使用g ++编译时静态函数的模板特化

Question

Why does the following code not work when compiling with:

$ g++ temp_main.cpp temp_spec.cpp
/tmp/ccirjc3Y.o:temp_spec.cpp:(.text+0x100): multiple definition of `my::say()'
/tmp/ccSo7IVO.o:temp_main.cpp:(.text$_ZN2myILi0EE3sayEv[my::say()]+0x0):
first defined here collect2: ld returned 1 exit status

I'm trying to have specialization of static function only when using paramteter 0.

temp_gen.h:

#ifndef temp_gen_h
#define temp_gen_h

#include <iostream>

template<int N>
struct my
{
   static void say();
};

template<int N>
void my<N>::say()
{
   std::cout << "generic " << N << std::endl;
}

#endif

temp_spec.h:

#ifndef temp_spec_h
#define temp_spec_h

#include <iostream>

#include "temp_gen.h"

template<>
void my<0>::say()
{
   std::cout << "specialized " << 0 << std::endl;
}

#endif

temp_spec.cpp:

#include "temp_spec.h"

temp_main.cpp:

#include "temp_gen.h"

int main(int argc, char* argv[])
{
   my<0>::say();  //should say "specialized 0"
   my<1>::say();  //should say "generic 0"
}

Answer 1

我相信，由于您的模板特化的实现位于标题中，因此您需要将其标记为inline以便编译器/链接器知道您可以违反一个定义规则。

Answer 2

From within your main.cpp , you did not specialize the template class, this is because you don't include the temp_spec.h .

As Vaughn Cato pointed out (see comments), you should move the definition of the specialized method (which is no template method anymore), to temp_spec.cpp .

I think (but I'm not an expert at this) you should always put specialization directly below the generic template (in the same header file), because whenever you include this, you also want the specialized ones to be defined, otherwise you will be confused when such errors happen. You can, however, just include the temp_spec.h at the bottom of temp_gen.h .

Also, you don't need the .cpp files for the template headers, as there will never be any code to be compiled separately (template classes are always compiled at the other .cpp file which uses it, and I think duplicates will be removed when linking, causing trouble at link time again when in one case you did not specialize it, one time you did ) [This paragraph only applies to the generic template class, not to (fully) specialized classes, as they require some code in their own compilation unit, see comments and the edit above.]

Answer 3

Using the n3337 version of the Standard (emphasis mine):

14.7.3 Explicit specialization [temp.expl.spec]

6/ If a template, a member template or a member of a class template is explicitly specialized then that specialization shall be declared before the first use of that specialization that would cause an implicit instantiation to take place, in every translation unit in which such a use occurs; no diagnostic is required.

Because in main the specialization of my<0>::say is not visible, implicit instantiation occurs and you end up in the case above: no diagnostic required (from the compiler).

Note that a specialization may only be declared after its generic counterpart has been declared.