Template static inheritance problem

[fgungor]

Hi I am having problems with the following code

class IRenderer
{
public:
virtual int Init() = 0;
virtual int Shutdown() = 0;
};

template <typename TConcreteDevice>
class Device
{
typename TConcreteDevice::TDevice *m_pDevicePtr;

public:

operator typename TConcreteDevice::TDevice *()
{
return m_pDevicePtr;
}
};

class RendererDevice : Device<RendererDevice>
{
public:
typedef IRenderer TDevice;
};

So basically I want to have several types of Devices and every Device
should expose the type of the underlying object it uses.

What is wrong with this code ? It tells me that RendererDevice has no
member named TDevice !?

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[Chris Uzdavinis]

On Dec 21, 11:16 pm, fgungor <neurore...@gmail.com> wrote:
> Hi I am having problems with the following code

...

> template <typename TConcreteDevice>
> class Device
> {
> typename TConcreteDevice::TDevice *m_pDevicePtr;

> };
>
> class RendererDevice : Device<RendererDevice>
> {
> public:
> typedef IRenderer TDevice;
>
> };
>
> So basically I want to have several types of Devices and every Device
> should expose the type of the underlying object it uses.
>
> What is wrong with this code ? It tells me that RendererDevice has no
> member named TDevice !?

At the point of declaring the base class for RendererDevice, the type
of
RendererDevice is still considered an incomplete type. Thus, when
Device<RendererDevice> attempts to extract a nested type, it cannot.

Chris

[Yechezkel Mett]

On Dec 22, 7:16 am, fgungor <neurore...@gmail.com> wrote:
> Hi I am having problems with the following code
>
> class IRenderer
> {
> public:
> virtual int Init() = 0;
> virtual int Shutdown() = 0;
>
> };
>
> template <typename TConcreteDevice>
> class Device
> {
> typename TConcreteDevice::TDevice *m_pDevicePtr;
>
> public:
>
> operator typename TConcreteDevice::TDevice *()
> {
> return m_pDevicePtr;
> }
>
> };
>
> class RendererDevice : Device<RendererDevice>
> {
> public:
> typedef IRenderer TDevice;
>
> };
>
> So basically I want to have several types of Devices and every Device
> should expose the type of the underlying object it uses.
>
> What is wrong with this code ? It tells me that RendererDevice has no
> member named TDevice !?

1. The line
> typename TConcreteDevice::TDevice *m_pDevicePtr;
has to be understood when Device<> is instantiated.
2. Device<RendererDevice> is instantiated in the line
> class RendererDevice : Device<RendererDevice>
3. In the above line RendererDevice is not yet complete, and you
cannot access any of its members.

A better compiler error message would have said explicitly that the
class is not complete.

Solutions:

1. Hold m_pDevicePtr by pointer-to-void. static_cast it to
TConcreteDevice::TDevice* in a member function. (Bodies of member
functions are instantiated later, even if they are written in the
class definition.) It's not very pleasant to have void* lying around,
but it is safe -- it's private and you can be sure to initialise and
access it correctly.

2. Move
> typedef IRenderer TDevice;
into a traits class.

3. Pass IRenderer explicitly as a parameter to Device<>.

Yechezkel Mett