member initializer does not name a non-static data

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Initialize parent's protected members with initialization list (C++) 4 answers

I'm having a hard time finding hits on google for this.

struct a {
    float m_x;
    float m_z;
public:
    a(float x): m_x(x) {}
};

class b : public a {
    b(float z): m_z(z) {}
};

On clang 3.2:

error: member initializer 'm_z' does not name a non-static data member or base class
    b(float z): m_z(z) {}

No you cannot initialize base class members from initializer list directly. This is because order of initialization proceeds in this way

C++ Standard n3337 § 12.6.2/10

In a non-delegating constructor, initialization proceeds in the following order:

— First, and only for the constructor of the most derived class (1.8), virtual base classes are initialized in the order they appear on a depth-first left-to-right traversal of the directed acyclic graph of base classes, where “left-to-right” is the order of appearance of the base classes in the derived class base-specifier-list.

— Then, direct base classes are initialized in declaration order as they appear in the base-specifier-list (regardless of the order of the mem-initializers).

— Then, non-static data members are initialized in the order they were declared in the class definition (again regardless of the order of the mem-initializers).

— Finally, the compound-statement of the constructor body is executed.

[ Note: The declaration order is mandated to ensure that base and member subobjects are destroyed in the reverse order of initialization. — end note ]

So you can specify a constructor in a base class (it can be protected) and use that one in initialization list of derived class (should be preferred) or you can assign to a base class member in derived class ctor body (different behaviour, different effect and also less efficient - you are assigning to default initialized (already has value) member).

In the former case you might write it this way:

struct A {
    float m_x;
    float m_z;
    A(){}
protected:
    A(float x): m_x(x) {}
};

class B : public A {
public:
    B(float z) : A(z) {}
    // alternatively
    // B(float z) {
    //     m_x = z;
    // }
};

int main(){
    B b(1);
    return 0;
}