const method modifies object using reference

Question

The following code calls a const method passing a reference to a member, which is then modified.

#include <iostream>

struct A {
  int i;
  A(int _x) : i(_x) { }
  void calc(int& j, const int value) const { j = value; }
  void set1() { calc(i, 1); }
};

int main()
{
  A a(3);
  std::cout << a.i << std::endl;
  a.set1();
  std::cout << a.i << std::endl;

  return 0;
}

The code compiles with gcc 6.4.0, and with clang 5.0.2, with no warnings.

Is the code legal? The const method calc is able to modify the object, when called from a non-const method.

Answer 1

const qualifier on a member function applies to the *this instance.

In calc(), this is a pointer to const A, but the parameter j is taken by non-const reference, so this is perfectly standard behaviour.

Now, if in calc you tried to assign to this->i, the code would not compile.

void A::calc(const int value) const
{
    i = value; // Compilation error here: i is a data member of a const instance
}

In the same way, if set1 was made a const member function, then, the code would not compile (because it would try to bind this->i to a parameter taken by non-const reference)

Answer 2

Sure. Marking the method const just makes *this const, i.e. the function promises not to modify the object by writing through this.

It's still possible to modify the object through other means (assuming they're not marked const as well, such as int& j in your example).