How Can `extern template` in C 11 Prevent Duplicate Template Instantiations and Reduce Compilation Time?

Using extern template to Avoid Instantiation in C 11

extern template is a powerful C 11 keyword that allows developers to prevent the instantiation of a template in a specific compilation unit. This can be particularly useful when working with large codebases where multiple source files may instantiate the same template with different parameters, leading to duplicate code and increased compilation time.

Function Templates

In the case of function templates, extern template can be used to force the compiler not to instantiate a particular template when it knows that the function will be instantiated elsewhere. Consider the following example:

// header.h
template<typename T>
void ReallyBigFunction() {
    // Body
}

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

// source2.cpp
#include "header.h"
extern template void ReallyBigFunction<int>();
void something2() {
    ReallyBigFunction<int>();
}

Without extern template, the compiler would compile ReallyBigFunction in both source1.cpp and source2.cpp, resulting in the following object files:

source1.o
    void something1()
    void ReallyBigFunction<int>()    // Compiled first time

source2.o
    void something2()
    void ReallyBigFunction<int>()    // Compiled second time

Linking these files together would result in one copy of ReallyBigFunction being discarded, wasting compile time and object file size.

To avoid this issue, we can use extern template in source2.cpp:

// source2.cpp
#include "header.h"
extern template void ReallyBigFunction<int>();
void something2() {
    ReallyBigFunction<int>();
}

This will result in the following object files:

source1.o
    void something1()
    void ReallyBigFunction<int>() // Compiled just one time

source2.o
    void something2()
    // No ReallyBigFunction<int> here because of the extern

When these object files are linked, the second object file will use the symbol from the first object file, avoiding duplicate code and reducing compilation time.

Class Templates

extern template can also be used with class templates to prevent the instantiation of specific class members. For example:

// header.h
template<typename T>
class Foo {
public:
    T f();
};

// source1.cpp
#include "header.h"
Foo<int> foo1;

// source2.cpp
#include "header.h"
extern template class Foo<int>;
Foo<int> foo2;

Without extern template, the compiler would compile the member function f for Foo in both source1.cpp and source2.cpp, resulting in duplicate code. Using extern template ensures that the member function is compiled only once.

Conclusion

extern template is a valuable tool for avoiding duplicate code and reducing compilation time in C 11 projects. By selectively using extern template to prevent unnecessary template instantiations, developers can optimize the performance of their code.

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