arrays of const, postblit and RefCounted
monarch_dodra
monarchdodra at gmail.com
Thu Nov 29 07:14:50 PST 2012
I investigating some weird behavior regarding arrays of const
object. I'm not 100% sure what I'm observing, but it would appear
that when adding objects to a const array, objects are not
postblit-ed? Is this the expected behavior?
In this program, "S" is a struct that just prints when postblit
is called:
//----
void main()
{
S[] arr;
foreach (i; 0 .. 5)
{
writeln(i);
arr ~= S(i);
}
}
//----
0
post
1
post
2
post
3
post
post
post
post
4
post
//----
Here, we can clearly see each element being postblit inserted. We
can also see the array re-locating.
However, if we change "S[] arr;" to "const(S)[] arr;", then the
output becomes:
//----
0
1
2
3
4
//----
Strange... right?
I was actually investigating the issue when trying to do a const
array of refcounted. It was behaving scary weird...:
//----
alias RCI = RefCounted!(int, RefCountedAutoInitialize.no);
void main()
{
const(RCI)[] arr;
foreach (i; 0 .. 3)
{
foreach (j; 0 .. i)
writefln("pay[%s]: %s", j, arr[j].refCountedPayload);
auto a = const(RCI)(i);
writefln("declared a: %s", a.refCountedPayload);
foreach (j; 0 .. i)
writefln("pay[%s] before: %s", j,
arr[j].refCountedPayload);
arr ~= a;
foreach (j; 0 .. i + 1)
writefln("pay[%s] after: %s", j,
arr[j].refCountedPayload);
writeln();
}
writeln("after");
foreach (i; 0 .. 3)
writeln("pay: ", arr[i].refCountedPayload);
}
//----
declared a: 0
pay[0] after: 0
pay[0]: 0
declared a: 1
pay[0] before: 1
pay[0] after: 1
pay[1] after: 1
pay[0]: 0
pay[1]: 0
declared a: 2
pay[0] before: 2
pay[1] before: 2
pay[0] after: 2
pay[1] after: 2
pay[2] after: 2
after
pay: 0
pay: 0
pay: 0
//----
See!?
The sole fact of declaring a on the stack modifies the values of
all the elements in the array... And as soon as it goes out of
scope, they all become 0 :/
The weirdest part (IMO), is that since "RefCountedAutoInitialize
== no", the value "0" is not a result of an auto initialize: It
is the actual allocated value of the RCI that is changing...
Changing it to non const "fixes" this, but I really don't even
begin to fathom what is happening...
Thoughts?
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