Re: const after initialization / pointers, references and values
First of all thanks for your replies, they are useful. @Philippe: A pure function is ok for initializing default_nodes but not for nodes because a pure function can't read a file. The static this has the problem it needs know the initializer at compile time but I wanted to choose the initializer at run time. I know default_nodes can be changed to immutable, but then it will require an explicit cast to remove the immutability. The final class may be a good point, I'll try this one. @Jakob: Thank you, but I'll go with Philippe's suggestion. @Wyatt: Certainly ref parameters help a lot, but I'm trying to get a Node by returning (a reference to) it. Does the ref keyword apply to the return type? Next time I'll post to D.learn, sorry for misplacing my question. Regards, Vicente.
Re: const after initialization / pointers, references and values
Indeed the ref can be applied to the return type:
http://dlang.org/function.html#ref-functions
So, does the following code copy any data from nodes?
If that is the case this solution avoids the class storage,
avoids pointers and nodes is encapsulated as read-only, that's
great.
The program I'm working on is still incomplete, so I still can't
benchmark it to compare with all proposed solutions.
@safe:
import std.stdio;
import std.string;
struct Node {
Node[] block;
uint num = 0;
};
immutable uint LAST = -1;
Node[] default_nodes = [
{num:3},
{block:[{num:4},{num:6},{block:[{num:5},{num:7}]}]},
// ...
{num:LAST},
];
class NodeProvider{
// change to const after constructor
private Node[] nodes;
private void parse_file(char[] file_name){
// initialize nodes from file
for(auto i=0; i3; i++){
nodes.length++;
nodes[$-1].num=i;
}
nodes.length++;
nodes[$-1].num=LAST;
}
this(){
nodes = default_nodes;
}
this(char[] file_name){
parse_file(file_name);
}
ref const(Node) get_node(const uint index){
return nodes[index];
}
}
string NodetoString(ref const(Node) n){
string str = format(%u{ , n.num);
foreach(b;n.block)
str ~= NodetoString(b);
str ~= } ;
return str;
}
@system: // for writeln
int main(char[][] args){
NodeProvider np;
uint i;
if(args.length==2)
np = new NodeProvider(args[1]);
else
np = new NodeProvider();
for(i=0;;i++){
const(Node) node = np.get_node(i);
if(node.num==LAST)
break;
writeln(NodetoString(node));
}
return 0;
}
Re: const after initialization / pointers, references and values
On Friday, 22 August 2014 at 18:21:06 UTC, Vicente wrote:
@Wyatt:
Certainly ref parameters help a lot, but I'm trying to get a
Node by returning (a reference to) it. Does the ref keyword
apply to the return type?
I poked it a bit and came out with this. I _think_ it's working
as expected:
import std.stdio;
import std.string;
// struct or class
struct Node {
Node[] block;
uint num = 0;
};
// static immutable
Node[] default_nodes = [
{num:3},
{block:[{num:4},{num:6},{block:[{num:5},{num:7}]}]},
// ...
];
class NodeProvider{
// change to const after constructor
private Node[] nodes;
private void parse_file(char[] file_name){
// initialize nodes from file
for(auto i=0; i3; i++){
nodes.length++;
nodes[$-1].num=i;
}
}
this(){
nodes = default_nodes;
}
this(char[] file_name){
parse_file(file_name);
}
auto ref opSlice(){return nodes[];};
}
string NodetoString(ref Node n){
string str = format(%u{ , n.num);
foreach(b;n.block)
str ~= NodetoString(b);
str ~= } ;
return str;
}
int main(char[][] args){
NodeProvider np;
if(args.length==2)
np = new NodeProvider(args[1]);
else
np = new NodeProvider();
foreach(node_ref; np){
writeln(NodetoString(node_ref));
}
return 0;
}
-Wyatt
Re: const after initialization / pointers, references and values
On Friday, 22 August 2014 at 20:12:39 UTC, Wyatt wrote:
I poked it a bit and came out with this. I _think_ it's
working as expected:
...
auto ref opSlice(){return nodes[];};
...
-Wyatt
Assuming it's working as expected, that is exactly what I was
looking for!
But the following code shows that at some point data gets copied.
By the way, neither works what I've posted previously.
In any case, thank you very much Wyatt.
Regards,
Vicente.
@safe:
import std.stdio;
import std.string;
struct Node { Node[] block; uint num = 0; };
/* immutable */ Node[] default_nodes = [ {num:3},
{block:[{num:4}]}, {num:6} ];
class NodeProvider{
private /* const */ Node[] nodes;
private Node[] tmp_nodes;
private void parse_file(char[] file_name){
tmp_nodes.length=1; tmp_nodes[0].num=5;
}
this(){ nodes = default_nodes; }
this(char[] file_name){
parse_file(file_name);
nodes = tmp_nodes;
}
auto ref opSlice(){return nodes[];};
}
@system: // for writeln
int main(char[][] args){
NodeProvider np;
if(args.length==2) np = new NodeProvider(args[1]);
else np = new NodeProvider();
// if they are real references this will modify the source data
foreach(node_ref; np) node_ref.num = 101;
// and will print all 101
foreach(node_ref; np) writeln(node_ref.num);
// but it prints unmodified 3, 0 and 6
return 0;
}
Re: const after initialization / pointers, references and values
If the foreach loop is replaced by: foreach(ref node_ref; np) Then it works like a charm!
