Returning local variable from a function in c++?

Question

This is excerpted from one of the c++ tutorials:

// vectors: overloading operators example
#include <iostream>
using namespace std;

class CVector {
  public:
    int x,y;
    CVector () {};
    CVector (int,int);
    CVector operator + (CVector);
};

CVector::CVector (int a, int b) {
  x = a;
  y = b;
}

CVector CVector::operator+ (CVector param) {
  CVector temp;
  temp.x = x + param.x;
  temp.y = y + param.y;
  return (temp);
}

int main () {
  CVector a (3,1);
  CVector b (1,2);
  CVector c;
  c = a + b;
  cout << c.x << "," << c.y;
  return 0;
}

In the operator overloading function, it creates a local var temp then returns it, I am quite confused, is this the right way?

We could give a more meaningful answer than "yes" if you explain what you're confused about. — interjay
– interjay, Commented Nov 9, 2013 at 22:05
If you don't count the fact that operator+ is typically implemented in terms of operator+=. — Qaz
– Qaz, Commented Nov 9, 2013 at 22:07

LihO · Accepted Answer · 2013-11-09 22:05:44Z

5

"is this the right way?"

Yes it is. Note that it is not local variable, but a copy of local variable that is actually returned, which is perfectly valid and right thing to do. Be cautious about returning local variables while returning by pointer or reference, not while returning by value.

answered Nov 9, 2013 at 22:05

LihO

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3 Comments

Michael Over a year ago

Perhaps such operation can cause undefined behavior. For instance, if class stores a pointer to some string which is allocated in constructor and if it has only default copy constructor.

LihO Over a year ago

@Michael: That is however not this case and quite unrelated to this matter.

Michael Over a year ago

Ok. Then using class described by the following link will be alright here: stackoverflow.com/questions/3278625/… ?

Kijewski · Accepted Answer · 2013-11-09 22:06:58Z

1

Yes, because it's returned by value. Had the function had the signature that follows, it would not have been correct:

CVector& CVector::operator+(CVector param);

By the way, a more efficient implementation would look like:

CVector CVector::operator+(const CVector &param) const;

edited Nov 9, 2013 at 22:06

Kijewski

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answered Nov 9, 2013 at 22:06

MDSL

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1 Comment

qed Over a year ago

In short, pass by reference, return by value.

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