What is the point of non-static const data member? [duplicate]

Question

For the following code:

class A
{
public:
    const int cx = 5;
};

Here, an instance of cx will be created for every object of A. Which seems a waste to me, as cx can never be modified. Actually, I don't see any reason why compiler shouldn't enforce making a const data member static. Can anybody please explain this to me ?

Answer 1

A const data member does not have to be the same for all instances. You can initialize it in the constructor.

class A
{
public:
    A(int n) :cx(n) {}

    const int cx;
};

int main()
{
    A a1(10);
    A a2(100);
}

Answer 2

Actually, I don't see any reason why compiler shouldn't enforce making a const data member static.

Have you considered that cx might be initialized in the constructor with a value known at run-time - and varying between different instances of A?
const members make assignment to them impossible, but sometimes having a member that cannot change its initial value proves useful.

Definitely not the best example, but to illustrate the idea:

struct Multiplier
{
    const int factor;

    Multiplier(int factor) : factor(factor) {}

    int operator()( int val ) const
    {
        return val * factor;
    }
};

std::vector<int> vec{1, 2, 3};
std::vector<int> vec2;

int i;
std::cin >> i;

std::transform( std::begin(vec), std::end(vec),
                std::back_inserter(vec2), Multiplier(i) );

// vec2 contains multiples of the values of vec