When Should I Use Explicit Template Instantiation?

Explicit Template Instantiation: Its Applications

When delving into the intricacies of templates, one often encounters explicit template instantiation. Understanding its purpose can be a puzzling task. This article aims to address this by exploring a scenario where its usage is justified.

Imagine creating a template class, StringAdapter, designed to handle various data types. However, there are certain cases where you may not require support for all data types within the template. For instance, you might want StringAdapter to work only with characters.

To achieve this, you can employ explicit template instantiation. By defining the template class in a header file, you can separate the template declaration from its implementation in a source file. Subsequently, you can explicitly instantiate the required versions within the source file, thus dictating which specific data types will be supported by the template.

Consider the following code example:

StringAdapter.h

template<typename T>
class StringAdapter
{
    // Class definition...
};

StringAdapter.cpp

template<typename T>
StringAdapter<T>::StringAdapter(T* data)
{
    // Constructor...
}

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

By explicitly instantiating StringAdapter for characters, you restrict the template to work solely with this data type. This allows you to maintain a cleaner and more focused implementation while ensuring that only the necessary versions are generated during compilation.

In conclusion, explicit template instantiation provides a mechanism for selectively defining template classes, enabling you to tailor them to specific scenarios where only a subset of data types is required. This technique can help in optimizing code, reducing compilation time, and improving overall efficiency.

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