[cfe-dev] A question about template class forwarding definition

[Nikola Smiljanic]

You can't use a forward declaration in contexts where object's size must be
known. Note that your member function g is fine as B only appears in the
parameter list. It can also appear as return type. Declaring pointers is
also fine but the moment you try to construct an actual object the type
will have to be fully defined. I think this is a gcc bug.

On Tue, Sep 9, 2014 at 5:00 PM, Jiangning Liu <liujiangning1 at gmail.com>
wrote:

> Hi,
>
> I have a class forwarding definition issue. For the following small test
> case, gcc can build pass, but clang(trunk) will build fail.
>
> ====================================
>
> template<class T> class B;
>
> template<class T>
> class A
> {
>   public:
>     void f(void) const { B<double> e; g(e); }
>     void g(B<double> &e) const;
> };
>
> template<class T>
> class B
> {
> public:
>   T x[9];
>
>   B() {
>     for(int i=0;i<9;i++) x[i]=0;
>   }
> };
> $ g++ -std=c++03 -c tt.cc
> $ clang++ -std=c++03 -c tt.cc
> tt.cc:7:36: error: implicit instantiation of undefined template 'B<double>'
>     void f(void) const { B<double> e; g(e); }
>                                    ^
> tt.cc:1:25: note: template is declared here
> template<class T> class B;
>                         ^
> 1 error generated.
>
> ====================================
>
> Is this bug in clang?
>
> If not, how should I write the forwarding declaration for class B, and why
> gcc can pass?
>
> Thanks,
> -Jiangning
>
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>
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