How Can `extern template` in C 11 Avoid Redundant Template Instantiations?

Avoiding Template Instantiation with extern template (C 11)

The extern template keyword in C 11 provides a mechanism to prevent unnecessary template instantiations, reducing compile time and object file size. In certain scenarios, it is beneficial to know that a template will be instantiated elsewhere and use this keyword to instruct the compiler not to perform the instantiation.

Usage for Function Templates

Contrary to the example provided in the question, the extern template keyword can also be used for function templates. Consider the following example:

// header.h
template<typename T>
void bigFunc();

// source1.cpp
#include "header.h"
void something1() {
    bigFunc<int>();
}

// source2.cpp
#include "header.h"
extern template void bigFunc<int>(); // This prevents its compilation in source2.cpp
void something2() {
    bigFunc<int>();
}

Without the extern template statement, bigFunc would be compiled in both source1.cpp and source2.cpp, resulting in redundant object code. By using extern template, the compiler is instructed not to instantiate bigFunc in source2.cpp, reducing compilation time and object file size.

Usage for Class Templates

As illustrated in Figure 2 of the question, extern template can also be used for class templates:

// header.h
template<typename T>
class myClass {
    T getValue();
};

// source1.cpp
#include "header.h"
extern template class myClass<int>; // Prevent instantiation here
void something1() {
    myClass<int> obj;
    obj.getValue();
}

In this scenario, the compiler is instructed not to instantiate myClass in source1.cpp, ensuring it is instantiated only once in the project.

Important Considerations

The use of extern template should be limited to situations where it is known that the template will be instantiated elsewhere. If it is not instantiated, the program will result in unresolved references. As a best practice, it is recommended to declare all template instantiations in a single header file, avoiding potential problems caused by multiple instantiations in different files.

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