www.digitalmars.com         C & C++   DMDScript  

digitalmars.D.learn - std.conv - custom string to struct conversion ?

reply "Douglas Petterson" <someone somewhere.nz> writes:
Is there a way to convert a string to a struct, similarly to the 
conversion of a struct to a custom string representation ?

The following code works fine when using format("%s",C) or 
to!string(C):

     struct sCustomConv
     {
         string toString()
         {
             return "My custom representation";
         }
     }

     sCustomConv C;
     string MyBackup = to!string(C);

But I can't find which class operator must be overloaded (or more 
simply how-to) to make this possible:

     C = to!sCustomConv(MyBackup);

Thx.
Aug 01 2013
parent reply Jonathan M Davis <jmdavisProg gmx.com> writes:
On Friday, August 02, 2013 06:50:01 Douglas Petterson wrote:

 Is there a way to convert a string to a struct, similarly to the
 conversion of a struct to a custom string representation ?
 
 The following code works fine when using format("%s",C) or
 to!string(C):
 
      struct sCustomConv
      {
          string toString()
          {
              return "My custom representation";
          }
      }
 
      sCustomConv C;
      string MyBackup = to!string(C);
 
 But I can't find which class operator must be overloaded (or more
 simply how-to) to make this possible:
 
      C = to!sCustomConv(MyBackup);


http://stackoverflow.com/questions/8351245/override-tot-for-used-defined-t-in-d

- Jonathan M Davis
Aug 01 2013
next sibling parent reply "monarch_dodra" <monarchdodra gmail.com> writes:
On Friday, 2 August 2013 at 05:09:14 UTC, Jonathan M Davis wrote:

 On Friday, August 02, 2013 06:50:01 Douglas Petterson wrote:

 Is there a way to convert a string to a struct, similarly to 
 the
 conversion of a struct to a custom string representation ?
 
 The following code works fine when using format("%s",C) or
 to!string(C):
 
      struct sCustomConv
      {
          string toString()
          {
              return "My custom representation";
          }
      }
 
      sCustomConv C;
      string MyBackup = to!string(C);
 
 But I can't find which class operator must be overloaded (or 
 more
 simply how-to) to make this possible:
 
      C = to!sCustomConv(MyBackup);


 http://stackoverflow.com/questions/8351245/override-tot-for-used-defined-t-in-d

 - Jonathan M Davis


There was a request for enhancement to provide a "fromString" for 
arbitrary types. The idea is that once we have that, then 
functions such as parse or to!S(string) will be generic, and work 
on mostly anything. Unfortunatly, (AFAIK), nobody is working on 
this.

Here is a discussion I opened:
http://forum.dlang.org/thread/gzodptjyzpqnhxctbbuv forum.dlang.org

...and I just noticed it ends with me saying "I'll try to write a 
DIP then :)" and then not doing it :/
Aug 01 2013
parent reply Jonathan M Davis <jmdavisProg gmx.com> writes:
On Friday, August 02, 2013 08:40:51 monarch_dodra wrote:

 There was a request for enhancement to provide a "fromString" for
 arbitrary types. The idea is that once we have that, then
 functions such as parse or to!S(string) will be generic, and work
 on mostly anything. Unfortunatly, (AFAIK), nobody is working on
 this.
 
 Here is a discussion I opened:
 http://forum.dlang.org/thread/gzodptjyzpqnhxctbbuv forum.dlang.org
 
 ...and I just noticed it ends with me saying "I'll try to write a
 DIP then :)" and then not doing it :/


What's the point of fromString when a constructor will do the job just fine?

- Jonathan M Davis
Aug 02 2013
parent reply "monarch_dodra" <monarchdodra gmail.com> writes:
On Friday, 2 August 2013 at 07:59:59 UTC, Jonathan M Davis wrote:

 On Friday, August 02, 2013 08:40:51 monarch_dodra wrote:

 There was a request for enhancement to provide a "fromString" 
 for
 arbitrary types. The idea is that once we have that, then
 functions such as parse or to!S(string) will be generic, and 
 work
 on mostly anything. Unfortunatly, (AFAIK), nobody is working on
 this.
 
