Re: Non-inline template specialization of member function okay?
"Niels Dekker - no return address" wrote:
Is it okay to put the implementation of a template
specialization of a member function in a CPP file, hoping to
have it externally linked? I tried to build the following
program, trying both VC++ 2003 and VC++ 2008, but it got me a
linker error.
[...]
VC++ 2008 got me the following: "error LNK2019: unresolved
external symbol "public: int __thiscall
Factory::Create<int>(void)const " (??$Create@H@Factory@@QBEHXZ)
referenced in function _main"
Interestingly the program *does* compile (and link), when the
class is declared as "__declspec(dllexport)". Is it really
necessary to use __declspec, in this case?
It seems that VC++ misinterprets the standard.
<quote>
14.7.3/2 "Explicit specialization":
An explicit specialization of a member function, member class or
static data member of a class template shall be declared in the
namespace of which the class template is a member. Such a
declaration may also be a definition. If the declaration is not a
definition, the specialization may be defined later in the
namespace in which the explicit specialization was declared, or in
a namespace that encloses the one in which the explicit
specialization was declared.
</quote>
So, definition of specialized member function in other translation
unit should work, since the definition belongs to enclosing
namespace (global namespace).
However, compiler erroneously treats member function
specialization in the same manner as if it were class template
specialization: i.e., if a definition of specialized template is
not included in a translation unit where it used, then no code is
generated.
In your example "Factory.obj" file won't contain any code at all.
By adding "__declspec(dllexport)" you force the compiler to
generate function's code by telling it that the function can be
used elsewhere.
I think it's a bug. In order to workarount it you can force a code
generation by referencing `Create<int>' member in "Factory.cpp":
// not used, just to trick the compiler
void ugly()
{
Factory().Create<int>();
}
Then the program links and runs successfully.
HTH
Alex