"+" operator overloading in C++

Question

4.11/5 (3 votes)

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C++

#include <iostream>

using namespace std;

class Test {
private:
	int mx;
	float my;
	int medata; //some extra data

public:
	Test() { mx = my = medata = 0; }
	Test(int x, float y, int edata) { mx = x; my = y; medata = edata; }

	void print() { cout << "x = " << mx << " y = " << my << " edata = " << medata << endl; }

	void modify_edata(int data) { medata = data; }

	Test operator+ (const Test& other)
	{
		Test sum;
		sum.mx = this->mx + other.mx;
		sum.my = this->my + other.my;
		return sum;
	}
	};

int main()
{
	Test t1(10, 50.5, 0), t2(20, 60.5, 0);
	Test t3;

	t3.modify_edata(1000);
	t3.print();

	t3 = t1 + t2;

	t3.print();

	return 0;
}

Question:
When writing operator "+" functionality, my intention would be to add few members of class but not all. The way we generally write "operator+" function will overwrite all the class members because we create a local object in "operator+ ()". Can we avoid this? How to retain the values of other class members in this case?

What I have tried:

Trying to understand the usage. May be this is how it works. Need confirmation.

Posted 28-Mar-19 0:45am

Member 13508014

Updated 28-Mar-19 22:58pm

KarstenK

v2

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CPallini · Accepted Answer · 2019-03-28T01:09:00

Solution 1

See the "Binary arithmetic operators" section of operator overloading - cppreference.com[^] for a guideline.

Posted 28-Mar-19 1:09am

CPallini

Comments

Member 13508014 28-Mar-19 7:40am

I read "Binary arithmetic operators" section of operator overloading - cppreference.com[^].
When we write a = b + c, according to the article, here 'b' will also get altered isn't it.

CPallini 28-Mar-19 7:49am

Nope, a copy of b whill be altered.

Maciej Los 29-Mar-19 13:23pm

5ed!

CPallini 30-Mar-19 6:59am

Thank you, Maciej.

Member 13508014 31-Mar-19 3:41am

Yes. Copy of b will be altered. I agree that.

But, I think, my question is not understood here.

My question is, in "a = b + c", lets assume that the object "a" is created a bit long and its state is modified now and when i call "a = b + c", i expect the addition operation to happen on only few members and those will get altered. Other member of object "a" for which "+" operation does not make sense should be same as before calling "a = b + c".

CPallini 31-Mar-19 5:55am

Possibly I actually misunderstood your original question.
If you are asking if 't3.edata' erase in
t3 = t1 + t2;
is correct, then yes, it is correct: it is so by the assignment operator semantics (t3 should be exactly the sum of t1 and t2).

Member 13508014 1-Apr-19 13:50pm

Yes. I too realized and agree that the point that t3 should be exactly the sum of t1 and t2. Thanks CPallini for your time in answering my question.

CPallini 1-Apr-19 15:50pm

You are welcome.

steveb · Accepted Answer · 2019-03-28T04:20:00

Solution 2

There are three rules for writing binary arithmetic operator:
1. You also must write a compound operator, += in your case.
2. Your compound operator must return a reference

MyClass & MyClass::operator+=(const MyClass &rhs) {
    ...
    return *this;
  }

3. Define your binary arithmetic operators using your compound assignment operators:

const MyClass MyClass::operator+(const MyClass &other) const {
    return MyClass(*this) += other;
  }

Posted 28-Mar-19 4:20am

steveb

v2

Comments

Stefan_Lang 29-Mar-19 4:41am

Good advice. But the most important part of it should be pointed out separately: that operator+ should be declared as a const member function! (the OP forgot to do that)

Maciej Los 29-Mar-19 13:23pm

5ed!

Stefan_Lang · Accepted Answer · 2019-03-28T22:59:00

It sounds like you're trying to merge to kinds of objects into one: one that has an intuitively understandable concept of summation, and another that doesn't. Still they are related somehow to the point that you want to have them inside a common ... container.

I'd suggest to separate your 'additive data' from the 'user data' like this:

C++

class MyValues {
  int mx;
  float my;
public:
  MyValues(int x, float y) : mx(x), my(y) {}
  MyValues(const MyValues& other) : mx(other.mx), my(other.my) {}
  MyValues& operator +=(const MyValues& other) {
    mx += other.mx;
    my += other.my;
    return *this;
  }
  MyValues operator+(const MyValues& other) const {
    return MyValues(*this)+=other;
  }
};
class MyData {
  int mdata;
public:
  MyData(int data) : mdata(data) {}
  MyData(const MyData& other) : mdata(other.mdata) {}
  void set(int data) { mdata = data; }
  int data const ( return mdata; }
};
class MyClass {
  MyValues mvalues;
  MyData mdata;
public:
  MyClass(int x, float y, int data) : mvalues(x, y), mdata(data) {}
  void addValues(const MyClass& other) {
    mvalues += other.mvalues;
  }
};

This allows you to separate user data functionality from numerical data functionality.