digitalmars.D - Serialization/deserialization of templated class

Dmitry Solomennikov (29/29) Jun 27 2017 Hi, guys!

Eugene Wissner (20/49) Jun 27 2017 Templates are a compile time feature. You won't be able to

Dmitry Solomennikov (8/22) Jun 27 2017 It is possible, but it is not a general solution. I've posted

Eugene Wissner (7/30) Jun 27 2017 As far as I know, Java can modify the byte code at runtime. D is
Jacob Carlborg (20/41) Jun 28 2017 It's possible to instantiate classes using reflection in D, but not for
Steven Schveighoffer (15/41) Jun 29 2017 No, because the class itself doesn't eixst until you instantiate it.

Dmitry Solomennikov <dmitrys99 yandex.ru> writes:

Hi, guys!

I have templated classes like this:

class Some(T1) {
   T1 value;
   this(T1 _value){
     value = _value;
   }
}
class Pair(T1, T2) {
   T1 first;
   T2 second;
   this(T1 _first, T2 _second){
     first = _first;
     second = _second;
   }
}

and a lot of serialised data, which I have to read and 
instantiate into given classes.
Serialized data looks like this: 'Some(Pair("df", 5.0))' or 
'Pair(Some(true), 11)', i.e. order of structs, its types and 
inclusion order are unknown at compile time.

I know there are several serialisation libraries, but I found 
they work only with non-templated classes, like:

class Foo {
   int a;
}

I can read input data and parse it, but have no idea how to 
construct object of templated class in this situation. Please 
give a hint.

Jun 27 2017

Eugene Wissner <belka caraus.de> writes:

On Wednesday, 28 June 2017 at 04:41:25 UTC, Dmitry Solomennikov 
wrote:
 Hi, guys!

 I have templated classes like this:

 class Some(T1) {
   T1 value;
   this(T1 _value){
     value = _value;
   }
 }
 class Pair(T1, T2) {
   T1 first;
   T2 second;
   this(T1 _first, T2 _second){
     first = _first;
     second = _second;
   }
 }

 and a lot of serialised data, which I have to read and 
 instantiate into given classes.
 Serialized data looks like this: 'Some(Pair("df", 5.0))' or 
 'Pair(Some(true), 11)', i.e. order of structs, its types and 
 inclusion order are unknown at compile time.

 I know there are several serialisation libraries, but I found 
 they work only with non-templated classes, like:

 class Foo {
   int a;
 }

 I can read input data and parse it, but have no idea how to 
 construct object of templated class in this situation. Please 
 give a hint.

Templates are a compile time feature. You won't be able to 
initialize the classes if you don't know the type at compile time.

You can use some data structure like Variant that can save values 
of different types.
You could use union for data storage in you class and list all 
possible types as union members.
Probably if you have serialized data, you convert strings to 
other types, so it may be possible to perfom if-checks:
if (myDataIsStringAndDouble(data))
{
   auto var = new Some!(Pair!(string, double))(new Pair!(string, 
double)("df", 5.0));
}
else if (myDataIsStringAndInt(data))
{
   auto var = new Some!(Pair!(string, int))(new Pair!(string, 
int)("df", 5));
}

Jun 27 2017

Dmitry Solomennikov <dmitrys99 yandex.ru> writes:

On Wednesday, 28 June 2017 at 05:01:17 UTC, Eugene Wissner wrote:
 On Wednesday, 28 June 2017 at 04:41:25 UTC, Dmitry Solomennikov 
 wrote:


 Probably if you have serialized data, you convert strings to 
 other types, so it may be possible to perfom if-checks:
 if (myDataIsStringAndDouble(data))
 {
   auto var = new Some!(Pair!(string, double))(new Pair!(string, 
 double)("df", 5.0));
 }
 else if (myDataIsStringAndInt(data))
 {
   auto var = new Some!(Pair!(string, int))(new Pair!(string, 
 int)("df", 5));
 }

It is possible, but it is not a general solution. I've posted 
couple of sample classes, but there are more complicated cases, 
of course, and it well be combinatorial explosion here.

I got the Variant idea, I'll give it a try.
 From other point of view, is there a reflection, say

auto i = newInstance("Pair!(int, string)(10, \"asdf\")"),

something like in Java?

