When is a non-static const data member more useful than a const static one?

Question

In C++ there are static and non-static const data members. When I want a constant, I always make it static because it does not make sense to have multiple copies (one per each instance of the class) if the item cannot be modified. I would like to know why are there non-static const members?

Answer 1

1. A static const applies to the class
2. A const applies to the object

E.g.

class Verticies {
    public:
       const int colour;
       static const int MAX_VERTICIES = 100;
       Point points[MAX_VERTICIES];
       Verticies(int c) : colour(c) { }
       // Etc
};

Here MAX_VERTICIES applies to all objects of type Verticies. But different objects could have different colours and that colour is fixed on construction

Answer 2

class MyClass
{
    public:
       MyClass(int val): myVal(val) {}
       const int myVal;
};

In this small example, myVal is readable by everyone without a getter method, but it's not modifiable as soon as MyClass is instantiated.

You just declare data types as const if you want to make sure they are not modified later on.

Because myVal is not static, it can have different values for every different instance of MyClass. If myVal would be static, it would have the same value for every instance and would not be assignable by the constructor.

Take the following example

#include <iostream>
using namespace std;

class MyClass
{
    public:
        MyClass(int val) : myConst(val) {}
        void printVars() {
            cout << "myStatic " << myStatic << endl;
            cout << "myConst " << myConst << endl;
        }

        static const int myStatic;
        const int myConst;
};

const int MyClass::myStatic = 5; //Note how this works without 
                                 //an instance of MyClass

int main()
{
    MyClass ca(23);
    MyClass cb(42);
    cout << "ca: " << endl;
    ca.printVars();
    cout << "cb: " << endl;
    cb.printVars();
    return 0;
}

Results:

ca: 
myStatic 5
myConst 23
cb: 
myStatic 5
myConst 42

The following would result in a compilation error because the variables are const:

ca.myConst = 11;
ca.myStatic = 13;