 Here is a discussion I opened:
 http://forum.dlang.org/thread/gzodptjyzpqnhxctbbuv forum.dlang.org
 
 ...and I just noticed it ends with me saying "I'll try to 
 write a
 DIP then :)" and then not doing it :/


 What's the point of fromString when a constructor will do the 
 job just fine?

 - Jonathan M Davis


Really? Because it's not generic. Unless I'm missing something, 
I'm sure you can appreciate that the goal is to allow:

to!S("string")
or
myFile.readf("Entry: %s", &s);

And a simple constructor doesn't (*CAN'T*) allow that.
Aug 02 2013
parent reply Jonathan M Davis <jmdavisProg gmx.com> writes:
On Friday, August 02, 2013 10:35:22 monarch_dodra wrote:

 On Friday, 2 August 2013 at 07:59:59 UTC, Jonathan M Davis wrote:

 On Friday, August 02, 2013 08:40:51 monarch_dodra wrote:

 There was a request for enhancement to provide a "fromString"
 for
 arbitrary types. The idea is that once we have that, then
 functions such as parse or to!S(string) will be generic, and
 work
 on mostly anything. Unfortunatly, (AFAIK), nobody is working on
 this.
 
 Here is a discussion I opened:
 http://forum.dlang.org/thread/gzodptjyzpqnhxctbbuv forum.dlang.org
 
 ...and I just noticed it ends with me saying "I'll try to
 write a
 DIP then :)" and then not doing it :/


 
 What's the point of fromString when a constructor will do the
 job just fine?
 
 - Jonathan M Davis


 
 Really? Because it's not generic. Unless I'm missing something,
 I'm sure you can appreciate that the goal is to allow:
 
 to!S("string")


As long as S defines a constructor which takes a string, this works just fine. 
All fromString would be doing would be to declare a function which did the 
same thing as the constructor.


 or
 myFile.readf("Entry: %s", &s);
 
 And a simple constructor doesn't (*CAN'T*) allow that.


Really? All it would have to do wolud be to pass the string that is the user 
input into the constructor for typeof(s) and set s to that.

- Jonathan M Davis
Aug 02 2013
parent reply "monarch_dodra" <monarchdodra gmail.com> writes:
On Friday, 2 August 2013 at 09:11:26 UTC, Jonathan M Davis wrote:

 On Friday, August 02, 2013 10:35:22 monarch_dodra wrote:

 On Friday, 2 August 2013 at 07:59:59 UTC, Jonathan M Davis 
 wrote:

 On Friday, August 02, 2013 08:40:51 monarch_dodra wrote:

 There was a request for enhancement to provide a 
 "fromString"
 for
 arbitrary types. The idea is that once we have that, then
 functions such as parse or to!S(string) will be generic, and
 work
 on mostly anything. Unfortunatly, (AFAIK), nobody is 
 working on
 this.
 
 Here is a discussion I opened:
 http://forum.dlang.org/thread/gzodptjyzpqnhxctbbuv forum.dlang.org
 
 ...and I just noticed it ends with me saying "I'll try to
 write a
 DIP then :)" and then not doing it :/


 
 What's the point of fromString when a constructor will do the
 job just fine?
 
 - Jonathan M Davis


 
 Really? Because it's not generic. Unless I'm missing something,
 I'm sure you can appreciate that the goal is to allow:
 
 to!S("string")


 As long as S defines a constructor which takes a string, this 
 works just fine.
 All fromString would be doing would be to declare a function 
 which did the
 same thing as the constructor.