Re: const after initialization / pointers, references and values
On Wednesday, 20 August 2014 at 21:26:55 UTC, Philippe Sigaud via Digitalmars-d wrote: If you want reference semantics but do not want to have pointers in your code, yes classes are your best choice. Alternatively, isn't this a good place to use ref parameters? Or is there some semantic I'm missing here? http://dlang.org/function.html#parameters BTW, Vicente, the D.learn section is a great place to get help. :) -Wyatt
Re: const after initialization / pointers, references and values
On Wed, Aug 20, 2014 at 11:49 PM, Jakob Ovrum via Digitalmars-d digitalmars-d@puremagic.com wrote: Certainly the easiest, but I don't think it's always the best. If light-weightedness is desired, make the struct contain the reference, effectively making the struct a reference type Well, yes. But the OP explicitly asked not to have to deal with pointers. So...
const after initialization / pointers, references and values
Hello,
I have some questions on how to do a few things in D.
Below is an example code on which the questions are based.
I need read access to a big and complex (i.e.: nested) data
structure of type Node.
But first it needs to be initialized. That can be done with a
default initializer or from a file.
After initialization there is no need to modify the data anymore.
My first question is: can the data be changed to const once
initialized?
if no, is there any alternative to acomplish that?
note that encapsulating into a class with only get_* methods
does not help because the get_node method returns a pointer to
the data
Related to that, how can the default initializer be changed to
immutable?
The second question is related to pointers, references and values
I know that structs by default use value semantics, but as data
is large I want to avoid data copying.
But I would like to avoid use of pointers, so, is there a way of
using reference semantics in the example code?
Maybe is as simple as changing from struct Node to class
Node, but seems intuitive that classes carry more overhead than
structs.
How much overhead carries a class in comparison to a struct?
Regards,
Vicente.
import std.stdio;
import std.string;
// struct or class
struct Node {
Node[] block;
uint num = 0;
};
// static immutable
Node[] default_nodes = [
{num:3},
{block:[{num:4},{num:6},{block:[{num:5},{num:7}]}]},
// ...
];
class NodeProvider{
// change to const after constructor
private Node[] nodes;
private void parse_file(char[] file_name){
// initialize nodes from file
for(auto i=0; i3; i++){
nodes.length++;
nodes[$-1].num=i;
}
}
this(){
nodes = default_nodes;
}
this(char[] file_name){
parse_file(file_name);
}
Node* get_node(const uint index){
if(index=nodes.length)
return null;
return (nodes[index]);
}
}
string NodetoString(Node n){
string str = format(%u{ , n.num);
foreach(b;n.block)
str ~= NodetoString(b);
str ~= } ;
return str;
}
int main(char[][] args){
NodeProvider np;
Node* node_ptr;
uint i;
if(args.length==2)
np = new NodeProvider(args[1]);
else
np = new NodeProvider();
i = 0;
while((node_ptr=np.get_node(i))!=null){
writeln(NodetoString(*node_ptr));
i++;
}
return 0;
}
Re: const after initialization / pointers, references and values
After initialization there is no need to modify the data anymore.
My first question is: can the data be changed to const once
initialized?
You can have const or immutable data, that can be initialized once.
I tend to use pure functions to do that:
struct Node {
Node[] block;
uint num = 0;
}
Node makeDefaultNode() pure {
return Node(null, 1);
}
void main(){
immutable Node n = makeDefaultNode();
import std.stdio;
writeln(n);
writeln(typeof(n).stringof);// immutable(Node)
}
You can also use module constructors:
module foo;
immutable myDefault;
static this() {
myDefault = ...
}
/* rest of code */
I cannot comment much more. I use functions to initialize my immutable
values. I don't know much about module constructors.
Related to that, how can the default initializer be changed to
immutable?
In your example code, your default value can be marked immutable, if
you want it.
Just use:
immutable Node[] default_nodes = [
{num:3},
{block:[{num:4},{num:6},{block:[{num:5},{num:7}]}]},
// ...
];
The second question is related to pointers, references and values
I know that structs by default use value semantics, but as data
is large I want to avoid data copying.
But I would like to avoid use of pointers, so, is there a way of
using reference semantics in the example code?
Maybe is as simple as changing from struct Node to class
Node, but seems intuitive that classes carry more overhead than
structs.
If you want reference semantics but do not want to have pointers in
your code, yes classes are your best choice.
How much overhead carries a class in comparison to a struct?
There is an overhead when calling functions, due to the virtual table.
But that's mainly when you construct hierarchies of classes, so that
the runtime can determine which method to call for each object. If you
don't need derived classes, then I guess using a final class is your
best bet. I think it discards the virtual table.
final class { ... }
I'd say: test it. Duplicate your module, change your structs to final
classes and compare the speed of the two modules.
Re: const after initialization / pointers, references and values
On Wednesday, 20 August 2014 at 21:26:55 UTC, Philippe Sigaud via
Digitalmars-d wrote:
If you want reference semantics but do not want to have
pointers in
your code, yes classes are your best choice.
Certainly the easiest, but I don't think it's always the best. If
light-weightedness is desired, make the struct contain the
reference, effectively making the struct a reference type:
---
private struct Data
{
int a;
string b;
double c;
}
/// Lorem ipsum ...
struct UserFacingType
{
private Data* data;
// use data.foo
}
---
Of course, this means UserFacingType.init is essentially `null`,
just like class references. By using std.typecons.RefCounted, the
user-facing type can also easily be made a reference-counted
reference.
Structs can do everything classes can with enough boilerplate.
Leverage templated types like RefCounted to remove the
boilerplate. Classes are just an in-built convenience feature to
handle the boilerplate of virtual functions.
