How Can Friend Operator Functions Enable Implicit Type Conversions in C Templates?

Implicit Type Conversion in Templates with Friend Operator Functions

To utilize implicit type conversion with templates, a technique frequently employed is the definition of a non-member friend function within the template class's definition.

In C , template functions are not instantiated until they are invoked with specific types. During overload resolution, the compiler deduces the types of the template parameters and eliminates any templates that don't exactly match the argument types, excluding implicit conversions.

The solution lies in utilizing a friend operator function defined inside the template class definition. For each template instantiation, the compiler generates a free non-template function with a signature matching the substituted types.

For instance, consider:

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

When the template is instantiated with int, the compiler creates a non-template function:

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

This function, available through argument-dependent lookup, performs implicit conversions on its arguments. This is because it is defined outside the template class and does not require exact type matches during overload resolution.

Thus, implicit type conversions become possible when calling the operator on objects of type test, allowing for expressions like:

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

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