When should I use C++ private inheritance?

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I use it all the time. A few examples off the top of my head:

  • When I want to expose some but not all of a base class’s interface. Public inheritance would be a lie, as Liskov substitutability is broken, whereas composition would mean writing a bunch of forwarding functions.
  • When I want to derive from a concrete class without a virtual destructor. Public inheritance would invite clients to delete through a pointer-to-base, invoking undefined behaviour.

A typical example is deriving privately from an STL container:

class MyVector : private vector<int>
{
public:
    // Using declarations expose the few functions my clients need 
    // without a load of forwarding functions. 
    using vector<int>::push_back;
    // etc...  
};
  • When implementing the Adapter Pattern, inheriting privately from the Adapted class saves having to forward to an enclosed instance.
  • To implement a private interface. This comes up often with the Observer Pattern. Typically my Observer class, MyClass say, subscribes itself with some Subject. Then, only MyClass needs to do the MyClass -> Observer conversion. The rest of the system doesn’t need to know about it, so private inheritance is indicated.

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