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Overloading Operator Problem

Started by utwo007 Apr 10, 2002 at 3:52 PM 7 replies 150+ views

Original Post

Apr 10, 2002 at 3:52 PM

Got a little problem. I''m creating a String class, and I''m getting a weird compiler error. Here''s what I have so far. The problem is with overloading the ''+'' operator. I want to be able to do something like this:

string3 = string1 + string2;

And I figured that the best way to do it would be to create a temporary string object, set that object to the union of string1 and string2, and then return that temporary object. It would look like this. I wont put the whole code here, but rather just the general logic of it so you get the gist:

string operator + (string &string2)
{

 string temp;
 
 // Set temp.length equal to the added lengths of both strings involved.

 // Use strcopy''s to copy data from both strings into temp''s buffer.

 return temp;

}

So, the above expression would essentialy end up like this:

string3 = temp;

And then I''ve overloaded the ''='' operator to do what''s expected. The problem is that I am getting this error that says "cannot return temporary" from the Bloodshed''s IDE (I forget which compiler it uses -- starts with M). Why the hell can''t I? This seems like the simplest, most logical thing to do. I''ve persued other methods, such as using a global temporary string object, as well as altering the calling object (in this case string1) so that it equals the result of string1 + string2. Nothing works. I even tried having the overloaded ''+'' function return a char*, and I overloaded ''='' to accept a char*, and I got the same dang message. Anyone know what I''m doing wrong? ---signature--- People get ready. I''m ready to play.

SabreMan

Apr 10, 2002 at 5:50 PM

Are you sure the compiler is complaining about the piece of code you showed? You might want to try posting the smallest code sample you can which recreates the error. The code you have showed shouldn''t cause the diagnostic you have described.

krez

Apr 10, 2002 at 6:36 PM

i believe the temp variable goes out of scope when the function exits, so you are returning something that is (might be?) immediately destroyed.

EDIT: sorry, my bad. evercrack has turned my brains to mush

[edited by - krez on April 10, 2002 7:45:49 PM]

Oluseyi

Apr 10, 2002 at 6:41 PM

quote:
Original post by krez
i believe the temp variable goes out of scope when the function exits, so you are returning something that is (might be?) immediately destroyed.

utwo007

Apr 10, 2002 at 7:59 PM

ok here is the code and the compiler error:


// In the main cpp driver the call      

s3 = s2 + s1; // where s1 s2 and s3 are all of user defined        
              // type String

And here's the overloaded operator function:


const String &String::operator+( const String &right )
{

    String Temp;

    Temp.sptr = new char[length + right.length+1];
    strcpy(Temp.sptr, sptr);
    strcat(Temp.sptr, right.sptr);
    return Temp;

}

//so s1 + s2     should essentially be replaced by Temp,  which would then read s3 = Temp        
  

So then this function is called:



[code]
const String &String::operator=( const String &right ){

    cout << "Operator = called ";

    if ( &right != this )
    {
        delete [] sptr;
        length=right.length;
        setString( right.sptr );

    }

    else
        cout << "Attempted to assign a string to itself";
    return *this; //enables cascading assignment

//the setstring function is this

void String::setString(const char *string2 ){

	sptr=new char [ length +1 ];
	assert( sptr != 0 );
	strcpy( sptr, string2);
}

the warning given is
97 functions5.cpp
warning: reference to local variable `Temp' returned

Anyone have any ideas?

EDIT: Added some text-formatting.

[edited by - utwo007 on April 10, 2002 9:14:12 PM]

Dragonskin

Apr 10, 2002 at 8:27 PM

Remove the "&" in the return type of the addition operator. That's what's giving you the problem (so it doesn't make the copy, it's going out of scope as krez mentioned).

[edited by - dragonskin on April 10, 2002 9:28:19 PM]

mmelson

Apr 10, 2002 at 8:28 PM

const String &String::operator+( const String &right )

should be

const String String::operator+( const String &right )

you were trying to return a reference to a local var, and that''s a no-no.

utwo007

Apr 10, 2002 at 8:39 PM

Crap, that causes another problem but ill work on that now. Thanks for the help mates!

SabreMan

Apr 11, 2002 at 4:10 AM

quote:
Original post by mmelson
should be

const String String::operator+( const String &right )

Not quite. By a strange quirk of C++, the return value should be non-const value. This allows you to do form degenerate structures like:

(s1 + s2) = s3;

Completely stupid, but its what the built-in types do, and its what the Standard Library is allowed to expect of your UDTs.

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