Why Must C Template Class Implementations and Declarations Reside in the Same Header File?

The Interdependence of Template Class Implementation and Declaration in Header Files

In the realm of C , template classes possess a unique characteristic: the implementation and declaration must reside within the same header file. This requirement stems from the compiler's inability to generate code for each instantiation of a template without full access to its definition.

Consider the example:

// my_template.h

template <typename T>
class MyClass {
public:
    T value;
};

// my_template.cpp

#include "my_template.h"

template <typename T>
void MyClass<T>::foo(T& value) {
    this->value = value;
}

In this example, my_template.h contains both the declaration and implementation of the MyClass template. This is necessary because when you instantiate MyClass with a specific type, such as MyClass, the compiler needs to have access to both the signature and the implementation of the template to generate the correct instantiation code.

If the implementation were moved to a separate source file, the compiler would not be able to find it during the instantiation process. Consequently, the program would fail to compile.

Therefore, it is crucial to maintain the implementation and declaration of a template class within the same header file to ensure successful compilation and code generation for any future instantiations.

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