C++ templates were the first language feature to require more intelligence from the environment than was traditionally found on a UNIX system. Somehow the compiler and linker have to make sure that each template instance occurs exactly once in the executable if it is needed, and not at all otherwise. There are two basic approaches to this problem, which are referred to as the Borland model and the Cfront model.
G++ implements the Borland model on targets where the linker supports it, including ELF targets (such as GNU/Linux), Mac OS X and Microsoft Windows. Otherwise G++ implements neither automatic model.
You have the following options for dealing with template instantiations:
Duplicate instances of a template can be avoided by defining an explicit instantiation in one object file, and preventing the compiler from doing implicit instantiations in any other object files by using an explicit instantiation declaration, using the
extern template int max (int, int);
This syntax is defined in the C++ 2011 standard, but has been supported by G++ and other compilers since well before 2011.
Explicit instantiations can be used for the largest or most frequently duplicated instances, without having to know exactly which other instances are used in the rest of the program. You can scatter the explicit instantiations throughout your program, perhaps putting them in the translation units where the instances are used or the translation units that define the templates themselves; you can put all of the explicit instantiations you need into one big file; or you can create small files like
#include "Foo.h" #include "Foo.cc" template class Foo<int>; template ostream& operator << (ostream&, const Foo<int>&);
for each of the instances you need, and create a template instantiation library from those.
This is the simplest option, but also offers flexibility and fine-grained control when necessary. It is also the most portable alternative and programs using this approach will work with most modern compilers.
This can be a suitable option for application code written for the Borland model, as it usually just works. Code written for the Cfront model needs to be modified so that the template definitions are available at one or more points of instantiation; usually this is as simple as adding
to the end of each template header.
For library code, if you want the library to provide all of the template instantiations it needs, just try to link all of its object files together; the link will fail, but cause the instantiations to be generated as a side effect. Be warned, however, that this may cause conflicts if multiple libraries try to provide the same instantiations. For greater control, use explicit instantiation as described in the next option.
If you are using Cfront-model code, you can probably get away with not using -fno-implicit-templates when compiling files that don't ‘ #include ’ the member template definitions.
If you use one big file to do the instantiations, you may want to compile it without -fno-implicit-templates so you get all of the instances required by your explicit instantiations (but not by any other files) without having to specify them as well.
In addition to forward declaration of explicit instantiations (with
), G++ has extended the template instantiation syntax to support instantiation of the compiler support data for a template class (i.e. the vtable) without instantiating any of its members (with
), and instantiation of only the static data members of a template class, without the support data or member functions (with
inline template class Foo<int>; static template class Foo<int>;