You can only apply enable_if to template arguments of the function and not of the class which contains the function, but you can't declare a virtual function template... So this is your dilemma. Regards, Ovanes On Tue, Aug 18, 2009 at 7:23 PM, topoden <_dvt_@mail.ru> wrote:

Hello,

I'm trying to use boost::enable_if to customize function declaration for a particular set of template arguments. But instead of using SFINAE to remove incorrect declaration it produces error. See the example below to see better what I mean:

#include <boost/utility.hpp> #include <boost/type_traits.hpp>

template<class A, class B> struct BaseListener { typename boost::disable_if<boost::is_void< B >, void>::type void virtual process(A *, B *) = 0;

typename boost::enable_if<boost::is_void< B >, void>::type void virtual process(A *) = 0; };

struct Request { };

struct Response { };

typedef BaseListener<Response, void> BaseResponseListener; typedef BaseListener<Request, BaseResponseListener> BaseRequestListener;

struct RequestListener : public BaseRequestListener { void process(Request *, BaseResponseListener *) { } };

struct ResponseListener : public BaseResponseListener { void process(Request *) { } };

int main(int argc, char * argv[]) { RequestListener reqListener; ResponseListener resListener;

return 0; }

error message: error C2039: 'type' : is not a member of 'boost::enable_if<Cond,T>' 1> with 1> [ 1> Cond=boost::is_void<BaseResponseListener>, 1> T=void 1> ] 1> c:\!projects\temp\consoleeventhandler\consoleeventhandler.cpp(134) : see reference to class template instantiation 'BaseListener<A,B>' being compiled 1> with 1> [ 1> A=Request, 1> B=BaseResponseListener 1> ] -- View this message in context: http://www.nabble.com/boost%3A%3Aenable_if%2C-did-I-get-it-wrong---tp2502791... Sent from the Boost - Users mailing list archive at Nabble.com.

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-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 On Tuesday 18 August 2009, topoden wrote:

Hello,

I'm trying to use boost::enable_if to customize function declaration for a particular set of template arguments. But instead of using SFINAE to remove incorrect declaration it produces error.

What you are doing is an error (and not just the extra void you have in the return types). SFINAE drops template functions from consideration during overload resolution. You would need to make the process member functions templates (but you can't because there are no virtual template methods in C++). It looks like what you are trying to do is provide a partial template specialization of BaseListener when the second template parameter is void.

template<class A, class B> struct BaseListener { typename boost::disable_if<boost::is_void< B >, void>::type void virtual process(A *, B *) = 0;

typename boost::enable_if<boost::is_void< B >, void>::type void virtual process(A *) = 0; }; -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.9 (GNU/Linux)

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