Accessing the name of a private-inherited class from a subclass

Question

The following code fails to compile with this error with g++ and a similar error with clang.

main.cpp:16:19: error: 'QByteArray' is a private member of 'QByteArray'         Container2(const QByteArray &ba);                          ^ main.cpp:3:18: note: constrained by private inheritance here class Container1:private QByteArray                  ^~~~~~~~~~~~~~~~~~ ../edge-clang/qt5/include/QtCore/qbytearray.h:167:21: note: member is declared here class Q_CORE_EXPORT QByteArray                     ^ main.cpp:18:30: error: 'QByteArray' is a private member of 'QByteArray' Container2::Container2(const QByteArray &ba):Container1(ba){}                              ^ main.cpp:3:18: note: constrained by private inheritance here class Container1:private QByteArray                  ^~~~~~~~~~~~~~~~~~ ../edge-clang/qt5/include/QtCore/qbytearray.h:167:21: note: member is declared here class Q_CORE_EXPORT QByteArray                     ^ 

I don't understand why this is not allowed for Container2 to inherit Container1 and initialize it through a public constructor, even though the inheritance of QByteArray is private.

#include <QByteArray>  class Container1:private QByteArray { public:      Container1() {}     Container1(const QByteArray &ba); };  Container1::Container1(const QByteArray &ba):QByteArray(ba){}  class Container2:public Container1 { public:     Container2() {}     Container2(const QByteArray &ba); }; Container2::Container2(const QByteArray &ba):Container1(ba){}  int main(int argc,char *argv[]) {     QByteArray ba;     Container1 c1(ba);     Container2 c2(ba); } 

Answer 1

This is a great bug!

In the context of C, "A" means "the injected-class-name A that I get from my base".
You could think of it as C::B::A.
Of course, that base is inaccessible due to private inheritance:

class A {};  class B : private A { public:    B() {}    B(const A&) {}   };  class C : public B { public:    C(const A&) {} };  int main() {    A a;    B b(a);    C c(a); }  // main.cpp:11:13: error: 'class A A::A' is inaccessible within this context //      C(const A&) {} //              ^ // main.cpp:1:9: note: declared here //  class A {}; //        ^ 

(live demo)

You can fix this by calling it ::A, taking a more indirect route to actually name the exact same type:

class A {};  class B : private A { public:    B() {}    B(const A&) {}   };  class C : public B { public:    C(const ::A&) {} };  int main() {    A a;    B b(a);    C c(a); }  // OK 

(live demo)

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As an aside, the exact same logic applies to privately inherited member variables:

int x = 1;  class A { private:    int x = 2; };  class B : A { public:    B() { int y = x; } };  int main() {    A a;    B b(a); }  // main.cpp: In constructor 'B::B()': // main.cpp:11:17: error: 'int A::x' is private within this context //    B() { int y = x; } 

(live demo)

It all does seem pretty stupid on the face of it, but there's probably a good reason.

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In your case, then:

Container2(const ::QByteArray &ba);