c++ - Is the order of initialization guaranteed by the standard?

Question

In the following code snippet d1's initializer is passed d2 which has not been constructed yet (correct?), so is the d.j in D's copy constructor an uninitialized memory access?

struct D
{
    int j;

    D(const D& d) { j = d.j; }
    D(int i) { j = i; }
};

struct A
{
    D d1, d2;
    A() : d2(2), d1(d2) {}
};

Which section of C++ standard discusses order of initialization of data members?

Answer 1

I don't have the standard handy right now so I can't quote the section, but structure or class member initialisation always happens in declared order. The order in which members are mentioned in the constructor initialiser list is not relevant.

Gcc has a warning -Wreorder that warns when the order is different:

       -Wreorder (C++ only)
           Warn when the order of member initializers given in the code does
           not match the order in which they must be executed.  For instance:

                   struct A {
                     int i;
                     int j;
                     A(): j (0), i (1) { }
                   };

           The compiler will rearrange the member initializers for i and j to
           match the declaration order of the members, emitting a warning to
           that effect.  This warning is enabled by -Wall.