C++ const function error

Question

I am quite new to oop in c++ and I came across the following piece of code while surfing the net:

#include<iostream>
using namespace std;

class cls
{int x;
public: 
    cls(int i=3) {x=i;}

    int &f() const{ return x;}
};
int main()
{
    const cls a(-3);
    int b=a.f();
    cout<<b;
    return 0;
}

And when I try to run the code, it crashes due to the f function. Now I am not quite sure what is going on there and why it crashes, so I would need someone to enlighten me a little bit on the subject.

Possible duplicate of [Is the practice of returning a C++ reference variable, evil?](http://stackoverflow.com/questions/752658/is-the-practice-of-returning-a-c-reference-variable-evil) — Robert Columbia, Aug 30 '16 at 15:35
On the other hand, your code [doesn't even compile](https://ideone.com/C7iIyn). What compiler has been used to compile this code? — Algirdas Preidžius, Aug 30 '16 at 15:36
@FirstStep 1st statement - construction of const object. 2nd statement - definition of constructor having an optional parameter. What exactly is wrong with those 2 lines, in your opinion? — Algirdas Preidžius, Aug 30 '16 at 15:39
_@Vasile_ Please remember always to post the exact error messages you get from your compiler. — πάντα ῥεῖ, Aug 30 '16 at 15:39
@AlgirdasPreidžius The question actually asks about how to fix the compiler error :P — πάντα ῥεῖ, Aug 30 '16 at 15:44
"And when I try to run the code, it crashes due to the f function." <- no it doesn't... the code, as shown, doesn't even *COMPILE*. — Nik Bougalis, Aug 30 '16 at 15:47
@πάνταῥεῖ As mentioned by NikBougalis - no, it doesn't. It talks about crashing (_And when I try to run the code, it crashes due to the f function._), and crashing != compiler error. — Algirdas Preidžius, Aug 30 '16 at 20:46
@AlgirdasPreidžius Define _crashing_ please. I don't know such term used seriously or professionally at all. — πάντα ῥεῖ, Aug 30 '16 at 20:47

πάντα ῥεῖ · Accepted Answer · 2016-08-30T15:41:13.693

4

As your function is declared as const you can't return a non const reference to a member variable unless you mark it as mutable.

To fix your code write either

class cls
{ 
    mutable int x;
 // ^^^^^^^
public: 
    cls(int i=3) {x=i;}

    int &f() const{ return x;}
};

or return a const reference.

class cls
{ 
    int x;
public: 
    cls(int i=3) {x=i;}

    const int &f() const{ return x;}
 // ^^^^^
};

Using mutable needs to be taken with a grain of salt, it would break up the encapsulation of your class, and allow your class internals to be changed via the reference you handed out.

edited Aug 30 '16 at 15:41

answered Aug 30 '16 at 15:35

πάντα ῥεῖ

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3

int f() const{ return x;} also ok – Aug 30 '16 at 15:44
2

@KenmanTsang yes in _this_ case it is ok and, in fact, it's probably advisable (copying an `int` is easy). But returning a copy isn't always a good idea or even possible. – Nik Bougalis Aug 30 '16 at 15:46

score 1 · Answer 2 · answered Aug 30 '16 at 15:49

You can't return a non const reference to a const. The const member function makes the variable x non const when it accesses it.

There seems no need to return a reference, and in fact it is a bad practice. If you need to change your internal int it makes more sense to add a setter:

class cls
{
    int x;
public: 
    cls(int i=3) { set(i); }
    void set (const int val) { x=val; }
    int f() const{ return x; }
};

C++ const function error

2 Answers2