How to hide Iterator and container implemtation in a custom class (w/o boost)

I have been wracking my brain over this for the last day or so. I am trying to figure out how to return an iterator from a class while hiding the type of container the class is using. An example would be I have a class canvas that holds widgets with the same interface and they are stored privately in a std::vector. So....

Simplified code

class Canvas  
{  
public:  
    WidgetIterator begin();  
    WidgetIterator end();  
private:
    class Impl;
    std::unique_ptr<Impl> mImpl;  
};

class Canvas::Impl  
{
public:  
    std::vector<widget> mWidget;  
    //how to return mWidget.begin(), mWidget.end() up as widget iterator??  
}  

usage:  
int main()  
{  
    Canvas canvas();
    ...Fill canvas with widgets...  
    //iterate over widgets  
    for(auto& a_widget : canvas)  
    {  
        //do something with / to a_widget.  User does not know or care  
        //that underlying iterator is std::vector<widget>::iterator 
    }  
    ...Do more stuff....  
    return 0;  
}  

Basically, I would like to somehow alias mWidget.begin() and mWidget.end() up through Canvas::begin() and Canvas::end(). The user knows that the iterator is to a widget, they just don't need to know that the iterator is std::vector::iterator. I am trying to both hide my implementations using PIMPL and keep information about how things are stored within the class.

I can't seem to find the right "formula". I have looked at type-erasure and trying to return function pointers through an interface, but I can't seem to think of a way to keep std::vector::iterator out of the header, and everything I have seen so far in looking this up don't seem to fit what I am trying to do. Can someone point me in the right direction? Reading material? Is there a concept I am missing? Oh - and I have seen some use boost int he examples that I couldn't figure out how to make work in my situation. I would like to avoid that because I am trying to reduce outside dependencies.

Thank you very much in advance!!!