Submitter: Willem Wakker
Submission Date: 2012-01-27
Source: WG14
Reference Document:
N1601
Version: 1.3
Date: October 2013
Subject: initialization
Summary
Consider the following code: typedef struct {
int k;
int l;
int a[2];
} T;
typedef struct {
int i;
T t;
} S;
T x = {.l = 43, .k = 42, .a[1] = 19, .a[0] = 18 };
void f(void)
{
S l = { 1, .t = x, .t.l = 41, .t.a[1] = 17};
}
The question is: what is now the value of l.t.k? Is it
42 (due to the initialization of .t = x) or is it 0 (due
to the fact that .t.l starts an incomplete initialization
of .t?
19 The initialization shall occur in initializer list order, each initializer provided for a particular subobject overriding any previously listed initializer for the same subobject;151) all subobjects that are not initialized explicitly shall be initialized implicitly the same as objects that have static storage duration.
Suggested Technical Corrigendum
Committee Discussion
Oct 2012 meeting
- It was noted that this is basically similar to dr_253.
- The following was proposed, but there was no consensus for adoption.
The initialization shall occur in initializer list order, each initializer provided for a particular subobject overriding any previously listed initializer for the same subobject 151). Subsequently, all subobjects that are not initialized explicitly previously shall be initialized implicitly the same as objects that have static storage duration.
Committee Discussion
Apr 2013 meeting
- The original author intended the result to be 42 by the following reasoning:
- 6.7.9 paragraphs 17-18 specify that each designator list affects only the smallest subobject to which the designator list refers. As a result, the second clause of paragraph 19 occurs once for the greater object as a whole, filling in only those parts of the whole object that were never initialized explicitly.
- gcc and some IBM compilers give the result as 0, although it is not believed that there is code dependent on this interpretation.
- David Keaton proposed N1659.
- This, however, does not clarify the interpreted conflict of paragraph 19 "subobjects that are not initialized explicitly [shall be set to zero]" applied "recursively to subaggregates" (paragraph 20).
- Adding the example is a desired outcome.
Committee Discussion
Oct 2013 meetingThere was no work performed on this DR. Although both GCC and six compilers from IBM provide the unintended answer, it is believed to be such a rarely used feature that it is not depended upon to a great degree, and the compiler venders are willing to change their behavior appropriately.
Committee Discussion
There has been considerable discussion and several proposals ( N1659, N1749) to clarify the standard to no avail. Upon reflection, and consultation with the author, we believe that the proper course of action is twofold. First, simply answer the question asked as the committee believes that the standard already specifies correctly. To add clarification to the standard we will also add the examples from N1659 that leads to this answer.
Proposed Committee Response
The proper answer to the question raised according to the standard is that the value of l.t.k is 42, because implicit initialization does not override explicit initialization. We will also provide a non-normative example to further clarify the intent.Proposed Committee Corrigendum
Add the following example to 6.7.9:
typedef struct {
int k;
int l;
int a[2];
} T;
typedef struct {
int i;
T t;
} S;
T x = {.l = 43, .k = 42, .a[1] = 19, .a[0]
= 18 };
void f(void)
{
S l = { 1, .t = x, .t.l = 41,
.t.a[1] = 17};
}
The value of l.t.k is 42, because implicit initialization does not override explicit initialization.