CODEWITHSUNDEEP
d
Maik Klein
Maik Klein

Reputation: 16148

How do I get the address of an object with an interface?

This is related to Disposing a class with an interface?

interface Bar{
}
class Foo: Bar{
    int i;
    this(int _i){
        i = _i;
    }
}

void main(){
    import std.experimental.allocator.mallocator;
    import std.experimental.allocator;
    auto f = Mallocator.instance.make!Foo(42);
    Bar b = f;
    void* p = (cast(void*)b);
    void* p1 = (cast(void*)f);
    writeln(p);
    writeln(p1);
    Mallocator.instance.dispose(b);// Bad
}

Prints:

1EBE438 
1EBE420

So the address of Bar has an offset of 24 bytes. That can't be right. How do I get the correct address from an interface?

Upvotes: 2

Views: 107

Answers (1)

user6479258
user6479258

Reputation: 26

From https://dlang.org/spec/abi.html#classes:

Casting a class object to an interface consists of adding the offset of the interface's corresponding vptr to the address of the base of the object. Casting an interface ptr back to the class type it came from involves getting the correct offset to subtract from it from the object.Interface entry at vtbl[0].

I don't know why this is necessary, but it seems to explain the different addresses.

Upvotes: 1

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