How does the C++ keyword export help with template linker errors?, C++ FAQ

C++ FAQ / Section 35 / FAQ 35.14

Section 35:

35.1	What's the idea behind templates?
35.2	What's the syntax / semantics for a "class template"?
35.3	What's the syntax / semantics for a "function template"?
35.4	How do I explicitly select which version of a function template should get called?
35.5	What is a "parameterized type"?
35.6	What is "genericity"?
35.7	My template function does something special when the template type `T` is `int` or `std::string`; how do I write my template so it uses the special code when `T` is one of those specific types?
35.8	Huh? Can you provide an example of template specialization that doesn't use `foo` and `bar`?
35.9	But most of the code in my template function is the same; is there some way to get the benefits of template specialization without duplicating all that source code?
35.10	All those templates and template specializations must slow down my program, right?
35.11	So templates are overloading, right?
35.12	Why can't I separate the definition of my templates class from its declaration and put it inside a .cpp file?
35.13	How can I avoid linker errors with my template functions? Updated!
35.14	How does the C++ keyword `export` help with template linker errors? Updated!
35.15	How can I avoid linker errors with my template classes? Updated!
35.16	Why do I get linker errors when I use template friends?
35.17	How can any human hope to understand these overly verbose template-based error messages?
35.18	Why am I getting errors when my template-derived-class uses a nested type it inherits from its template-base-class?
35.19	Why am I getting errors when my template-derived-class uses a member it inherits from its template-base-class?
35.20	Can the previous problem hurt me silently? Is it possible that the compiler will silently generate the wrong code?
35.21	How can I create a container-template that allows my users to supply the type of the underlying container that actually stores the values?
35.22	Follow-up to previous: can I pass in the underlying structure and the element-type separately?
35.23	Related: all those proxies must negatively reflect on the speed of my program. Don't they?

[35.14] How does the C++ keyword export help with template linker errors? Updated!

[Recently updated due to C++11 syntax. Click here to go to the next FAQ in the "chain" of recent changes]

This answer will be updated due to C++11 extern template. Watch this space for updates in the near future!!

The C++ keyword export was originally designed to eliminate the need to include a template definition (either by providing the definition in the header file or by including the implementation file). However, only a few compilers ever supported this capability, such as Comeau C++ and Sun Studio, and the general concensus was that it was not worth the trouble.

Because of that, the C++11 standardization committee has voted to remove the export feature from the language. Assuming this meets final approval, export will remain a reserved word but it will no longer have any meaning in the standard.

If you are working with a compiler that supports the export keyword, it will probably continue to support the keyword via some sort of compiler option or extension until its users migrate away from it. If you already have code that uses export, you can use a fairly simple discipline to allow your code to easily migrate if/when your compiler stops supporting it entirely. Just define your template header-files like this:

// File Foo.h

#ifdef USE_EXPORT_KEYWORD
export
#endif
template<typename T>
class Foo {
  ...
};

#ifndef USE_EXPORT_KEYWORD
  #include "Foo.cpp"
#endif

And define your non-inline functions in a source-file like this:

// File Foo.cpp

#ifdef USE_EXPORT_KEYWORD
export
#endif
template<typename T> ...

Then compile with -DUSE_EXPORT_KEYWORD, or whatever equivalent compiler option lets you set a preprocessor symbol like USE_COMPILER_KEYWORD, and if/when your compiler removes support for export, just remove that compiler option.