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Taming Friends for Use in Templates-4 : Page 4




Building the Right Environment to Support AI, Machine Learning and Deep Learning

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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