Why Should Template Class Implementations and Declarations Be in the Same Header File?

Benefits of Co-Locating Template Class Implementation and Declaration in Header Files

When working with template classes, it's recommended to define both the implementation and declaration in the same header file for the following reasons:

Compiler Visibility:

The compiler requires the complete template definition, not just its signature, to generate code for each template instantiation. Defining the function implementations within the header ensures that the compiler has access to the necessary information.

Example:

Consider the following template class:

template <typename T>
class MyClass {
public:
    void doSomething(T value);
};

If we were to define the implementation of doSomething outside the header:

#include "MyClass.h"
template <typename T>
void MyClass<T>::doSomething(T value) {
    // Implementation
}

The compiler would not be able to find the definition when it tries to instantiate MyClass, resulting in an error.

The Inclusion Model:

The C standard defines the "Inclusion Model," which dictates how template definitions are instantiated. When a template class is instantiated, the compiler needs to include the template definition from a header file. Co-locating the implementation within the header guarantees that the compiler has access to the complete definition during instantiation.

Avoid Symbol Collision:

Placing the implementation in a header helps prevent symbol collision issues when multiple translation units attempt to define the same template class. Defining the template in a header ensures that it's defined consistently across all units.

Modularization and Reusability:

By keeping the implementation and declaration together, you promote modularization and reusability. Other developers can easily include the header file to access the complete definition of the template class without having to search for its implementation separately.

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