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Tip of the Day
Language: C++
Expertise: Beginner
May 6, 1997

Overloading the assignment operator "="

Question:
I am puzzled at the way in which the assignment operator is overloaded in C++. Would you please show an example as to what efficient code for overloading this operator would look like. Specifically, my confusion is about how this function would copy old resources into new resources before return *this.

Answer:
The copy assignment operator for a class must do the following.

  1. make sure that all members are copied.
  2. make sure it calls assignment on any base classes it inherits from
  3. be safe from assignment to self.
lets consider a sample string class. e.g.
// class string inherits from some class called datatype
class string : public datatype
{
public:
	string(char *str,int len)
	{
		data_ = new char [len+1];
		strncpy(data_,str,len)
		data_[len] = '\0';
	}

	~string() {delete [] data_;}

	const string& operator = (const string &rhs)
	{
		// this is necessary else stuff later will destroy the 
		// data_ so we'll loose information
		if(&rhs != this)
		{
			// call base classes assignment operator
			datatype::operator =(rhs);

			// copy over data members
			delete [] data_;
			data_ = new char[strlen(rhs.data_)];
			strcpy(data_,rhs.data_);
		}

		// finally return referance to self
		return *this;
	}
private:
	char * data_;
};
Note that it is very important to check for &rhs != this, other wise if some one wrote a = a where a is a string object we would delete data_ and thus invalidate the object.. also note that I prefer to return a const referance to self from the copy assignment operator. This prevents code like (a = b) = c from compiling. hope that helps -bom C++Pro
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