Order of initialization of class member variables

Question

Consider the following code snippet:

class A
{
public:
    A(int a, int b) : j(a), i(j + b) {}
    int i, j;
};

int main()
{
    A a(10, 20);
    std::cout << a.i << " " << a.j << std::endl;
    return 0;
}

The standard says that the order of initialization of the member variables is the order in which they are declared. In this case, i will be initialized before j. Since j isn't initialized yet, i = *a garbage value* + 20, and then j is initialized with 10.

The code prints 20 10.

i.e., j was considered to be 0 when calculating i.

Does the standard guarantee to use default values for built-in types in such a scenario? Or is it just that the garbage value happened to be 0? Or is it undefined behavior?

"Or is it just that the garbage value happened to be 0?" - Probably in over 90% of the cases this (still UB of course). — Hawky, Dec 20 '19 at 13:32
This should answer your question: https://stackoverflow.com/a/6032889/5728381 — Ultramoi, Dec 20 '19 at 13:35

walnut · Answer 1 · 2019-12-22T08:05:46.360

j + b is undefined behavior, because it uses an indeterminate value (that of j before its initialization) outside of one of the specific instances where doing so is allowed (all of which relate to char variants and std::byte, not int, indeterminate values).

In C++20 it will also have undefined behavior, because you are accessing j outside its lifetime. From what I can tell in C++17 and before the lifetime of j begins with allocation of its storage, because the initialization of j is vacuous. The definitions of vacuous initialization and lifetime change in C++20, though.

So in any case use of a variable before it is initialized is very ill-advised.

score 1 · Accepted Answer · answered Dec 20 '19 at 13:42

Does the standard guarantee to use default values for built-in types in such a scenario?

No. The value is indeterminate in this case.

Or is it undefined behavior?

Yes. The behaviour of reading an indeterminate value is undefined (except for narrow character types IIRC).

Order of initialization of class member variables

2 Answers2