Trying to understand Pimpl

[Paul]

Here is some basic pimpl code below:
I don't understand why this is legal. It seems that class widget{..}
defines widget as a base class.
And then class widget is then redefined as a class privately inheriting
from impl.
Why isn't widget being illegally defined twice?
Also, can someone explain pimpl to me or direct me to a good explanation?

Thanks

Paul

class widget {
public:
widget();
~widget();
private:
class impl;
unique_ptr<impl> pimpl;
};

// in implementation file
class widget::impl {
// :::
};

widget::widget() : pimpl{ new impl{ /*...*/ } } { }
widget::~widget() { }

[Ian Collins]

On 02/25/17 09:49 AM, Paul wrote:
> Here is some basic pimpl code below:
> I don't understand why this is legal. It seems that class widget{..}
> defines widget as a base class.

Where?

> And then class widget is then redefined as a class privately inheriting
> from impl.

Where?

> Why isn't widget being illegally defined twice?
> Also, can someone explain pimpl to me or direct me to a good explanation?

I think you need to explain where your conclusions (above) came from,
the code does not support them.

> class widget {

No base class...

> public:
> widget();
> ~widget();
> private:
> class impl;
> unique_ptr<impl> pimpl;

Contains a nested impl class, no inheritance.

--
Ian

[Paul]

Ok. I get it now. Thanks.
I read it as class widget : private impl {//};
I'm nervous about an upcoming job interview, and
it's affecting my thinking.

Paul