Is taking the address of a member of an uninitialized object well defined?

Question

Consider the following example. When bar is constructed, it gives it's base type (foo) constructor the address of my_member.y where my_member is data member that hasn't been initialized yet.

struct foo {
    foo(int * p_x) : x(p_x) {}
    int * x;
};

struct member {
    member(int p_y) : y(p_y) {}
    int y;
};

struct bar : foo
{
    bar() : foo(&my_member.y), my_member(42) {}
    member my_member;
};

#include <iostream>

int main()
{
    bar my_bar;
    std::cout << *my_bar.x;
}

Is this well defined? Is it legal to take the address of an uninitialized object's data member? I've found this question about passing a reference to an uninitialized object but it's not quite the same thing. In this case, I'm using the member access operator . on an uninitialized object.

It's true that the address of an object's data member shouldn't be changed by initialization, but that doesn't necessarily make taking that address well defined. Additionally, the ccpreference.com page on member access operators has this to say :

The first operand of both operators is evaluated even if it is not necessary (e.g. when the second operand names a static member).

I understand it to mean that in the case of &my_member.y my_member would be evaluated, which I believe is fine (like int x; x; seems fine) but I can't find documentation to back that up either.

Answer 1

First let's make accurate the question.

What you are doing isn't using an uninitialized object, you are using an object not within its lifetime. my_member is constructed after foo, therefore the lifetime of my_member hasn't begun in foo(&my_member.y).

From [basic.life]

before the lifetime of an object has started but after the storage which the object will occupy has been allocated [...], any glvalue that refers to the original object may be used but only in limited ways. [...] such a glvalue refers to allocated storage, and using the properties of the glvalue that do not depend on its value is well-defined. The program has undefined behavior if:

• the glvalue is used to access the object, or [...]

Here accessing it means specifically to either read or modify the value of the object.

The evaluation of my_member yields a lvalue, and there is nothing necessitating a conversion to prvalue, hence it stays a lvalue. Likewise, the evaluation of my_member.y is also a lvalue. We then conclude that no object's value have been accessed, this is well-defined.

Answer 2

Yes you are allowed to pass &my_member.y to foo's constructor, and even copy the pointer - which you do with x(p_x).

The behaviour on dereferencing that pointer though in foo's constructor is undefined. (But you don't do that.)