Assignment in Const C++ functions

Question

I have an assignment in which I have to write a simple class. This class must hold an array of strings, and contain an overload of the '+' operator, which combines the elements of two arrays into a new array and returns it.

Additionally, this function must be 'const', which is where I ran into problems. When trying to change the class "size" attribute and the array it is holding, I get errors. I also get an error when trying to return the object. I understand the cause for the first two errors is because I have declared the function 'const', but my question is, what is the proper way to reassign these values inside of a const method?

Don't tear into me too bad, I've just begun learning C++, and the concepts are a bit new to me. I've tried researching the subject, but I haven't found any useful answers. That being said, I really need to get this function working, so any help will be much appreciated.

Here is the class:

class FirstClass{

private:
    string* sList;
    unsigned int _size;

public:
    FirstClass(const unsigned int initSize = 1);
        // copy, assignment, and destructor 
    FirstClass& operator+(const FirstClass& add)const;
};





    FirstClass::FirstClass(const unsigned int initSize):_size(initSize)
{
    sList = new string[initSize];
    return;
}



FirstClass& FirstClass::operator+(const FirstClass& add)
const{
    if(add.sList){
        int prevSize = this->_size;
        this->_size += add._size;   // can't do this
        string newList[this->_size];


        for(int a=0; a<prevSize; a++){
            newList[a] = this->sList[a];
        }
        for(int b=0; b<add._size; b++){
            sList[b+prevSize] = add.sList[b];
        }
        delete [] sList;
        this->sList = newList; // or this
    }

    return *this; // or this
}

EDIT: Thanks for the quick response, and clearing up what I was doing. Here is my revised code.

FirstClass FirstClass::operator+(const FirstClass& add)
const{
    FirstClass ma(this->_size + add._size); 
    if(add.sList){

        for(int a=0; a<this->_size; a++){
            ma.sList[a] = this->sList[a];
        }
        for(int b=0; b<add._size; b++){
            ma.sList[b+this->_size] = add.sList[b];
        }
    }

    return ma;
}

Hint: if I do: int a; int b; int c = a + b; does a or b get modified? — Pubby
– Pubby, Commented Apr 25, 2013 at 16:49

juanchopanza · Accepted Answer · 2014-01-06 23:22:46Z

4

An addition operator should return a new object, not a reference to one of its operands. It doesn't make any sense for A+B to modify A or B, or return a reference to one of them.

With this in mind, it is easy to see how the operator can be const.

FirstClass operator+(const FirstClass& rhs) const;

You could implement a mutating operator+=, and implement operator+ in terms of that:

FirstClass& operator+=(const FirstClass& rhs);

FirstClass operator+(const FirstClass& rhs) const 
{
  FirstClass ret = rhs;
  ret += *this;
  return ret;
}

or as a non-member:

FirstClass operator+(const FirstClass& lhs, const FirstClass& rhs)
{ 
  FirstClass ret = lhs;
  ret += rhs;
  return ret; 
}

edited Jan 6, 2014 at 23:22

answered Apr 25, 2013 at 16:49

juanchopanza

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3 Comments

dousin Over a year ago

Your non-member version modifies lhs, which is operator+ not supposed to do. In addition, using const on a non-member function is not allowed.

juanchopanza Over a year ago

@dousin Thanks. It takes the LHS by value, so the first part is not an issue. What is an issue is that it inhibits return value optimization. I will fix that now.

juanchopanza Over a year ago

@dousin It was good that you brought my attention to this. I must have been drunk when I posted the original answer!

Jack · Accepted Answer · 2013-04-25 16:50:18Z

1

The + operator shouldn't return a reference to an object but rather a new object. Don't confuse +() with +=() operator. The latter applies the operation to the lval of the expression while the first returns a new object.

You should return:

FirstClass FirstClass::operator+(const FirstClass& add) const

Assuming this you can understand why the const requirement has a sense. Without it you would be allowed to modify any of the variables of the implicit this object which is not allowed. The compilation error is there exactly to tell you: "look you are trying to modify the size but you should return a new object so operands must not be modified".

answered Apr 25, 2013 at 16:50

Jack

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