Why is it possible to declare static constant member in a class but not constant member?

Do you mean initialize the variable in-class ?

A static const is a variable that is not going to change its value for all instances of the class.

A const variable on the other hand can have a value that doesn't change for one instance of the class, but can be different for different instances.

A static const example -

class A
{
public:
    static const int var = 100;
};

A const example -

class A
{
public:
    const int var;

    A(int a) : var(a) {}
};

int main()
{
    A a(10);
    A b(20);
}

I want to see it
just as an optimization of the memory usage... :-D