How to Achieve Elegant Implicit Type Conversion with Template Friend Functions?

Elegant Implicit Type Conversion with Template Friend Functions

When dealing with template classes, enabling implicit type conversion can be challenging. Consider the class template A with a constructor that takes an int and an operator overload for addition:

template <unsigned int m>
class A {
public:
    A(int) {}
};

template<unsigned int m>
A<m> operator+(const A<m>&, const A<m>&) {
    return A<m>(0);
}

Normally, calling the constructor with an int requires explicit casting. However, the following example raises compilation errors:

A<3> a(4);
A<3> b = a + 5;
A<3> c = 5 + a;

The Problem of Exact Type Matching

The issue lies in how overload resolution works for template functions. During type deduction, the compiler seeks exact matches for template parameters. In our case, it fails to match the int argument to the constructor because the types do not align exactly.

The Solution: Non-Member Friend Functions

One elegant solution is to define a non-member friend function inside the class definition:

template <typename T>
class test {
    friend test operator+(test const &, test const &) {
        return test();
    }
};

For each template instantiation, the compiler generates a separate non-template function at namespace level with the appropriate signature:

test<int> operator+(test<int> const &, test<int> const &) { 
   return test<int>();
}

Advantages of Non-Member Friend Functions

• Genericity: They provide non-template functions for all instantiated types, making them generically usable.
• Implicit Conversions: As non-template functions, they can leverage implicit conversions for both arguments, resolving the type mismatch issue.

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