Right. That's what to! does. In that case, I'll correct myself, 
and say:

parse!S("string")

wouldn't work.


 or
 myFile.readf("Entry: %s", &s);
 
 And a simple constructor doesn't (*CAN'T*) allow that.


 Really? All it would have to do wolud be to pass the string 
 that is the user
 input into the constructor for typeof(s) and set s to that.

 - Jonathan M Davis


It wouldn't because "myFile.readf("Entry: %s %s", &s, &s);" 
wouldn't know *what* to pass to the constructor. AFAIK. I could 
be wrong.

Regardless, a constructor can only be explicit, whereas a generic 
"fromString" can be implemented by default for structs, just the 
same way you don't ask users to write a toString.

Given:

struct Person
{
   string name;
}

Then:
parse!Person(`Person("Jonathan")`);
Should "just work".

Finally, a constructor is meant to construct an object, and not 
unserialize it. There could be notable differences in both.

auto myName = to!Person("Jonathan"); //Fine
parse!Person(`Person("Jonathan")`);

As you can see, the strings don't match. A constructor may not be 
the reciprocal of toString. You simply *can't* call a constructor 
in a parse operation.
Aug 02 2013
next sibling parent "bearophile" <bearophileHUGS lycos.com> writes:
At a first sight the idea of having a standard fromString seems 
good.

I think Haskell has something similar, the standard Read 
typeclass:

http://hackage.haskell.org/packages/archive/base/latest/doc/html/Prelude.html#t:Read

Bye,
bearophile
Aug 02 2013
prev sibling parent reply "Dicebot" <public dicebot.lv> writes:
On Friday, 2 August 2013 at 09:59:12 UTC, monarch_dodra wrote:

 It wouldn't because "myFile.readf("Entry: %s %s", &s, &s);" 
 wouldn't know *what* to pass to the constructor. AFAIK. I could 
 be wrong.


It should. Pattern matching is not target-aware for readf 
patterns. It will simply find a whitespace separators and pass 
everything to the left to one constructor and everything to the 
right to the second one (trimming extra whitespaces).
Aug 02 2013
parent "monarch_dodra" <monarchdodra gmail.com> writes:
On Friday, 2 August 2013 at 14:02:17 UTC, Dicebot wrote:

 On Friday, 2 August 2013 at 09:59:12 UTC, monarch_dodra wrote:

 It wouldn't because "myFile.readf("Entry: %s %s", &s, &s);" 
 wouldn't know *what* to pass to the constructor. AFAIK. I 
 could be wrong.


 It should. Pattern matching is not target-aware for readf 
 patterns. It will simply find a whitespace separators and pass 
 everything to the left to one constructor and everything to the 
 right to the second one (trimming extra whitespaces).


Right, but in that case, the constructor isn't
this(string iString)
{
   //Copy string here
}

but rather
this(ref string ioString)
{
     //Use the string, and pop off all used elements.
}

There's a pretty big difference implementation wise.

Also, since we passed a S*, we can't call the constructor anymore 
(unless you emplace I guess)
Aug 02 2013
prev sibling parent "Douglas Petterson" <someone somewhere.nz> writes:
On Friday, 2 August 2013 at 05:09:14 UTC, Jonathan M Davis wrote:

 On Friday, August 02, 2013 06:50:01 Douglas Petterson wrote:

 Is there a way to convert a string to a struct, similarly to 
 the
 conversion of a struct to a custom string representation ?
 
 The following code works fine when using format("%s",C) or
 to!string(C):
 
      struct sCustomConv
      {
          string toString()
          {
              return "My custom representation";
          }
      }
 
      sCustomConv C;
      string MyBackup = to!string(C);
 
 But I can't find which class operator must be overloaded (or 
 more
 simply how-to) to make this possible:
 
      C = to!sCustomConv(MyBackup);


 http://stackoverflow.com/questions/8351245/override-tot-for-used-defined-t-in-d

 - Jonathan M Davis


That perfectly fits to my need, thx.
Aug 02 2013