How to understand "most modern linkers will remove redundant code like template instantiations."

Question

From here:

Great answer. Additionally most modern linkers will remove redundant code like template instantiations.

• What does redundant template instantiations here mean?
• Aside of template instantiations, what else codes will be redundant when link-time?

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_{I will try to answer the first question on my own, if anywhere is wrong, please correct me. BTW, please forgive my poor English :-)}

Answer 1

• What does redundant template instantiations here mean?

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Assume we have a template <typename T> class A {}; which is declared in the a.hpp. We have two ways to instantiate an object in the a.cpp:

1. explicit instantiation

• Explicit instantiation definition: template class A <type>;
• Explicit instantiation declaration: extern template class A <type>;

2. implicit instantiation

• When code refers to a template in context that requires a completely defined type, or when the completeness of the type affects the code, and this particular type has not been explicitly instantiated, implicit instantiation occurs

When the TUs cannot find the explicit instantiation definition or declaration, tA<int> a; will invoke implicit instantiation.

_{difference between specialization and instantiate}

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Let's return the Question: What does redundant template instantiations here mean?

I will divide it into several cases within one a.cpp:

Case 1 ---- implicit instantiation

#include "a.h"
A<int> a;

=>

class A<int>
{

};

A<int> a;

Case 2 ---- implicit instantiation

#include "a..h"
A<int> a;
A<int> b;

=>

class A<int> 
{

};

A<int> a;
A<int> b;

Case 3 ---- Explicit instantiation definition:

#include "a.h"
template class A<int>;
A<int> a;

=>

#include "a.h"
template class A<int>;
A<int> a;

• _{Explicit instantiation declaration in the head file will behavior like Case3 ---- no redundant template instantiated.}

• _{member function have other rulers, take a look: https://en.cppreference.com/w/cpp/language/class_template}

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Conclusion:

• Implicit instantiation will produce redundant template instantiations while explicit instantiation definition/declaration won't.
• Linker will remove such redundant template instantiations, but it will increase link-time.

_{If you wonder how does the linker work to remove redundant template instantiations, please read this thread: How does the linker handle identical template instantiations across translation units?}