当试图用另一个类模板部分专门化一个类模板时，在VS2013中出现错误

Question

给定以下类和函数模板:

template <typename WrappedType, ParameterType ParamType, bool IsOutputParameter>
    class WrappedParameter; // Definition left out for brevity
template <typename T>
struct ParameterUnwrapper
{
    static T UnwrapParameter(const T& in_param)
    {
        return in_param;
    }
};
template <typename T, ParameterType ParamType, bool IsOutputParameter>
struct ParameterUnwrapper<WrappedParameter<T, ParamType, IsOutputParameter>>
{
    static T UnwrapParameter(const WrappedParameter<T, ParamType, IsOutputParameter>& in_param)
    {
        return in_param.GetWrapped();
    }
};
template <typename T>
T UnwrapParameter(T in_param)
{
    return Impl::ParameterUnwrapper<T>::UnwrapParameter(in_param);
}
template <typename T>
Impl::WrappedParameter<T, Impl::POINTER_PARAMETER, true> WrapOutputPointerParameter(T in_param)
{
    return Impl::WrappedParameter<T, Impl::POINTER_PARAMETER, true>(in_param);
}
template <typename MemFunc, typename ...Args>
HRESULT ExecuteAndLog(
    MemFunc in_memberFunction,
    const std::string& in_methodName,
    Args... args) //-> decltype((m_wrapped->*in_memberFunction)(UnwrapParameter(args)...))
{
    return ExecuteFunctorAndLog(
        [&]() { return (m_wrapped->*in_memberFunction)(UnwrapParameter(args)...); },
        in_methodName,
        args...);
}

下面的调用:(The ExecuteAndLog)

HRESULT STDMETHODCALLTYPE AccessorWrapper::AddRefAccessor(
    HACCESSOR hAccessor,
    DBREFCOUNT *pcRefCount)
{
    return ExecuteAndLog(
        &IAccessor::AddRefAccessor,
        "AddRefAccessor",
        hAccessor,
        WrapOutputPointerParameter(pcRefCount));
}

给出错误:

error C2664: 'HRESULT (HACCESSOR,DBREFCOUNT *)' : cannot convert argument 2 from 'Impl::WrappedParameter<DBREFCOUNT *,POINTER_PARAMETER,true>' to 'DBREFCOUNT *'
No user-defined-conversion operator available that can perform this conversion, or the operator cannot be called
see reference to function template instantiation 'ExecuteAndLog<HRESULT(__stdcall IAccessor::* )(HACCESSOR,DBREFCOUNT *),HACCESSOR, Impl::WrappedParameter<DBREFCOUNT *,POINTER_PARAMETER,true>>(MemFunc,const std::string &,HACCESSOR,Impl::WrappedParameter<DBREFCOUNT *,POINTER_PARAMETER,true>)' being compiled
with
[
    MemFunc=HRESULT (__stdcall IAccessor::* )(HACCESSOR,DBREFCOUNT *)
]
see reference to function template instantiation 'ExecuteAndLog<HRESULT(__stdcall IAccessor::* )(HACCESSOR,DBREFCOUNT *),HACCESSOR,Impl::WrappedParameter<DBREFCOUNT *,POINTER_PARAMETER,true>>(MemFunc,const std::string &,HACCESSOR,Impl::WrappedParameter<DBREFCOUNT *,POINTER_PARAMETER,true>)' being compiled
with
[
    MemFunc=HRESULT (__stdcall IAccessor::* )(HACCESSOR,DBREFCOUNT *)
]

我想我搞砸了ParameterUnwrapper的部分专门化(或者我的方法是错误的)。任何建议吗?

更多信息:

Impl是一个嵌套的命名空间(除了ExecuteAndLog之外，所有提供的模板都在命名空间中)

m_wrapped在本例中是IAccessor* (COM接口)类型。

        enum ParameterType
        {
            POINTER_PARAMETER,
            ARRAY_PARAMETER
        };

更新:下面是一个独立的例子:http://codepad.org/lwTzVImb

我在VS2013中得到的错误是:

error C2664: 'int (int,int **,size_t *)' : cannot convert argument 2 from 'WrappedParameter<int **,ARRAY_PARAMETER,true>' to 'int **'
No user-defined-conversion operator available that can perform this conversion, or the operator cannot be called
see reference to function template instantiation 'int ExecuteAndLog<int(__thiscall A::* )(int,int **,size_t *),int,WrappedParameter<int **,ARRAY_PARAMETER,true>,size_t*>(MemFunc,const std::string &,int,WrappedParameter<int **,ARRAY_PARAMETER,true>,size_t *)' being compiled
with
[
    MemFunc=int (__thiscall A::* )(int,int **,size_t *)
]

Answer 1

我明白了!

问题是UnwrapParameter的返回类型。一旦我把它的声明改成

    template <typename T>
    auto UnwrapParameter(T in_param) -> decltype(Impl::ParameterUnwrapper<T>::UnwrapParameter(in_param))

它编译。可惜编译器没有抱怨这个函数的定义，而是相信它声明的返回值。

我现在还有一些其他的问题，但至少我已经取得了进步。