CODEWITHSUNDEEP
c++referenceconstants
ajayg
ajayg

Reputation: 51

A reference data-member changing another data member inside a const function

I apologize if this question has already been asked, but I could not find the same question. Please redirect me to the relevant question.

#include<iostream>

using namespace std;

class ABC
{
    int a;
    int &ref;
public:    
    ABC(int arg = 0):a(arg), ref(a){}
    void mutate_func(int arg) const {
        ref = arg;
    }
    void print_val() {
        cout << endl << &a << "\t" << &ref;
        cout << endl << a << "\t" << ref;
    }
};

int main()
{
    ABC abc_obj(5);
    cout << sizeof(abc_obj);

    abc_obj.print_val();
    abc_obj.mutate_func(10);
    abc_obj.print_val();
    return 0;
}

I am trying to modify a data-member of a class inside a const member function through a reference variable which is part of the same class only.

I have two questions -

  1. Why it is not throwing compilation error.

  2. I am printing address of both variables and as expected both are showing same address, but still sizeof() for the instance is showing size as 8 bytes.

Upvotes: 2

Views: 160

Answers (2)

Thelvyn
Thelvyn

Reputation: 264

I can answer the sizeof and address part.

A reference can be seen like a pointer with a lighter syntax so it needs storage space equivalent to a pointer (4 bytes here).

A reference is modified only during its initialization ref(a). After its initialization, any operation on the reference is performed on the referenced value, so &ref gives the address of the referenced value and not the reference itself. So &a == &ref makes perfect sense.

Upvotes: 1

Johannes Schaub - litb
Johannes Schaub - litb

Reputation: 506897

  1. The compiler cannot track reference destinations completely at compile time.
  2. Because somewhere it needs to be stored where a reference refers to.

This is a class making it very easy to trigger undefined behavior. Just create a const object of it and call the const member function.

C++ supports the mutable keyword for a supported mechanism of changing the bits of an object in a const member function.

Upvotes: 2

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