What is the use of const in the given function when the member variables are changed by the function?

Question

class StatDemo
{
   private: static int x;
   int y;
   public: void setx(int a) const { x = a; }
   void sety(int b) const { y = b; }
   int getx() {return x; }
   int gety() {return y; }
} ;

What is the use of const when the member variables are changed by the function??

It looks like you've mistakenly made the setters const, and the getters non-const. It should be the other way around. And this stuff should all be _inside_ the class definition, not after it. — paddy, Sep 19 '16 at 02:43

xaxxon · Accepted Answer · 2019-08-15T20:13:34.420

1

methods not marked const cannot be called on a const object (or ref or pointer to a const object).

StatDemo sd;
StatDemo const & sdr = sd;
sdr.get(x); // error because getx isn't marked const

However, that means that all the data members accessed from within a method marked const are also const, so you cannot change them (without playing tricks).

That's why your setx won't compile -- x is const within those methods.

edited Aug 15 '19 at 20:13

answered Sep 19 '16 at 02:43

xaxxon

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`x` is static, so it isn't affected by `const` qualification on `setx` – Caleth Aug 15 '19 at 08:35
@Caleth fixed, thank you :) I didn't catch that it was static. – xaxxon Aug 15 '19 at 20:14

score 0 · Answer 2 · answered Aug 15 '19 at 20:35

What is the use of const when the member variables are changed by the function?

As @songyuanyao correctly mentioned, to cause compile errors.

Yet, it is rather a convention. You may still modify members via const_cast on this, or via marking members mutable.

There's a difference between logical and physical constness, as discussed here.

Why may we still modify non-const static members in const methods?

A non-static method of a class has this as a parameter. const qualifier on a method makes this constant (and triggers a compile error when the convention's violated).

A static member isn't related to this in any way: it is the only one for every object of a class. That's why the constness of a method (i.e. the constness of this) has no impact on static members of a class.

What is the use of const in the given function when the member variables are changed by the function?

2 Answers2