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Taming Friends for Use in Templates-4 : Page 4


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Aren't Friends Electric?
We're reaching the last twist in this thick plot. Suppose you want to define an overloaded == function template that accepts different types of arguments:

// asymmetric, used in Vector<int> == Vector<double> etc.

template <typename T, typename T2> 
 bool operator== (const Vector<T> &a, 
                  const Vector<T2> &b);
Such an asymmetric overloaded operator probably reminds you the asymmetric assignment operator discussed here recently. Here again you have two distinct template parameters, T and T2. How do you declare such an asymmetric overloaded operator as a friend? Here's the answer:

template <typename T> class Vector {
 T val;
public:
 explicit Vector(T v = 0): val(0){}
 ~Vector(){}
 
  //asymmetric operator ==
 template <typename T2> 
  friend bool operator==(const Vector <T> &a, 
                         const Vector <T2> &b);
};

//definition of asymmetric operator==

template <typename T, typename T2> 
 bool operator== (const Vector<T> &a,                  
                  const Vector<T2> &b) 
{
 return a.val == b.val;
}


Vector<int> vi, vi2;
Vector<double> vd;

bool equal = 
 vi == vi2; // OK
equal = 
 vi == vd;// OK 



Danny Kalev is a certified system analyst and software engineer specializing in C++. He was a member of the C++ standards committee between 1997 and 2000 and has since been involved informally in the C++0x standardization process. He is the author of "The ANSI/ISO Professional C++ Programmer's Handbook" and "The Informit C++ Reference Guide: Techniques, Insight, and Practical Advice on C++."
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