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How to implement a copy
Mar 18, 2010
Paul D. Anderson
Mar 18, 2010
bearophile
Mar 18, 2010
Paul D. Anderson
Mar 19, 2010
Pelle Månsson
Mar 19, 2010
Ali Çehreli
```If I'm implementing a struct and want to provide for duplication, is there a standard way to implement this?

Here's an example:

//-------------------------------

struct S {

// members of the struct -- three integer values
int a;
int b;
int c;

// here's a copy constructor
this(S s) {
this.a = s.a;
this.b = s.b;
this.c = s.c;
}

// here's the dup property
S dup() {
S s;
result.a = this.a;
result.b = this.b;
result.c = this.c;
return s;
}

// here's opAssign for S
void opAssign(S s) {
this.a = s.a;
this.b = s.b;
this.c = s.c;
}

} // end struct S

// and here's a copy function
S copy(S s) {
S t;
t.a = s.a;
t.b = s.b;
t.c = s.c;
return t;
}

//-------------------------------

Which of these three calls is "better" (more efficient, more intuitive, more consistent...)?

S s;    // the original struct

S t = s.dup;     // copied via dup
S u = S(s);      // copied via copy constructor
S v = s;           // copied via opAssign
S w = copy(s);  // copied via copy function

Or is this a distinction without a difference?

Paul

```
```Paul D. Anderson:
> Or is this a distinction without a difference?

For POD structs like this one I suggest to implement nothing, and just let the compiler copy the struct by itself.
If the struct is not a POD then I like the dup property. Each of those other ways can be OK, according to the syntax you prefer to use :-) The free copy function is probably not necessary.

If the struct has alignment "holes", like:
struct Foo { double x; short y; }
Then different copying methods are not the same. The holes are meant to be filled with bytes set to zero, but there can be situations where this can be false. So in those situations copying the whole memory block of a struct or copying just the members is not the same thing. The default D copy copies the whole block of memory. You can see the situation with this (D2 code, but I think it's the same in D1):

import std.c.string: memset;
import std.c.stdio: printf;

struct Foo {
double d;
short s;

static if (1) {
void opAssign(Foo other) {
this.d = other.d;
this.s = other.s;
}
}
}

void showStruct(T)(T s) if (is(T == struct)) {
foreach (b; cast(ubyte[T.sizeof])s)
printf("%d ", b);
printf("\n");
}

void fillStruct(T)(ref T s, ubyte value) if (is(T == struct)) {
memset(&s, value, T.sizeof);
}

void main() {
Foo f, g;

showStruct(f);
showStruct(g);

fillStruct(f, ubyte.min);
fillStruct(g, ubyte.max);

showStruct(f);
showStruct(g);

g = f;

showStruct(f);
showStruct(g);
}
/*
Without opAssign:

0 0 0 0 0 0 252 127 0 0 0 0 0 0 0 0
0 0 0 0 0 0 252 127 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
255 255 255 255 255 255 255 255 255 255 255 255 255 255 255 255
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

lea ESI,-020h[EBP]
lea EDI,-010h[EBP]
movsd
movsd
movsd
movsd

-----------------------------

With opAssign:

0 0 0 0 0 0 252 127 0 0 0 0 0 0 0 0
0 0 0 0 0 0 252 127 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
255 255 255 255 255 255 255 255 255 255 255 255 255 255 255 255
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0 0 255 255 255 255 255 255

Inside the Dmain_:
...
lea EAX,-010h[EBP]
call    near ptr _D4bug23Foo8opAssignMFS4bug23FooZv
...

_D4bug23Foo8opAssignMFS4bug23FooZv  comdat
L0:     enter   4,0
mov -4[EBP],EAX
cmp dword ptr -4[EBP],0
jne L2C
push    9
push    dword ptr _D4bug23Foo6__initZ[01Ch]
push    dword ptr _D4bug23Foo6__initZ[018h]
push    dword ptr _D4bug23Foo6__initZ[034h]
push    dword ptr _D4bug23Foo6__initZ[030h]
call    near ptr __d_assert_msg
L2C:        fld qword ptr 8[EBP]
mov EAX,-4[EBP]
fstp    qword ptr [EAX]
mov CX,010h[EBP]
mov 8[EAX],CX
leave
ret 010h

opAssign compiled with dmd -O -release:

_D4bug23Foo8opAssignMFS4bug23FooZv  comdat
push    EAX
fld qword ptr 8[ESP]
mov CX,010h[ESP]
mov 8[EAX],CX
fstp    qword ptr [EAX]
pop EAX
ret 010h

*/

The performance too is not the same, the built-in copy is probably a little faster (you can see it from the asm too). If you want to be sure you can write a little benchmark.

Bye,
bearophile
```
```bearophile Wrote:

> Paul D. Anderson:
> > Or is this a distinction without a difference?
>
> For POD structs like this one I suggest to implement nothing, and just let the compiler copy the struct by itself.
> If the struct is not a POD then I like the dup property. Each of those other ways can be OK, according to the syntax you prefer to use :-) The free copy function is probably not necessary.
>

Thanks for the help.

```
```On 03/18/2010 05:43 PM, Paul D. Anderson wrote:
> If I'm implementing a struct and want to provide for duplication, is there a standard way to implement this?
>
> Here's an example:
>
> //-------------------------------
>
> struct S {
>
>      // members of the struct -- three integer values
>      int a;
>      int b;
>      int c;
>
>      // here's a copy constructor
>      this(S s) {
>          this.a = s.a;
>          this.b = s.b;
>          this.c = s.c;
>      }
>
>      // here's the dup property
>      S dup() {
>          S s;
>          result.a = this.a;
>          result.b = this.b;
>          result.c = this.c;
>          return s;
>      }
>
>      // here's opAssign for S
>      void opAssign(S s) {
>          this.a = s.a;
>          this.b = s.b;
>          this.c = s.c;
>      }
>
>
> } // end struct S
>
> // and here's a copy function
> S copy(S s) {
>      S t;
>      t.a = s.a;
>      t.b = s.b;
>      t.c = s.c;
>      return t;
> }
>
> //-------------------------------
>
> Which of these three calls is "better" (more efficient, more intuitive, more consistent...)?
>
> S s;    // the original struct
>
> S t = s.dup;     // copied via dup
> S u = S(s);      // copied via copy constructor
> S v = s;           // copied via opAssign
> S w = copy(s);  // copied via copy function
>
> Or is this a distinction without a difference?
>
> Paul
>
>
>
>

this(this) is the copy constructor, I think.

Try using that :)
```
```� wrote:

> this(this) is the copy constructor, I think.

this(this) is not the copy constructor. It is post-blit, which is useful for when corrections need to be done after the automatic blitting:

http://digitalmars.com/d/2.0/struct.html#StructPostblit

Ali
```