class operator +

Hi!

I'm trying to learn operator overloading. But I have some problems grasping the basic idea.

Let's say we have this class:

Code:

class tutClass
{
  char * data;

public:
  tutClass() : data(NULL) {};
  tutClass(char *);
  ~tutClass();

  conversion operator + (char *);
  conversion operator + (tutClass &);
}

tutClass::tutClass(char * str)
{
  data = new char [ strlen(str) + 1 ];
  strcpy(data, str);
}

~tutClass()
{
  if (data)
    delete [] data;
}

tutClass tutClass::operator + (char * str)
{
  //??? what goes here?
  //??? something like this?

  char * temp = new char [ strlen(str) + strlen(data) + 1 ];
  strcpy(temp, data);
  strcat(temp, str);

  tutClass result(str);

  delete [] temp;

  return result; //this isn't right, returning a local var
//but how to do it right?
}
//same for tutClass::operator + (tutClass & tut)

I probably should implement operator = also to make this work, but this is just to show "+"...

And should it return tutClass, or tutClass &

Any help would be appreciated!

Joren

Check out how microsoft does it....
where string1 is the left and string2 is the right
then a new object (s) is created and returned...found it the file

C:\Program Files\Microsoft Visual Studio\VC98\MFC\SRC\strcore.cpp

Code:

CString AFXAPI operator+(const CString& string1, const CString& string2)
{
	CString s;


	s.ConcatCopy(string1.GetData()->nDataLength, string1.m_pchData,
		string2.GetData()->nDataLength, string2.m_pchData);


	return s;
}

void CString::ConcatCopy(int nSrc1Len, LPCTSTR lpszSrc1Data,
	int nSrc2Len, LPCTSTR lpszSrc2Data)
{
  // -- master concatenation routine
  // Concatenate two sources
  // -- assume that 'this' is a new CString object

	int nNewLen = nSrc1Len + nSrc2Len;
	if (nNewLen != 0)
	{
		AllocBuffer(nNewLen);
		memcpy(m_pchData, lpszSrc1Data, nSrc1Len*sizeof(TCHAR));
		memcpy(m_pchData+nSrc1Len, lpszSrc2Data, nSrc2Len*sizeof(TCHAR));
	}
}

But isn't s a local variable? And you shouldn't return a local variable, because it goes out of scope on return s; If not, I don't understand anything anymore. At least not about scope

>But isn't s a local variable?

Yes, but when you're returning by value a copy is made. If the code was returing a pointer or reference to a local object created on the stack you'd be correct.

try to think of operator overloading as a way of relating 2 of the same objects with operators eg =, +. / etc, etc. for example with 3d vectors my + operator is like this:

Code:

Vector3d operator +( const Vector3d& a, const Vector3d& b )
{
	Vector3d temp;
	int i;

	for( i = 0; i < 3; i++ )
	{
		temp.triple[ i ] = b.triple[ i ] + a.triple[ i ];
	}

	return temp;
}

the vector is like an x, y, z coordinate and this is the way to add 2 togethor - dead simple.

Lets try another with the following class....

Code:

class Date
{
public:
      Date( );
      friend ostream& operator << ( ostream& os, const Date );

private:
     int day;
     int month;
     int year;
}

//implementation of <<
ostream& operator << ( ostream& os, const Date )
{
      cout << day << " : " << month << " : " << year;
}

this is a simple example of insertion overloading but I hope you get the idea