Nested class accessing private member of parent

In the following  code, nested class B is accessing private member of
A. Is this allowed? This code compiles on Comeau online test compiler
and Visual Studio 2005.

class A

{
    class B
   {
       A* a;
       int bVal;
       B() { bVal = a->aVal; } // This compiles
    };

private:

    int aVal;
};


int main(int argc, char** argv) {}

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