Why can I call a method that changes a member from a const method?

Question

I'm not sure to understand why I can modify an object from a const method, look:

#include <iostream>

struct Foo {
    int a = 0;
    void change() {
        a = 3;
    }
};

struct Test {
    Foo* f;

    Test(): f{new Foo} {}

    void test() const {
        f->change();
    }
};

int main()
{
  Test t;
  std::cout << "before: " << t.f->a << "\n";
  t.test();
  std::cout << "after: " << t.f->a << "\n";

}

Not only it compiles but it prints:

0
3

So I was able to modify the logical state of an object from a const method. Is that because I used a pointer?

Answer 1

The const applies to the pointer itself, f, not to what this pointer is pointing to. The type of f inside your const-qualified member function, Test::test(), is Foo* const (i.e., const pointer to Foo), not const Foo* (i.e., pointer to const Foo). That's why you can modify what the pointer is pointing to in spite of the const qualification of the member function.

---

Note that the following sample member function, Test::test2(), does fail to compile since it is const-qualified and tries to modify the pointer data member, f:

void Test::test2() const {
        f = nullptr; // <-- error
}