Calling a base class's protected constructor from a subclass does not work

Question

I have a C++/WinRT base class that I need to subclass. My problem is that I don't manage to call the base class's protected constructor from the subclass.

That base class is defined in MIDL as follows:

namespace My.Custom.WindowsRuntimeComponent
{
  unsealed runtimeclass BaseClass : Windows.UI.Xaml.Controls.SwapChainPanel
  {
  }
}

From it, the following header and implementation is created:

#pragma once

#include <DirectXMath.h>
#include "BaseClass.g.h"

namespace winrt::My::Custom::WindowsRuntimeComponent::implementation
{
  struct BaseClass : BaseClassT<BaseClass>
  {
  protected:
    BaseClass();
    ::DirectX::XMFLOAT3 ProtectedMethod();
  };
}

#include "pch.h"
#include "BaseClass.h"
#include "BaseClass.g.cpp"

namespace winrt::My::Custom::WindowsRuntimeComponent::implementation
{
  BaseClass::BaseClass()
  {
    // Important stuff happening here
  }

  ::DirectX::XMFLOAT3 BaseClass::ProtectedMethod()
  {
    return ::DirectX::XMFLOAT3();
  }
}

The subclass's MIDL, header and implementation are defined as follows:

import "BaseClass.idl";

namespace My.Custom.WindowsRuntimeComponent
{
  runtimeclass SubClass : BaseClass
  {
    SubClass();
    void UseProtectedMethod();
  }
}

#pragma once

#include "BaseClass.h"
#include "SubClass.g.h"

namespace winrt::My::Custom::WindowsRuntimeComponent::implementation
{
  struct SubClass : SubClassT<SubClass, My::Custom::WindowsRuntimeComponent::implementation::BaseClass>
  {
    SubClass();
    void UseProtectedMethod();
  };
}

namespace winrt::My::Custom::WindowsRuntimeComponent::factory_implementation
{
  struct SubClass : SubClassT<SubClass, implementation::SubClass>
  {
  };
}

#include "pch.h"
#include "SubClass.h"
#include "SubClass.g.cpp"

namespace winrt::My::Custom::WindowsRuntimeComponent::implementation
{
  SubClass::SubClass()
  {
  }

  void SubClass::UseProtectedMethod()
  {
    ::DirectX::XMFLOAT3 value = ProtectedMethod();
  }
}

The above example compiles. However, if I attempt to call the protected base class constructor from the initializer list of the subclass as shown below I receive a compiler error.

#include "pch.h"
#include "SubClass.h"
#include "SubClass.g.cpp"

namespace winrt::My::Custom::WindowsRuntimeComponent::implementation
{
  SubClass::SubClass() : BaseClass() // This line does not compile
  {
  }

  void SubClass::UseProtectedMethod()
  {
    ::DirectX::XMFLOAT3 value = ProtectedMethod();
  }
}

The compiler error is as follows: error C2614: 'winrt::My::Custom::WindowsRuntimeComponent::implementation::SubClass': illegal member initialization: 'BaseClass' is not a base or member

Both in the MIDL and the header of SubClass I specify that it inherits from BaseClass so it is unclear to me why the compiler emits that error.

I could work around that problem, I guess, but I'm curious about what exactly is going on here. Any hints?

Answer 1

The SubClass in the winrt::... namespace derives from SubClassT<...>, not from SubClass.
The SubClass that is outside the winrt:: namespace derives from BaseClass, but thats a different one. Your naming is easy to get lost in.