常量类成员不是常量-C ++

Question

I wonder when I pass a value to a constant variable, why this variable can change.

#include <cstdio>

struct point
{
    int x, y, z;
};

class A
{
    public:
        A(const point &p) :
           p(p)
        {
             printf("(%d,%d,%d)\n", p.x, p.y, p.z);
        }

        void do_smth()
        {
            printf("(%d,%d,%d)\n", p.x, p.y, p.z);
        }

        const point &p;
};

int main(int argc, const char *argv[])
{
    point p = {1, 1, 1};
    A a(p);
    p.y = 4;
    a.do_smth();
    return 0;
}

stdout:

(1, 1, 1)
(1, 4, 1)

I use g++ v4.7 with no extra arguments to compile this code.

Answer 1

Well, the Point inside the A class is constant, but as it's a reference to p which it not constant, you can change p as much as you like and the all references to p will change as well. To simplify, references are just a kind of fancy pointer.

Answer 2

p itself is not const. Ap is... Referencing a non- const variable with a const reference doesn't magically make it const .

Answer 3

const表示该值不能通过此引用更改，而不是根本不会更改。

Answer 4

You change the variable in place, where it is not constant. You cannot change it in do_smth() method, try it.

Passing to some class as constant doesn't change the object at all.