extern(C++) cast from interface to child class returns wrong pointer value

[dlangBugzillaToGithub]

Atila Neves (@atilaneves) reported this on 2018-09-26T11:50:29Z

Transferred from https://issues.dlang.org/show_bug.cgi?id=19267

CC List

• Suleyman Sahmi (سليمان السهمي)

Description

The code at the end with extern(C++) prints:

fooImpl @ 0x7fa6503c1000
foo     @ 0x7fa6503c1008

foo     @ 0x7fa6503c1008
fooImpl @ 0x7fa6503c1008


If the interface and class are extern(D) it (correctly) prints:

fooImpl @ 0x7fa6503c1000
foo     @ 0x7fa6503c1008

foo     @ 0x7fa6503c1008
fooImpl @ 0x7fa6503c1000


Casting from the class to the interface works in both cases and adds 8 bytes to the pointer. Casting back from interface to the class only works for extern(D) and fails for extern(C++): there's no way to get the original pointer back except for manually subtracting 8 bytes.

Code to reproduce:



import core.stdc.stdio: printf;

extern(C++) interface Foo {
    int foo();
}

extern(C++) class FooImpl: Foo {
    override int foo() { return 42; }
}

void main() {
    auto fooImpl = new FooImpl;
    printf("fooImpl @ %p
", fooImpl);

    Foo foo = fooImpl;
    printf("foo     @ %p
", foo);

    printf("
");
    takingFoo(foo);
}


void takingFoo(Foo foo) {
    printf("foo     @ %p
", foo);
    FooImpl fooImpl = cast(FooImpl) foo;  // oops
    printf("fooImpl @ %p
", fooImpl);

}

[dlangBugzillaToGithub]

sahmi.soulaimane commented on 2019-05-22T06:08:15Z

This would require dynamic casting which means druntime must know about cpp rtti.