Template method implementation without <>

Question

I have a template method in class:

template<class T>
    A& doSomething(T value)

Then I have an implementation

template<class T>
    A& A::doSomething(T value){
        // do something with value
        return *this;
    }

Then I have a specialization for lets say bool

template<>
    A& A::doSomething(bool value){
        // do something special with value of type bool
        return *this
    }

This is what I understand, now in the code there is something like this which I don't know what mean:

template A& A::doSomething(char const*);
template A& A::doSomething(char);
template A& A::doSomething(int);
template A& A::doSomething(int*);
template A& A::doSomething(double);
...

What is the exact meaning of those last 5 lines with template?

Thanks

score 5 · Accepted Answer · edited May 23 '17 at 12:31

5

It's Explicit instantiation, which forces the instantiation of the templates with the specified template arguments, and prevents their implicit instantiations.

An explicit instantiation definition forces instantiation of the function or member function they refer to. It may appear in the program anywhere after the template definition, and for a given argument-list, is only allowed to appear once in the program.

An explicit instantiation declaration (an extern template) prevents implicit instantiations: the code that would otherwise cause an implicit instantiation has to use the explicit instantiation definition provided somewhere else in the program.

And see Explicit instantiation - when is it used?

^{Because you tagged c++03, please note that extern template mentioned in the linked page was introduced from c++11.}

edited May 23 '17 at 12:31

Community

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answered Mar 03 '16 at 08:28

songyuanyao

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There is no standard `extern` template with c++03 – Garf365 Mar 03 '16 at 08:30
@Garf365 Added to the answer. – songyuanyao Mar 03 '16 at 08:35
Thanks, now i see. I think the use case for this is to avoid undefined reference when you link to a library which has its templates implemented in cpp files. Am I right? – Jan Mar 03 '16 at 08:45
@Jan Yes, that is indeed a use case for explicit instantiation. – Angew is no longer proud of SO Mar 03 '16 at 08:52
@Jan Yes, from the view of the library developer, you can restrict end user to use only the definitive versions. Use others will get a link error. – songyuanyao Mar 03 '16 at 08:54
@Jan See [Explicit instantiation - when is it used?](http://stackoverflow.com/q/2351148/3309790) for the use cases of explicit instantiation. – songyuanyao Mar 03 '16 at 08:59

score 2 · Answer 2 · edited Mar 03 '16 at 08:42

2

This lines aren't implementation. There only say to compiler to "create" code for doSomething for types char const*, char, int, int * and double

template A& A::doSomething(char const*);
template A& A::doSomething(char);
template A& A::doSomething(int);
template A& A::doSomething(int*);
template A& A::doSomething(double);

edited Mar 03 '16 at 08:42

Jarod42

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answered Mar 03 '16 at 08:25

Garf365

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Template method implementation without <>

2 Answers2