Why must initializer list order match member declaration order?

Question

Why does gcc throw a hissy fit if the initializer list order doesn't match variable order in the class?

class myClass
{
public:
   int A;
   int B;
   myClass();
};

myClass::myClass() :
B(1),
A(2)
{}

will result in:

file.h:274: warning: 'myClass::A' will be initialized after
file.h:273: warning:   'int myClass::B
file.cpp:581: warning:   when initialized here

Is there any specific reason why this kind of warning is issued? Are there any risks associated with initializing variables of a class in order different than they are defined within the class?

(note, there is a question which touches the subject, but the answers are pretty much "because it should be so" without giving any rationale as to why it should be ordered, or what's wrong with this being out of order - I'd like to know why such a restriction exists - could someone give an example where it may backfire maybe?)

Answer 1

The warning is trying to prevent situations where you might be relying on the wrong ordering of the data members. Say you think B is initialized before A, and then you do something like this:

myClass::myClass() :
B(42), A(B) {}

Here, you have undefined behaviour because you are reading from an uninitialized B.