Strange Error with Partial and explicit template specialization

Question

The following gives an "Internal Compiler Error" on MSVC++ 10 Sp1.

And on gcc:

b.cpp:16:12: error: explicit specialization in non-namespace scope ‘struct A::B’

b.cpp:16:28: error: template-id ‘f’ in declaration of primary template

//class template
template< class T>
struct A{

    //struct B  {};  //Remove the comment and it will compile!
};

//partial specialization
template<  class T >
struct A< T* >
{
    struct B  {

        template<class C>   void f(){}

            //"Internal Compiler Error"
        template<>          void f<int>(){};

    };
};

However, if the comments before struct B is removed it will compile!

I don't understand the problem!

First, it is not supposed to compile. And you are supposed to file a bug report. There is no question here, and we can't explain bugs in code we don't have the source of. — Alexandre C., Nov 08 '11 at 21:34
possible duplicate of [C++ syntax for explicit specialization of a template function in a template class?](http://stackoverflow.com/questions/2097811/c-syntax-for-explicit-specialization-of-a-template-function-in-a-template-clas) — Ben Voigt, Nov 08 '11 at 21:52
@Alexandre C. I will file a bug, if I know that it is a bug. But with C++ it can get quite tricky sometimes — Johannes Gerer, Nov 08 '11 at 21:54

Marcelo Cantos · Accepted Answer · 2011-11-08T21:55:52.833

2

You have a bug in your code and MSVC++ isn't coping with it. The gcc compile produces this:

$ make parspec.o
g++    -c -o parspec.o parspec.cc
parspec.cc:17: error: explicit specialization in non-namespace scope ‘struct A<T*>::B’
make: *** [parspec.o] Error 1

In short, you can't specialise inside a class or struct.

EDIT: A quick Google around suggests that MSVC++ allows such non-conforming constructs, but I guess they didn't do a very good job of it.

edited Nov 08 '11 at 21:55

answered Nov 08 '11 at 21:40

Marcelo Cantos

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But what about the fact, that it compiles, if the indicated comment is removed? – Johannes Gerer Nov 08 '11 at 21:45
@user578832: That's a bug in GCC, it should not compile. – Mooing Duck Nov 08 '11 at 21:48
@user578832: Perhaps my memory is failing me, since I haven't looked at this stuff for a long time, but the last time I checked, I'm pretty sure the standard didn't allow it. – Marcelo Cantos Nov 08 '11 at 21:48
@MooingDuck: Did you mean to say that's a bug in *MSVC++*? – Marcelo Cantos Nov 08 '11 at 21:50
@MarceloCantos: to be clearer, there's a bug in both that allows it to compile, and a bug in MSVC that sometimes causes the compiler to crash. – Mooing Duck Nov 08 '11 at 21:52
@user578832: I can't speak for MSVC++. Clearly something is broken in the relevant portion of the compiler. – Marcelo Cantos Nov 08 '11 at 21:53
@MooingDuck: I don't understand; gcc *doesn't* allow it to compile. – Marcelo Cantos Nov 08 '11 at 21:53
@MarceloCantos: I interpreted the OP as saying both compilers compiled the code when he uncommented the lines. That might be incorrect. – Mooing Duck Nov 08 '11 at 22:00
@MooingDuck: Ah, that makes sense now. I didn't notice the OP had added the gcc output to the question. – Marcelo Cantos Nov 08 '11 at 22:05

score 0 · Answer 2 · answered Nov 08 '11 at 21:48

You cannot specialize a member function for multiple instances of a class template. This is allowed:

template<typename T>
struct A{
};

template<typename T>
struct A<T*>
{
    struct B  {
        template<class C>   void f();
    };
};

template<typename T>
template<typename C>
void A<T*>::B::f() {}

template<>
template<>
void A<char*>::B::f<int>() {}

But this is not:

template<typename T>
template<>
void A<T*>::B::f<int>() {}

Strange Error with Partial and explicit template specialization

2 Answers2