extern templates

This is a discussion on extern templates within the C++ Programming forums, part of the General Programming Boards category; I dont understand how I can improve compilation time with 'extern template' which is introduced in C++0x. I can just ...

  1. #1
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    extern templates

    I dont understand how I can improve compilation time with 'extern template' which is introduced in C++0x.

    I can just put extern before a template object in a header anbd then use it as normal in the source file? like this:

    Code:
    #include <vector>
    
    // bla.h
    struct bla
    {
      void f();
      extern template std::vector<int> vec;
    };
    
    //bla.cpp
    void bla::f()
    {
      vec.push_back(3);
    }

  2. #2
    Master Apprentice phantomotap's Avatar
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    Basically.

    The idea is to tell the compiler "Do not fully instantiate this template in this translation unit; it will be instantiated elsewhere.". (There remains some confusion about this when it comes against `inline' methods.) It works almost like "`extern' variable" declarations do for variables.

    You will need to force a full instantiation in a separate translation unit.

    Soma

  3. #3
    the hat of redundancy hat nvoigt's Avatar
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    I put "extern template" into Google and the very first link gave a detailed explanation. Try it, it really works
    hth
    -nv

    She was so Blonde, she spent 20 minutes looking at the orange juice can because it said "Concentrate."

    When in doubt, read the FAQ.
    Then ask a smart question.

  4. #4
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    duh ofcourse i did that but I still dont understand. Take the IBM example.

    Code:
    template<class C> C foo(C c) { return c; }
    
    extern template int foo<int>(int);  // extern explicit template instantiation
    
    int i = foo(1);  // does not cause instantiation of the body of foo<int>
    ok I get this, the template function is not instantiated here.
    But if you dont instantiate it somewhere dont you get a linker error then? Or what do you have to do to get it to compile and link? nothing?

  5. #5
    the hat of redundancy hat nvoigt's Avatar
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    The short version is, you need to instantiate it somewhere, but not everywhere in your code, so you don't need the linker to sort out all the duplicate instantiations in different source files later.

    This is a direct quote from the first hit in my Google search:

    When the compiler encounters an extern template directive, it suppresses the generation of code for that particular specialization. However, an explicit instantiation of the entity must appear in another translation unit, or later in the same translation unit. An explicit instantiation definition of a given entity shall appear at most once in the program.
    hth
    -nv

    She was so Blonde, she spent 20 minutes looking at the orange juice can because it said "Concentrate."

    When in doubt, read the FAQ.
    Then ask a smart question.

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