If you’re using public inheritance in C++, the compiler will implicitly “upcast” from a Derived class pointer to a Base class pointer. So I thought a std::auto_ptr to a Derived class would have a similar implicit upcast. It does…but only for assignment and construction!

The example shows that if you try to pass an auto_ptr<Derived> to a function that takes an auto_ptr<Base>, it fails to convert due to an ambiguity. You must put an explicit static_cast< auto_ptr<Base> > at the calling site.

This “minor” problem led me down an investigation of the state of auto_ptr and its alternative, unique_ptr. I sought the wisdom of friends and people on the Freenode IRC. Wound up even building a newer version of gcc than came with the latest Kubuntu distribution! Though my findings are not the most exciting subject for a blog, I thought writing them up might help someone.

So I have good news, and bad news…

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