How Can Implicit Type Conversion Be Enabled in Template Classes?

Implicit Type Conversion with Templates

The query explores the issue of enabling implicit type conversion in template classes. Consider the case of a template class A with a constructor that accepts an integer.

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

Furthermore, there's an operator ' ' that returns an instance of A given two A objects.

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

The problem arises when attempting to implicitly convert an integer to an A object. For instance, the following code attempts to do so, but the compiler throws an error:

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

Solution

The solution lies in exploiting a feature of the language that allows the definition of non-member friend functions inside a class definition. In the case of templates, for each instantiation of the template, the compiler generates a free non-template function with a signature obtained by substituting the real types of the instantiation in the friend declaration:

template <typename T>
class test {
    friend test operator+(test const &, test const &); // [1]
};
test<int> t; // [2]

In [1], the compiler allows the definition of the friend function inside the class scope. Then, in [2], when the template is instantiated, the compiler generates a free function:

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

This non-template function is always defined, regardless of whether it's used or not.

Magic of Implicit Conversion

The "magic" here lies in the following aspects:

• Generic Definition: The non-template function is generically defined for each instantiated type, providing both genericity and the ability to use it when arguments are not perfect matches.
• Implicit Conversions: Because it's a non-template function, the compiler can call implicit conversions on both arguments, enabling the expected behavior.
• Argument Dependent Lookup: The function can only be found by argument dependent lookup, which implies that it's only considered when at least one of the arguments is of the desired type.

However, this solution also has some limitations:

• It limits the visibility of the function since it's only accessible through ADL.
• It prevents obtaining a function pointer to it.

Despite these limitations, this solution provides an elegant way to enable implicit conversion within template classes, allowing for more flexible and convenient code.

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