On Monday, 27 July 2015 at 12:03:06 UTC, Vlad Leberstein wrote:
Hi! My use case requires interaction with C API which in turn
implies storing object instance reference as void *. I'm using
gdc 4.9.2 and everything worked fine with "object -> void * ->
object" conversion, but "object -> void * -> interface" failed.
The stripped-down example is something like this:
interface TestInterface {
void testMethod();
}
class TestImpl : TestInterface {
void testMethod() {
writefln("TestImpl::testMethod\n");
}
};
void testDispathcer(void *rawSelf) {
auto self = cast(TestInterface) rawSelf;
// nothing happens
self.testMethod();
}
int main(string[] args) {
auto t = new TestImpl();
testDispathcer(cast(void *) t);
return 0;
}
However this works:
void testDispathcer(TestImpl rawSelf) {
auto self = cast(TestInterface) rawSelf;
// TestImpl::testMethod is printed to stdout
self.testMethod();
}
int main(string[] args) {
auto t = new TestImpl();
testDispathcer(t);
return 0;
}
Is there any way to handle such situation without resorting to
templating dispatcher with every *Impl type? As second example
works as expected I presume that runtime loses some type
information to perform downcast and maybe it's possible to
extract it and store externally and merge back later?
I'm quite new to D and sorry if the question is dumb. Any help
would be greatly appreciated! Thanks in advance!
In the first example, you pass a pointer to a class instance. You
cannot get the vtbl entry for the interface like this. Instead
try to do this in 2 steps:
---
import std.stdio;
interface TestInterface {
void testMethod();
}
class TestImpl : TestInterface {
void testMethod() {
writefln("TestImpl::testMethod\n");
}
};
void testDispathcer(void *rawSelf) {
auto obj = cast(Object) rawSelf;
auto self = cast(TestInterface) obj;
// nothing happens
self.testMethod();
}
int main(string[] args) {
auto t = new TestImpl();
testDispathcer(cast(void *) t);
return 0;
}
---
