https://issues.dlang.org/show_bug.cgi?id=22633
Issue ID: 22633
Summary: Associative array require and update don't work if the
value type can't be reassigned or copied.
Product: D
Version: D2
Hardware: x86_64
OS: Linux
Status: NEW
Severity: normal
Priority: P1
Component: druntime
Assignee: [email protected]
Reporter: [email protected]
The program below should work, but instead fails to compile with various errors
of the form:
Error: cannot modify struct instance `*p` of type `S` because it contains
`const` or `immutable` members
///////////////////////////////////////////////////
module app;
import std.stdio : writeln;
struct S {
const(int) a;
}
void main() @safe {
S[string] aa;
aa.require("x", S(1));
writeln(aa["x"]);
aa.update("y", () => S(2), (ref const(S)) { });
writeln(aa["y"]);
}
///////////////////////////////////////////////////
The cause of the error is that object.require and object.update both use
assignment for initialization, even though it is known at compile time that the
target is new and has not yet been constructed.
The fix is to use core.lifetime.moveEmplace for initialization, instead:
///////////////////////////////////////////////////
ref V require(K, V)(ref V[K] aa, K key, lazy V value = V.init)
{
bool found;
// if key is @safe-ly copyable, `require` can infer @safe
static if (isSafeCopyable!K)
{
auto p = () @trusted
{
return cast(V*) _aaGetX(cast(AA*) &aa, typeid(V[K]), V.sizeof,
&key, found);
} ();
}
else
{
auto p = cast(V*) _aaGetX(cast(AA*) &aa, typeid(V[K]), V.sizeof, &key,
found);
}
if (found)
return *p;
else
{
import core.internal.traits : hasElaborateMove;
static if(hasElaborateMove!V)
() @system { } ();
(V value) @trusted
{
import core.lifetime : moveEmplace;
moveEmplace(value, *p);
} (value());
return *p; // this might not return a ref to the left-hand side.
}
}
// ...
void update(K, V, C, U)(ref V[K] aa, K key, scope C create, scope U update)
if (is(typeof(create()) : V) && (is(typeof(update(aa[K.init])) : V) ||
is(typeof(update(aa[K.init])) == void)))
{
bool found;
// if key is @safe-ly copyable, `update` may infer @safe
static if (isSafeCopyable!K)
{
auto p = () @trusted
{
return cast(V*) _aaGetX(cast(AA*) &aa, typeid(V[K]), V.sizeof,
&key, found);
} ();
}
else
{
auto p = cast(V*) _aaGetX(cast(AA*) &aa, typeid(V[K]), V.sizeof, &key,
found);
}
if (!found)
{
import core.internal.traits : hasElaborateMove;
static if(hasElaborateMove!V)
() @system { } ();
(V value) @trusted
{
import core.lifetime : moveEmplace;
moveEmplace(value, *p);
} (create());
}
else
{
static if (is(typeof(update(*p)) == void))
update(*p);
else
*p = update(*p);
}
}
///////////////////////////////////////////////////
Forum thread: https://forum.dlang.org/post/[email protected]
--
