Re: Ref local variables?
On Sun, 08 Jan 2012 12:54:13 -0500, Ben Davis ent...@cantab.net wrote:
Hi,
Is there a reason 'ref' is disallowed for local variables? I want to
write something like:
MapTile[] map; // It's a struct
ref MapTile tile=map[y*w+x];
tile.id=something;
tile.isWall=true;
My actual case is more complicated, so inlining the expression
everywhere would be messy. I can't use 'with' because I sometimes pass
'tile' to a function (which also takes it as a ref). I don't want to
make it a class since the array is quite big and that would be a lot of
extra overhead. For now I'm using pointers, but this is forcing me to
insert or * operators sometimes, and it also reduces the temptation to
use 'ref' in the places where it IS allowed, since it's inconsistent.
I hope it's not a stupid question - it's my first one - but I couldn't
find an answer anywhere. I like most of what I've seen of D so far, and
I'm very glad to be able to leave C and C++ (mostly) behind!
My first inclination is to use pointers. D doesn't have - operator, so
pointers are seamless for your small example:
auto tile = map[y*w+x];
tile.id = something;
tile.isWall = true;
When you need to pass it to a ref function, or if you do any operators on
it, you would need to use *.
Another horribly inefficient option (if you don't use -inline) is this:
@property ref tile() { return map[y*w+x]; }
With new = syntax (in git head), this would probably be:
@property ref tile = map[y*w+x];
-Steve
Re: Ref local variables?
Steven Schveighoffer: With new = syntax (in git head), this would probably be: @property ref tile = map[y*w+x]; That's not currently supported: http://d.puremagic.com/issues/show_bug.cgi?id=7176 Bye, bearophile
Re: Ref local variables?
On Mon, 09 Jan 2012 09:27:06 -0500, bearophile bearophileh...@lycos.com wrote: Steven Schveighoffer: With new = syntax (in git head), this would probably be: @property ref tile = map[y*w+x]; That's not currently supported: http://d.puremagic.com/issues/show_bug.cgi?id=7176 The given function is not a method, but it is an inner-class function. But you are right, it's a named function not a lambda. I'm not too concerned about whether this version works or not, the other version is reasonably short. I wonder if this would work, but it wouldn't be inlined like my original example: auto tile = @property ref () = map[y*w+x]; -Steve
Ref local variables?
Hi, Is there a reason 'ref' is disallowed for local variables? I want to write something like: MapTile[] map; // It's a struct ref MapTile tile=map[y*w+x]; tile.id=something; tile.isWall=true; My actual case is more complicated, so inlining the expression everywhere would be messy. I can't use 'with' because I sometimes pass 'tile' to a function (which also takes it as a ref). I don't want to make it a class since the array is quite big and that would be a lot of extra overhead. For now I'm using pointers, but this is forcing me to insert or * operators sometimes, and it also reduces the temptation to use 'ref' in the places where it IS allowed, since it's inconsistent. I hope it's not a stupid question - it's my first one - but I couldn't find an answer anywhere. I like most of what I've seen of D so far, and I'm very glad to be able to leave C and C++ (mostly) behind! Thanks, Ben :)
Re: Ref local variables?
MapTile[] map; // It's a struct ref MapTile tile=map[y*w+x]; tile.id=something; tile.isWall=true; MapTile* tile = map[y*w+x];
Re: Ref local variables?
Sorry, didn't read the rest. ^^
Re: Ref local variables?
On 08.01.2012 18:54, Ben Davis wrote:
Hi,
Is there a reason 'ref' is disallowed for local variables? I want to
write something like:
MapTile[] map; // It's a struct
ref MapTile tile=map[y*w+x];
tile.id=something;
tile.isWall=true;
My actual case is more complicated, so inlining the expression
everywhere would be messy. I can't use 'with' because I sometimes pass
'tile' to a function (which also takes it as a ref). I don't want to
make it a class since the array is quite big and that would be a lot of
extra overhead. For now I'm using pointers, but this is forcing me to
insert or * operators sometimes, and it also reduces the temptation to
use 'ref' in the places where it IS allowed, since it's inconsistent.
I hope it's not a stupid question - it's my first one - but I couldn't
find an answer anywhere. I like most of what I've seen of D so far, and
I'm very glad to be able to leave C and C++ (mostly) behind!
Thanks,
Ben :)
I got something working, but only when using templates. Take the
following with a grain of salt as I'm a newbie myself.
struct MapTile {
string id;
}
enum w = 80, h = 25;
MapTile[w*h] map;
ref MapTile getTile(int x, int y) {
return map[y*w+x];
}
void f(T)(ref T tile) {
tile.id = f();
}
void g(ref MapTile tile) {
tile.id = g();
}
void main() {
// You can use auto ref return to set values directly
getTile(10,10).id = a;
assert(getTile(10,10).id == a);
// And using templated ref arguments, you can pass by reference
// note that I need to take the reference even when
// using auto ref return
auto tile = getTile(1,1);
f(tile);
assert(tile.id == f());
// But you'll need a dereference if not using a template
g(*tile);
assert(tile.id == g());
assert(getTile(1,1).id == g());
}
Re: Ref local variables?
I also meant to say: 80x25? Epic :D
On 08/01/2012 20:25, Ben Davis wrote:
There are two things going on in your example:
1. Use of 'auto' where I'm currently having to write 'MapTile*' and
wanted to write 'ref MapTile'. It will infer 'MapTile*'. Or, if you
forget the , then it will infer 'MapTile' and do the copy that I want
to avoid. So it might be slightly more error-prone this way.
