When and Why Use Explicit Template Instantiation in C ?

Explicit Template Instantiation: When and Why

After a brief hiatus, let's delve into the realm of templates with "Templates – The Complete Guide." This discussion focuses on the concept of explicit template instantiation, a mechanism that sparks curiosity about its practical applications.

Explicit instantiation involves explicitly defining a specific instantiation of a template class, ensuring its availability without complete compilation of the template for every instantiation. This is particularly useful when:

1. Restricting Template Use:

• Example: Consider a template function that performs a specific operation. By explicitly instantiating it for a particular set of types, you can limit its usage to those types only, enhancing code optimization.

2. Limiting Template Expansion:

• Example: When dealing with templates that generate a significant amount of code, explicitly instantiating only the necessary specializations can save compilation time and reduce executable size.

3. Enhancing Linker Optimization:

• Example: If a template class is declared in a header file, the linker may not be able to determine which instantiations are actually used. Explicit instantiation informs the linker about the specific instantiations, allowing it to perform better optimizations.

4. Supporting Different Implementations for Different Types:

• Example: You can create multiple explicit instantiations of a template class, each implementing the template differently for different data types, providing tailored solutions.

Process:

To explicitly instantiate a template, you must:

1. Declare the template in a header file.
2. Provide a definition for the template in a source file.
3. At the end of the source file, explicitly instantiate the desired template specializations using the template class syntax.

Example:

Consider the following template class:

template<typename T>
class MyTemplate {
public:
    // ...
};

To explicitly instantiate a specialization for int, you would add the following line to the source file:

template class MyTemplate<int>;

Conclusion:

Explicit template instantiation is a powerful mechanism that allows for fine-tuning template usage, optimizing code, and supporting flexible implementations. By understanding its applications, you can leverage this feature to enhance the efficiency and versatility of your C codebase.

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