Automatic opApply iteration counter

bearophile bearophileHUGS at
Wed Apr 14 05:46:49 PDT 2010

This thread is now fit for D.learn, but I answer it here to not split it up.

Steven Schveighoffer:

> Don't forget, opApply is just a function, you can use it as such :)

Right, I forget it often... I will keep in mind your example, thank you.

> Oh, and don't forget to always make opApply take a scope delegate!  
> Otherwise, it allocates the calling function's frame on the heap (in D2 at  
> least).

To test it, I have compiled this program with and without the two scope annotations (I use printf to remove lot of noise from the resulting asm, because writeln adds tons of functions):

import std.c.stdio: printf;
alias int Int;
struct Xfibonacci {
    Int max;

    int opApply(scope int delegate(ref Int) dg) {
         int _dg(ref int i, ref Int l) { return dg(l); }
         return opApply(&_dg);

    int opApply(scope int delegate(ref int, ref Int) dg) {
        int result, index;
        Int a=0, b=1;
        while (a < max) {
            result = dg(index, b);
            if (result) break;
            a += b;
            result = dg(index, a);
            if (result) break;
            b += a;
        return result;

void main() {
    foreach (f; Xfibonacci(200))
        printf("%d\n", f);

    foreach (i, f; Xfibonacci(200))
        printf("%d %d\n", i, f);

This is one of the functions, using the scope attributes:

    push EAX
    mov ECX,offset FLAT:_D11xfibonacci210Xfibonacci7opApplyMFMDFKiZiZi3_dgMFKiKiZi
    mov EAX,0[ESP]
    push ECX
    lea EDX,4[ESP]
    push EDX
    call near ptr _D11xfibonacci210Xfibonacci7opApplyMFMDFKiKiZiZi
    pop ECX
    ret 8

The same function, not using the scope attributes:

    push EAX
    push EBX
    push ESI
    push 010h
    call near ptr __d_allocmemory
    mov EBX,EAX
    mov EAX,0Ch[ESP]
    mov [EBX],EAX
    mov EDX,018h[ESP]
    mov ESI,014h[ESP]
    mov 8[EBX],ESI
    mov ECX,offset FLAT:_D11xfibonacci210Xfibonacci7opApplyMFDFKiZiZi3_dgMFKiKiZi
    mov 0Ch[EBX],EDX
    add ESP,4
    push ECX
    push EBX
    call near ptr _D11xfibonacci210Xfibonacci7opApplyMFDFKiKiZiZi
    pop ESI
    pop EBX
    pop ECX
    ret 8


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