[SO] Modifing local class instance in pure function

[bearophile]

div0:
> Objects are defined to have an identity, so by returning a new object
> from the function you are not getting the same result for the same inputs.
> Does D relax the definition of pure to say a function may return an
> equivalent object?

Interesting question, I have tried the following code with D2:

import std.stdio: writeln;

class C {}

pure C genc() { return new C; }

void main() {
    writeln(genc() is genc());
}


The generated asm shows that pure of genc is ignored:

With "pure":
main:
L0:     push    EBX
        mov EAX,offset FLAT:_D5temp31C7__ClassZ
        push    EAX
        call    near ptr __d_newclass
        add ESP,4
        mov ECX,offset FLAT:_D5temp31C7__ClassZ
        push    EAX
        sub ESP,4
        push    ECX
        call    near ptr __d_newclass
        add ESP,4
        add ESP,4
        mov EDX,EAX
        mov EBX,1
        pop EAX
        cmp EAX,EDX
        je  L32
        xor EBX,EBX
L32:        mov EAX,offset FLAT:_D3std5stdio6stdoutS3std5stdio4File
        push    EBX
        push    0Ah
        call    near ptr _D3std5stdio4File14__T5writeTbTaZ5writeMFbaZv
        xor EAX,EAX
        pop EBX
        ret

        
Without "pure":
main:
L0:     push    EBX
        mov EAX,offset FLAT:_D5temp31C7__ClassZ
        push    EAX
        call    near ptr __d_newclass
        add ESP,4
        mov ECX,offset FLAT:_D5temp31C7__ClassZ
        push    EAX
        sub ESP,4
        push    ECX
        call    near ptr __d_newclass
        add ESP,4
        add ESP,4
        mov EDX,EAX
        mov EBX,1
        pop EAX
        cmp EAX,EDX
        je  L32
        xor EBX,EBX
L32:        mov EAX,offset FLAT:_D3std5stdio6stdoutS3std5stdio4File
        push    EBX
        push    0Ah
        call    near ptr _D3std5stdio4File14__T5writeTbTaZ5writeMFbaZv
        xor EAX,EAX
        pop EBX
        ret        

I don't know what's happening under the hood into dmd here, Don may give a better answer.

Bye,
bearophile