functions (Java methods) within the class definition, in a header file (.h), and to define these member functions in a C++ file (.cpp, .cc, or .C, …) This reduces compilation time, when you change the body of a function, only the C++ file has to be compiled, whereas if you change something in the header file, all C++ files that include this header are to be compiled.

Answer 2

It's much cleaner if you only define prototypes in the class definition (which belongs into a header file) and implement the methods in a cpp file.

When you have very small classes doing everything in the class definition might sound easier since everything is at the same place, but as soon as your class grows any developer using it will hate you since he'll have your code between the method prototypes he might look at to find out something (not everyone uses an IDE which shows all available methods!).

However, there is one exception: Template class methods needs to be implemented in the header as they need to be compiled for every specialization of the template.