MSVC 2010 templates compiler problem

Question

1>C:\Program Files (x86)\Microsoft Visual Studio 10.0\VC\include\list(1194): error C2451: conditional expression of type 'void' is illegal
1>          Expressions of type void cannot be converted to other types
1>          C:\Program Files (x86)\Microsoft Visual Studio 10.0\VC\include\list(1188) : while compiling class template member function 'void std::list<_Ty>::remove(const _Ty &)'
1>          with
1>          [
1>              _Ty=ServerLoginResponseCallback
1>          ]
1>          c:\users\shawn\edu\csclient\ConfigurationServerClient.h(56) : see reference to class template instantiation 'std::list<_Ty>' being compiled
1>          with
1>          [
1>              _Ty=ServerLoginResponseCallback
1>          ]

here is the code that generates the error...

typedef std::shared_ptr<protocols::ServerLoginResponse> ServerLoginResponsePtr;
typedef std::function<void (ServerLoginResponsePtr)> ServerLoginResponseCallback;
typedef std::list<ServerLoginResponseCallback> ServerLoginResponseCallbackList;

So we have a list of functors that return void and take an argument of type shared_ptr. Does anyone know why the MSVC compiler is having trouble?

Answer 1

It seems you have problems with instantiation. I've just tried to reproduce your bug but my MSVC compiled this code successfully.

Please, show us more code )) For example, show us how you use this list after creation.

Answer 2

while compiling class template member function 'void std::list<_Ty>::remove(const _Ty &)' with [ _Ty=ServerLoginResponseCallback ]

You are instantiating std::list<std::function<void (ServerLoginResponsePtr)>>, and trying to call erase on it, and that depends on calling operator== on two std::function objects, but std::functions are not comparable (only to nullptr):

§20.8.14.2 [func.wrap.func] (from the final draft n3092):

Member functions:

// deleted overloads close possible hole in the type system
template<class R2, class... ArgTypes2> 
bool operator==(const function<R2(ArgTypes2...)>&) = delete;

template<class R2, class... ArgTypes2> 
bool operator!=(const function<R2(ArgTypes2...)>&) = delete;

These are free functions:

template<class R, class... ArgTypes> 
bool operator==(const function<R(ArgTypes...)>&, nullptr_t);

template<class R, class... ArgTypes>
bool operator==(nullptr_t, const function<R(ArgTypes...)>&);