using extern template C 11

Question

Figure 1  function templates  TemplHeader h  template lt typename T gt  void f      TemplCpp cpp  template lt typename T gt  void f                     explicit instantation template void f lt T gt       Main cpp   include  TemplHeader h  extern template void f lt T gt       is this correct  int main         f lt char gt         return 0      Is this the correct way to use extern template  or do I use this keyword only for class templates as in Figure 2   Figure 2  class templates  TemplHeader h  template lt typename T gt  class foo       T f         TemplCpp cpp  template lt typename T gt  void foo lt T gt   f                   explicit instantation template class foo lt int gt     Main cpp   include  TemplHeader h  extern template class foo lt int gt     int main         foo lt int gt  test      return 0      I know it is good to put all of this in one header file  but if we instantiate templates with the same parameters in multiple files  then we got multiple same definitions and the compiler will remove them all  except one  to avoid errors  How do I use extern template  Can we use it only for classes  or can we use it for functions too   Also  Figure 1 and Figure 2 may be expanded to a solution where templates are in a single header file   In that case  we need to use the extern template keyword to avoid multiple same instantations  Is this only for classes or functions too

User · Answer

The known problem with the templates is code bloating, which is consequence of generating the class definition in each and every module which invokes the class template specialization. To prevent this, starting with C++0x, one could use the keyword extern in front of the class template specialization

#include <MyClass>
extern template class CMyClass<int>;

The explicit instantion of the template class should happen only in a single translation unit, preferable the one with template definition (MyClass.cpp)

template class CMyClass<int>;
template class CMyClass<float>;

User · Answer

If  you have used extern for functions before  exactly same philosophy is followed for templates  if not  going though extern for simple functions may help  Also  you may want to put the extern s  in header file and include the header when you need it

User · Answer

You should only use extern template to force the compiler to not instantiate a template when you know that it will be instantiated somewhere else   It is used to reduce compile time and object file size   For example      header h  template lt typename T gt  void ReallyBigFunction            Body       source1 cpp   include  header h  void something1         ReallyBigFunction lt int gt           source2 cpp   include  header h  void something2         ReallyBigFunction lt int gt         This will result in the following object files   source1 o     void something1       void ReallyBigFunction lt int gt          Compiled first time  source2 o     void something2       void ReallyBigFunction lt int gt          Compiled second time   If both files are linked together  one void ReallyBigFunction lt int gt    will be discarded  resulting in wasted compile time and object file size   To not waste compile time and object file size  there is an extern keyword which makes the compiler not compile a template function   You should use this if and only if you know it is used in the same binary somewhere else   Changing source2 cpp to      source2 cpp   include  header h  extern template void ReallyBigFunction lt int gt     void something2         ReallyBigFunction lt int gt         Will result in the following object files   source1 o     void something1       void ReallyBigFunction lt int gt       compiled just one time  source2 o     void something2          No ReallyBigFunction lt int gt  here because of the extern   When both of these will be linked together  the second object file will just use the symbol from the first object file  No need for discard and no wasted compile time and object file size   This should only be used within a project  like in times when you use a template like vector lt int gt  multiple times  you should use extern in all but one source file   This also applies to classes and function as one  and even template member functions

User · Answer

extern template is only needed if the template declaration is complete  This was hinted at in other answers  but I don t think enough emphasis was given to it   What this means is that in the OPs examples  the extern template has no effect because the template definitions on the headers were incomplete    void f     just declaration  no body class foo  declares method f   but has no definition   So I would recommend just removing the extern template definition in that particular case  you only need to add them if the classes are completely defined   For example   TemplHeader h  template lt typename T gt  void f      TemplCpp cpp  template lt typename T gt  void f         Explicit instantiation for char  template void f lt char gt       Main cpp   include  TemplHeader h      Commented out from OP code  has no effect     extern template void f lt T gt       is this correct   int main         f lt char gt         return 0      compile and view symbols with nm   g   -std c  11 -Wall -Wextra -pedantic -c -o TemplCpp o TemplCpp cpp g   -std c  11 -Wall -Wextra -pedantic -c -o Main o Main cpp g   -std c  11 -Wall -Wextra -pedantic -o Main out Main o TemplCpp o echo TemplCpp o nm -C TemplCpp o   grep f echo Main o nm -C Main o   grep f     output   TemplCpp o 0000000000000000 W void f lt char gt    Main o                  U void f lt char gt      and then from man nm we see that U means undefined  so the definition did stay only on TemplCpp as desired   All this boils down to the tradeoff of complete header declarations    upsides    allows external code to use our template with new types we have the option of not adding explicit instantiations if we are fine with object bloat  downsides    when developing that class  header implementation changes will lead smart build systems to rebuild all includers  which could be many many files if we want to avoid object file bloat  we need not only to do explicit instantiations  same as with incomplete header declarations  but also add extern template on every includer  which programmers will likely forget to do    Further examples of those are shown at  Explicit template instantiation - when is it used   Since compilation time is so critical in large projects  I would highly recommend incomplete template declarations  unless external parties absolutely need to reuse your code with their own complex custom classes   And in that case  I would first try to use polymorphism to avoid the build time problem  and only use templates if noticeable performance gains can be made   Tested in Ubuntu 18 04

User · Answer

Wikipedia has the best description     In C  03  the compiler must instantiate a template whenever a fully specified template is   encountered in a translation unit  If the template is instantiated with the same types in   many translation units  this can dramatically increase compile times  There is no way to   prevent this in C  03  so C  11 introduced extern template declarations  analogous to extern   data declarations       C  03 has this syntax to oblige the compiler to instantiate a template     template class std  vector lt MyClass gt         C  11 now provides this syntax     extern template class std  vector lt MyClass gt         which tells the compiler not to instantiate the template in this translation unit    The warning  nonstandard extension used     Microsoft VC   used to have a non-standard version of this feature for some years already  in C  03   The compiler warns about that to prevent portability issues with code that needed to compile on different compilers as well   Look at the sample in the linked page to see that it works roughly the same way  You can expect the message to go away with future versions of MSVC  except of course when using other non-standard compiler extensions at the same time

[c++] using extern template (C++11)

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