Is it possible change value of Member variable within "const" function?

Question

Value of constant variable can be changed through pointer tricks, but is it possible to do something like this :

    class A (){
       int x;
    public:
       void func () const {
          //change value of x here
    }
}

*Value of constant variable can be changed through pointer tricks* - Sure, but it's not useful at all. Just because you can trick the compiler into that doesn't mean it's safe. — chris, Sep 21 '15 at 01:49
i understand, just asked out of curiosity and knowledge of tricks — , Sep 21 '15 at 01:51
@jacky666 You see, the use of `const` is generally optional, its mostly something you use to tell the compiler a determined method is "read-only" and guaranteed to execute without changing any data in the class. When you violate that premise you defeat the purpose of defining a method as `const` to begin with. Don't do it. If your method does in fact change data in the class, then don't make it `const`. — Havenard, Sep 21 '15 at 01:54
The search is your friend http://stackoverflow.com/questions/751681/meaning-of-const-last-in-a-c-method-declaration — StahlRat, Sep 21 '15 at 02:08

score 66 · Answer 1 · answered Sep 21 '15 at 01:54

66

declare x mutable

class A (){
   mutable int x;
public:
   void func () const {
      //change value of x here
   }
};

answered Sep 21 '15 at 01:54

john zhao

986
1
6
8

This saved me a ton of time! Thank you for this! – Aviv Moshe Albeg Sep 19 '21 at 04:09

bku_drytt · Answer 2 · 2015-09-21T02:00:52.943

13

You have two options:

class C
{
public:
    void const_f() const
    {
        x = 5;                               // A
        auto* p_this = const_cast<C*>(this); // B
        p_this->y = 5;
    }

private:
    mutable int x;                           // A
    int y;
};

A: declare certain members mutable.
B: const_cast to remove constness from the this pointer.

edited Sep 21 '15 at 02:00

answered Sep 21 '15 at 01:55

bku_drytt

3,169
17
19

Isn't `p_this->y = 5;` undefined behavior? – Emil Laine Dec 29 '15 at 13:15
Your Plan B is UB if `this` really points to an immutable object. – Swordfish Aug 20 '20 at 17:04

Adeel Ahmad · Accepted Answer · 2015-09-21T01:52:19.640

8

Though this is not appreciated, but C++ provides “Backdoors” which can be used to breach its own regulations, just like dirty pointer tricks. Anyway, you can easily do this by using a casted version of “This” pointer :

class A (){
           int x;
        public:
           void func () const {
              //change value of x here
         A* ptr =  const_cast<A*> (this);
         ptr->x= 10;     //Voila ! Here you go buddy 
        }
 }

edited Sep 21 '15 at 01:52

answered Sep 21 '15 at 01:49

Adeel Ahmad

990
8
22

2

I believe you meant `const_cast`, and this is an incomplete answer. – Havenard Sep 21 '15 at 01:51
and even if you `const_cast` ... doing it to an immutable object and modifying it is UB. – Swordfish Aug 20 '20 at 17:02

score 6 · Answer 4 · answered Sep 21 '15 at 02:29

The most important thing to understand here is bitwise/physical/concrete constness and conceptual/meaningwise/logical/abstract constness.

In short:

If the function is conceptually const, make the member data mutable.
Otherwise, make the function non-const.

score 0 · Answer 5 · answered Dec 27 '15 at 08:37

Just cast 'this', this would be a dirty way to implement your program, do avoid this if you are doing a project or teamwork as others would get confused by this.

    class CAST_CLASS (){
       int var;
    public:
       void change_CAST () const {

     CAST_CLASS* pointer =  const_cast<CAST_CLASS*> (this);
     pointer->var= 10;     
    }};

score 0 · Answer 6 · answered May 25 '20 at 19:39

The other answers don't mention this, but following also modifies "x" (definitely, not advisable):

class A {
    int x, &y{x}, *z{&x};
public:
    void func () const
    {
        y  = 42; // x is modified now!
        *z = 29; // x is modified again!!
    }
};

Is it possible change value of Member variable within "const" function?

6 Answers6