2. Use of a template function, allowing f() to take any type, not just
MapTile. Interestingly you're passing MapTile*, AND the parameter is
ref, so f() could change the pointer itself for the caller if it wanted
to! Also, if you called f() elsewhere with just a MapTile, you'd have
two copies if the function kicking around. So it saves us having to
write the *, but at the expense of some messiness. (Also prevents a lot
of the compile-time checks for f() from happening until f() is used.)
At least that's assuming D templates are like C++ ones. I haven't read
up on D templates yet. :)
So unfortunately, neither solution really solves the problem.
I suspect 'ref' for local variables could be added, and hasn't because
it didn't figure in the specific use cases that someone had in mind when
implementing it for parameters and 'foreach' variables. But it's also
possible that (unlike in Java) local variables can do stuff that
parameters can't (which I don't know about), which makes it impossible
to allow 'ref' ones without invalidating a compile-time check or
something. Hence why I was asking. :)
Anyway, I hope I've been able to help a bit with your own D adventure :)
Ben :)
On 08/01/2012 19:27, simendsjo wrote:
I got something working, but only when using templates. Take the
following with a grain of salt as I'm a newbie myself.
struct MapTile {
string id;
}
enum w = 80, h = 25;
MapTile[w*h] map;
ref MapTile getTile(int x, int y) {
return map[y*w+x];
}
void f(T)(ref T tile) {
tile.id = f();
}
void g(ref MapTile tile) {
tile.id = g();
}
void main() {
// You can use auto ref return to set values directly
getTile(10,10).id = a;
assert(getTile(10,10).id == a);
// And using templated ref arguments, you can pass by reference
// note that I need to take the reference even when
// using auto ref return
auto tile = getTile(1,1);
f(tile);
assert(tile.id == f());
// But you'll need a dereference if not using a template
g(*tile);
assert(tile.id == g());
assert(getTile(1,1).id == g());
}
Re: Ref local variables?
There are two things going on in your example:
1. Use of 'auto' where I'm currently having to write 'MapTile*' and
wanted to write 'ref MapTile'. It will infer 'MapTile*'. Or, if you
forget the , then it will infer 'MapTile' and do the copy that I want
to avoid. So it might be slightly more error-prone this way.
2. Use of a template function, allowing f() to take any type, not just
MapTile. Interestingly you're passing MapTile*, AND the parameter is
ref, so f() could change the pointer itself for the caller if it wanted
to! Also, if you called f() elsewhere with just a MapTile, you'd have
two copies if the function kicking around. So it saves us having to
write the *, but at the expense of some messiness. (Also prevents a lot
of the compile-time checks for f() from happening until f() is used.)
At least that's assuming D templates are like C++ ones. I haven't read
up on D templates yet. :)
So unfortunately, neither solution really solves the problem.
I suspect 'ref' for local variables could be added, and hasn't because
it didn't figure in the specific use cases that someone had in mind when
implementing it for parameters and 'foreach' variables. But it's also
possible that (unlike in Java) local variables can do stuff that
parameters can't (which I don't know about), which makes it impossible
to allow 'ref' ones without invalidating a compile-time check or
something. Hence why I was asking. :)
Anyway, I hope I've been able to help a bit with your own D adventure :)
Ben :)
On 08/01/2012 19:27, simendsjo wrote:
I got something working, but only when using templates. Take the
following with a grain of salt as I'm a newbie myself.
struct MapTile {
string id;
}
enum w = 80, h = 25;
MapTile[w*h] map;
ref MapTile getTile(int x, int y) {
return map[y*w+x];
}
void f(T)(ref T tile) {
tile.id = f();
}
void g(ref MapTile tile) {
tile.id = g();
}
void main() {
// You can use auto ref return to set values directly
getTile(10,10).id = a;
assert(getTile(10,10).id == a);
// And using templated ref arguments, you can pass by reference
// note that I need to take the reference even when
// using auto ref return
auto tile = getTile(1,1);
f(tile);
assert(tile.id == f());
// But you'll need a dereference if not using a template
g(*tile);
assert(tile.id == g());
assert(getTile(1,1).id == g());
}
Re: Ref local variables?
On Sun, 08 Jan 2012 18:54:13 +0100, Ben Davis ent...@cantab.net wrote:
Hi,
Is there a reason 'ref' is disallowed for local variables? I want to
write something like:
MapTile[] map; // It's a struct
ref MapTile tile=map[y*w+x];
tile.id=something;
tile.isWall=true;
My actual case is more complicated, so inlining the expression
everywhere would be messy. I can't use 'with' because I sometimes pass
'tile' to a function (which also takes it as a ref). I don't want to
make it a class since the array is quite big and that would be a lot of
extra overhead. For now I'm using pointers, but this is forcing me to
insert or * operators sometimes, and it also reduces the temptation to
use 'ref' in the places where it IS allowed, since it's inconsistent.
I hope it's not a stupid question - it's my first one - but I couldn't
find an answer anywhere. I like most of what I've seen of D so far, and
I'm very glad to be able to leave C and C++ (mostly) behind!
Thanks,
Ben :)
Quick hack:
struct Ref( T ) {
private:
T* data;
public:
this( ref T value ) {
data = value;
}
ref inout(T) get( ) inout {
return *data;
}
alias get this;
}
Ref!T byRef( T )( ref T value ) {
return Ref!T( value );
}
unittest {
int a = 3;
Ref!int b = a;
b = 4;
assert( a == 4 );
auto c = byRef( a );
c = 5;
assert( a == 5 );
assert( b == 5 );
}
