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c++constants

Same function with const and without - When and why?

T& f() { // some code ... }
const T& f() const { // some code ... }

I've seen this a couple of times now (in the introductory book I've been studying thus far). I know that the first const makes the return value const, in other words: unmodifiable. The second const allows that the function can be called for const declared variables as well, I believe.

But why would you have both functions in one and the same class definition? And how does the compiler distinguish between these? I believe that the second f() (with const) can be called for non-const variables as well.

Solution

  • But why would you have both functions in one and the same class definition?

    Having both allows you to:

    • call the function on a mutable object, and modify the result if you like; and
    • call the function on a const object, and only look at the result.

    With only the first, you couldn't call it on a const object. With only the second, you couldn't use it to modify the object it returns a reference to.

    And how does the compiler distinguish between these?

    It chooses the const overload when the function is called on a const object (or via a reference or pointer to const). It chooses the other overload otherwise.

    I believe that the second f() (with const) can be called for non-const variables as well.

    If that were the only overload, then it could. With both overloads, the non-const overload would be selected instead.