Access to protected constructor of base class

Question

A derived class can call a protected base class constructor in its ctor-initializer, but only for its own base class subobject, and not elsewhere:

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Answer 1

C++11 §11.2/5:

”
A member m is accessible at the point R when named in class N if

• m as a member of N is public, or

• m as a member of N is private, and R occurs in a member or friend of class N, or

• m as a member of N is protected, and R occurs in a member or friend of class N, or in a member or friend of a class P derived from N, where m as a member of P is public, private, or protected, or

• there exists a base class B of N that is accessible at R, and m is accessible at R when named in class B.

For your constructor invocation

Base b2;

the 3^rd point above applies. m is the Base constructor. N, the naming class, is Base. m as a member of Base is protected, and the declaration occurs in a member of class Derived derived from Base, but it's not the case that the Base constructor as a member of Derived is public, private or protected: it’s simply not a member of Derived, constructors are not implicitly inherited.

I think the language “is public, private, or protected” is pretty awkward; I can only surmise that it’s the result of some evolution of this paragraph.

I have yet to find an explanation of how formally the protected Base constructor is accessible in a member initializer list in Derived, but then I just started looking at this for this question.

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Update: I fail to find any language in the standard pertaining to access to constructors in an initializer list, and I fail to find any Defect Report about it. It’s quite possibly a defect.