When is Explicit Template Instantiation Beneficial in C ?

Explicit Template Instantiation: When is it Beneficial?

In the realm of C template programming, explicit template instantiation stands as a potent tool, albeit one that may seem enigmatic at first glance. This article aims to dispel any confusion and unveil the scenarios where explicit template instantiation proves its worth.

Explicit template instantiation involves explicitly specifying which instantiations of a template class should be compiled. This differs from implicit instantiation, where the compiler automatically instantiates templates as needed during compilation.

One compelling use case for explicit template instantiation is when a template class needs to be restricted to a limited set of types. Consider the following example:

template<typename T>
class StringAdapter {
    // ...
};
typedef StringAdapter<char> StrAdapter;
typedef StringAdapter<wchar_t> WStrAdapter;

Here, we have defined a template class StringAdapter that can adapt strings of various types. To support both character and wide character strings, we create type aliases StrAdapter and WStrAdapter respectively.

However, we only want the template class to work with characters. By explicitly instantiating only the StringAdapter and StringAdapter templates, we ensure that the compiler compiles only those specific instantiations.

// Explicitly instantiate only the desired instantiations
template class StringAdapter<char>;
template class StringAdapter<wchar_t>;

This explicit instantiation prevents the potential instantiation of StringAdapter for unintended types, enhancing code correctness and preventing unexpected behavior. It also offers performance benefits by reducing compile times and code bloat.

Another advantage of explicit template instantiation is when dealing with classes with member templates. In such cases, explicit instantiation can help ensure that the correct member templates are compiled for the desired types.

Overall, explicit template instantiation empowers developers to control template instantiation, restrict template functionality to specific types, and optimize compilation and execution. By understanding its benefits, programmers can harness this powerful feature to enhance their template-based designs.

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