OK, so structs are a different beast in D than they are in C++. This results in one of my most common pitfalls. I'll find myself writing:

struct A
{
   int a;
   int b;
}

A[] nameTooLong = ...;

foreach (whatever; thingie)
{
   nameTooLong[whatever.whatever].a = whatever.x*3;
   nameTooLong[whatever.whatever].b = whatever.y/3;

   // more of the same sort of stuff
}

So I get fed up typing 'nameTooLong[whatever.whatever]', and instead I write

foreach (whatever; thingie)
{
   A ntl = nameTooLong[whatever.whatever];
   ntl.a = whatever.x*3;
   ntl.b = whatever.y/3;

   // more of the same sort of stuff
}

Then I chase a bug in my program, which compiled OK. After some time, I realize that

   A ntl = nameTooLong[whatever.whatever];

is doing a copy, which is not what I was thinking about at all - old C++ habits.

ntl = ...;

has no effect whatsoever on nameTooLong[whatever.whatever].

So then - pissed off by that point - I rewrite it as:

foreach (whatever; thingie)
{
   A* ntl = &nameTooLong[whatever.whatever];
   // This suggests an ambiguity in the language?
   ntl.a = whatever.x*3;
   ntl.b = whatever.y/3;

   // more of the same sort of stuff
}

This works OK, but it's still not the D way to do things. Try:

foreach (whatever; thingie)
{
   alias nameTooLong[whatever.whatever] ntl;
   ntl.a = whatever.x*3;
   ntl.b = whatever.y/3;

   // more of yer same sort of stuff
}

Steve Teale wrote:
> OK, so structs are a different beast in D than they are in C++. This results in one of my most common pitfalls. I'll find myself writing:
> 
> struct A
> {
>    int a;
>    int b;
> }
> 
> A[] nameTooLong = ...;
> 
> foreach (whatever; thingie)
> {
>    nameTooLong[whatever.whatever].a = whatever.x*3;
>    nameTooLong[whatever.whatever].b = whatever.y/3;

with(nameTooLong[whatever.whatever]) {
  a = whatever.x*3;
  b = whatever.y/3;
}

On Mon, May 11, 2009 at 1:48 PM, Steve Teale <steve.teale@britseyeview.com> wrote:

> Then I chase a bug in my program, which compiled OK. After some time, I realize that
>
>   A ntl = nameTooLong[whatever.whatever];
>
> is doing a copy, which is not what I was thinking about at all - old C++ habits.

Um, C++ works exactly the same way, if you're using classes/structs by value.

Ary Borenszweig Wrote:

> Steve Teale wrote:
> > OK, so structs are a different beast in D than they are in C++. This results in one of my most common pitfalls. I'll find myself writing:
> > 
> > struct A
> > {
> >    int a;
> >    int b;
> > }
> > 
> > A[] nameTooLong = ...;
> > 
> > foreach (whatever; thingie)
> > {
> >    nameTooLong[whatever.whatever].a = whatever.x*3;
> >    nameTooLong[whatever.whatever].b = whatever.y/3;
> 
> with(nameTooLong[whatever.whatever]) {
>    a = whatever.x*3;
>    b = whatever.y/3;
> }

Ary,

Yes I use with quite often, it's when I have two of the beasts where I want to use with at the same time that I have fallen into this.

Now that I've rubbed my nose in it I'm sure I won't do it again.