Order of member initialization list

Question

After simplifying my code for many times, I found the following cause the problem.

class B {
public:
    B(const int x)
              :_x(x) {}
    const int _x;
};

class C {
public:
    C(const B& b) 
        : _b(b), _b2(_b._x) {}
    B _b2;       // line 1
    const B& _b; // line 2
};

int main() {
    B b(1);
    C c(b);
}

Warning (clang 8.0.0)

test16.cpp:11:22: warning: reference '_b' is not yet bound to a value when used here [-Wuninitialized]
        : _b(b), _b2(_b._x) {}
                     ^
1 warning generated.

g++-6 compiles the program. Running the program causes segmentation fault.

Does the initialization of members of a class follows the order of the member initialization list (: _b(b), _b2(_b._x)) or the order of the members in the class (like B _b2; const B& _b;) ?

Answer 1

Initialization of member variables occurs in the order that they are declared in the class.

http://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#Rc-order

and:

http://en.cppreference.com/w/cpp/language/initializer_list

3) Then, non-static data members are initialized in order of declaration in the class definition.

Answer 2

The order how you specify in the member initializer list doesn't matter, non-static members are initialized in order of declaration in the class definition.

The order of member initializers in the list is irrelevant: the actual order of initialization is as follows:

3) Then, non-static data members are initialized in order of declaration in the class definition.

That means _b2 will always be initialized before _b; and when _b is used to initialize _b2 it's still not initialized.

BTW: The similar rule is applied for the initializations of direct base classes.

2) Then, direct base classes are initialized in left-to-right order as they appear in this class's base-specifier list