Jun 27 2017

Eugene Wissner <belka caraus.de> writes:

On Wednesday, 28 June 2017 at 05:52:38 UTC, Dmitry Solomennikov 
wrote:
 On Wednesday, 28 June 2017 at 05:01:17 UTC, Eugene Wissner 
 wrote:
 On Wednesday, 28 June 2017 at 04:41:25 UTC, Dmitry 
 Solomennikov wrote:


 Probably if you have serialized data, you convert strings to 
 other types, so it may be possible to perfom if-checks:
 if (myDataIsStringAndDouble(data))
 {
   auto var = new Some!(Pair!(string, double))(new 
 Pair!(string, double)("df", 5.0));
 }
 else if (myDataIsStringAndInt(data))
 {
   auto var = new Some!(Pair!(string, int))(new Pair!(string, 
 int)("df", 5));
 }

 It is possible, but it is not a general solution. I've posted 
 couple of sample classes, but there are more complicated cases, 
 of course, and it well be combinatorial explosion here.

 I got the Variant idea, I'll give it a try.
 From other point of view, is there a reflection, say

 auto i = newInstance("Pair!(int, string)(10, \"asdf\")"),

 something like in Java?

As far as I know, Java can modify the byte code at runtime. D is 
a compiled language. It has good compile-time reflection. Anyway 
you have to find a way to initialize the classes with the types 
you need. If the type isn't initialized at compile-time, it isn't 
just there at runtime and can't be used.

Jun 27 2017

Jacob Carlborg <doob me.com> writes:

On 2017-06-28 07:52, Dmitry Solomennikov wrote:
 On Wednesday, 28 June 2017 at 05:01:17 UTC, Eugene Wissner wrote:
 On Wednesday, 28 June 2017 at 04:41:25 UTC, Dmitry Solomennikov wrote:


 Probably if you have serialized data, you convert strings to other
 types, so it may be possible to perfom if-checks:
 if (myDataIsStringAndDouble(data))
 {
   auto var = new Some!(Pair!(string, double))(new Pair!(string,
 double)("df", 5.0));
 }
 else if (myDataIsStringAndInt(data))
 {
   auto var = new Some!(Pair!(string, int))(new Pair!(string,
 int)("df", 5));
 }

 It is possible, but it is not a general solution. I've posted couple of
 sample classes, but there are more complicated cases, of course, and it
 well be combinatorial explosion here.

 I got the Variant idea, I'll give it a try.
  From other point of view, is there a reflection, say

 auto i = newInstance("Pair!(int, string)(10, \"asdf\")"),

 something like in Java?

It's possible to instantiate classes using reflection in D, but not for 
templated classes:

class Foo(T) {}
class Bar {}

void main()
{
     Foo!int a = new Foo!int;
     Bar b = new Bar;

     auto o1 = Object.factory("main.Foo!int.Foo"); // does not work for 
templated classes
     auto o2 = Object.factory("main.Bar"); // works for non-templated 
classes

     assert(o1 is null);
     assert(o2 !is null);
}

Actually, if you have the template instantiation available you can do this:

Object o = Foo!int.classinfo.create();

-- 
/Jacob Carlborg

Jun 28 2017

Steven Schveighoffer <schveiguy yahoo.com> writes:

On 6/28/17 1:52 AM, Dmitry Solomennikov wrote:
 On Wednesday, 28 June 2017 at 05:01:17 UTC, Eugene Wissner wrote:
 On Wednesday, 28 June 2017 at 04:41:25 UTC, Dmitry Solomennikov wrote:

 
 
 Probably if you have serialized data, you convert strings to other 
 types, so it may be possible to perfom if-checks:
 if (myDataIsStringAndDouble(data))
 {
   auto var = new Some!(Pair!(string, double))(new Pair!(string, 
 double)("df", 5.0));
 }
 else if (myDataIsStringAndInt(data))
 {
   auto var = new Some!(Pair!(string, int))(new Pair!(string, 
 int)("df", 5));
 }

 
 It is possible, but it is not a general solution. I've posted couple of 
 sample classes, but there are more complicated cases, of course, and it 
 well be combinatorial explosion here.
 
 I got the Variant idea, I'll give it a try.
  From other point of view, is there a reflection, say
 
 auto i = newInstance("Pair!(int, string)(10, \"asdf\")"),

No, because the class itself doesn't eixst until you instantiate it.

Certainly if you want to write out all the possible instantiations, it's 
possible, via the mechanisms already specified above. What I would do is 
create a newInstance *template* that takes a string and generates a 
function that would build it from that string. Then you can feed a 
sample file that contains all the possible instantiations to it at 
*compile time* (via import strings), and then you can automatically 
build the code that would be able to parse it. Finally, send the real 
file at runtime.

 something like in Java?

It's important to realize that Java's generics are completely different 
than D's templates. They are a compile-time wrapping around a runtime 
construct. Of course, it's possible to mimic Java, but you won't get 
templates out of it, more like Variant holders.

-Steve

Jun 29 2017

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