Comparing 2 objects of the same class (overriding == operator) c++

Question

I'm new to c++ (coming from Java and C#), and I'm trying to override the == operator in one of my classes so I can see if I have 2 object that have the same value for a given property. I've been doing a bunch of googling and trying to make something that works. What I need is for the == operator to return TRUE when 2 objects have the same _name text.

Here's the header file:

//CCity.h -- city class interface
#ifndef CCity_H
#define CCity_H

#include <string>

class CCity
{
friend bool operator ==(CCity& a,  CCity& b)
{
    bool rVal = false;
    if (!(a._name.compare(b._name)))
        rVal = true;
    return rVal;
}
private:
    std::string _name;
    double _x; //need high precision for coordinates.
    double _y;
public:
    CCity (std::string, double, double); //Constructor
    ~CCity (); //Destructor
    std::string GetName();
    double GetLongitude();    
    double GetLatitude();
    std::string ToString();
};
#endif

In my main() method:

    CCity *cit1 = new CCity("bob", 1, 1);
    CCity *cit2 = new CCity("bob", 2, 2);
    cout<< "Comparing 2 cities:\n";
    if (&cit1 == &cit2)
        cout<< "They are the same \n";
    else
        cout << "They are different \n";
    delete cit1;
    delete cit2;

The problem is that my code in the friend bool operator == block never gets executed. I feel like I'm doing something wrong with either how I'm declaring that operator, or how I'm using it.

Pubby · Accepted Answer · 2013-02-04 04:11:41Z

5

& takes the address of (you're comparing pointers), when really you want to dereference using *:

if (*cit1 == *cit2)
    cout<< "They are the same \n";

Anyway, there is absolutely no point to using pointers here, let alone dumb ones.

Here's how it would look without them (the proper way):

CCity cit1("bob", 1, 1);
CCity cit2("bob", 2, 2);
cout<< "Comparing 2 cities:\n";
if (cit1 == cit2)
    cout<< "They are the same \n";
else
    cout << "They are different \n";

Also, as WhozCraig mentions, consider using const-ref parameters for your operator== function since it shouldn't modify the arguments.

edited Feb 4, 2013 at 4:11

answered Feb 4, 2013 at 4:06

Pubby

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2 Comments

WhozCraig Over a year ago

+1, nor is there any point in declaring a free-function for this operator, nor non-const reference parameters, etc...

CurtisHx Over a year ago

Ah. Now I see. I was (trying) to pass pointers to ==. And yes, I could have gotten away without using pointers, but I needed to use the == in other places in my code where I will be using dynamic memory and pointers. Thank you.

Vaughn Cato · Accepted Answer · 2013-02-04 04:06:32Z

3

With this code:

CCity *cit1 = new CCity("bob", 1, 1);
CCity *cit2 = new CCity("bob", 2, 2);
cout<< "Comparing 2 cities:\n";
if (&cit1 == &cit2)
    cout<< "They are the same \n";
else
    cout << "They are different \n";

You are comparing the pointers to the pointers to the CCity instances.

You want something like this:

CCity *cit1 = new CCity("bob", 1, 1);
CCity *cit2 = new CCity("bob", 2, 2);
cout<< "Comparing 2 cities:\n";
if (*cit1 == *cit2)
    cout<< "They are the same \n";
else
    cout << "They are different \n";

answered Feb 4, 2013 at 4:06

Vaughn Cato

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1 Comment

Alex Over a year ago

I'd rather have this one as the answer than what Pubby put out.

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