boost::bind with protected members & context

Question

In the below code, there are two \"equivalent\" calls to std::for_each using boost:bind expressions. The indicated line compiles, the indicated fai

Answer 1

The reason for this restriction is enforcement of access control across different classes that share a common base.

This is reinforced by notes in Core Language Defects Report defect #385, the relevant part copied here for reference:

[...] the reason we have this rule is that C's use of inherited protected members might be different from their use in a sibling class, say D. Thus members and friends of C can only use B::p in a manner consistent with C's usage, i.e., in C or derived-from-C objects.

As an example of something this rule prevents:

class B {
protected:
    void p() { };
};

class C : public B {
public:
    typedef void (B::*fn_t)();
    fn_t get_p() {
        return &B::p; // compilation error here, B::p is protected
    }
};

class D : public B { };

int main() {
    C c;
    C::fn_t pbp = c.get_p();
    B * pb = new D();
    (pb->*pbp)();
}

The protected status of D::p is something we want the compiler to enforce, but if the above compiled that would not be the case.