When Should You Use Explicit Template Instantiation in C ?

Explicit Template Instantiation: Understanding Its Applications

In the realm of C templates, explicit template instantiation provides a mechanism to explicitly define template class or function specializations, making them available for compilation without relying solely on implicit instantiation. While the feature itself is straightforward, its practical applications may not be immediately apparent.

When to Use Explicit Template Instantiation

There are several scenarios where explicit template instantiation proves useful:

1. Restricting Template Specialization: When a template class or function is intended to work only for specific data types, explicit instantiation allows you to limit the available specializations. By explicitly defining the template for the desired types, you can prevent other types from being instantiated accidentally.
2. Controlling Code Bloat: Template metaprogramming and generic programming can lead to excessive code generation. Explicit template instantiation enables you to explicitly control which specializations are instantiated, reducing the amount of code generated and improving compilation speed.
3. Optimizing Performance: In certain cases, explicitly instantiating a template for specific types can optimize performance. This is because the compiler can inline template functions for the explicitly instantiated types, improving execution speed.

Example: Limiting Template Specializations

Consider the following example:

template<typename T>
class StringAdapter {
public:
    StringAdapter(T* data);
    void doAdapterStuff();
private:
    std::basic_string<T> m_data;
};

Suppose you want to use this template class only for characters, not other types. You can explicitly instantiate the template for char and wchar_t types:

// StringAdapter.cpp
template class StringAdapter<char>;
template class StringAdapter<wchar_t>;

By explicitly instantiating the templates, you ensure that the template class is defined for these two types and prevent it from being instantiated for other types.

Conclusion

Explicit template instantiation is a valuable technique in C programming that allows you to control the behavior and performance of templates. By restricting specializations, controlling code bloat, and optimizing performance, explicit instantiation provides flexibility and efficiency in working with templates.

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