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digitalmars.D.learn - writeln an object

reply "gedaiu" <szabobogdan yahoo.com> writes:
Hi,

how i can control what writeln outputs when I pass an object 
parameter?

Thanks,
Bogdan
Apr 18 2013
parent reply "JN" <666total wp.pl> writes:
On Thursday, 18 April 2013 at 17:36:10 UTC, gedaiu wrote:

 Hi,

 how i can control what writeln outputs when I pass an object 
 parameter?

 Thanks,
 Bogdan


You can override the toString() method, like this 
http://dpaste.dzfl.pl/db7dbe28
Apr 18 2013
parent reply "gedaiu" <szabobogdan yahoo.com> writes:
On Thursday, 18 April 2013 at 17:42:53 UTC, JN wrote:

 On Thursday, 18 April 2013 at 17:36:10 UTC, gedaiu wrote:

 Hi,

 how i can control what writeln outputs when I pass an object 
 parameter?

 Thanks,
 Bogdan


 You can override the toString() method, like this 
 http://dpaste.dzfl.pl/db7dbe28


i've done that but i get this error:

Error: function base.Value.Value.toString cannot override a 
non-virtual function
Error: function base.Value.Value.toString override only applies 
to class member functions
Apr 18 2013
parent reply Andrej Mitrovic <andrej.mitrovich gmail.com> writes:
On 4/18/13, gedaiu <szabobogdan yahoo.com> wrote:

 i've done that but i get this error:

 Error: function base.Value.Value.toString cannot override a
 non-virtual function
 Error: function base.Value.Value.toString override only applies
 to class member functions


If it's a struct then don't put "override".
Apr 18 2013
parent reply "John Colvin" <john.loughran.colvin gmail.com> writes:
On Thursday, 18 April 2013 at 18:04:03 UTC, Andrej Mitrovic wrote:

 On 4/18/13, gedaiu <szabobogdan yahoo.com> wrote:

 i've done that but i get this error:

 Error: function base.Value.Value.toString cannot override a
 non-virtual function
 Error: function base.Value.Value.toString override only applies
 to class member functions


 If it's a struct then don't put "override".


Just to provide a bit more info:

Classes all derive from Object, which defines toString. Hence, 
you need to override it to define your own.

Structs don't have a parent (or any inheritance at all) and hence 
you don't override anything, you just define the method.
Apr 18 2013
parent reply "gedaiu" <szabobogdan yahoo.com> writes:
On Thursday, 18 April 2013 at 18:25:21 UTC, John Colvin wrote:

 On Thursday, 18 April 2013 at 18:04:03 UTC, Andrej Mitrovic 
 wrote:

 On 4/18/13, gedaiu <szabobogdan yahoo.com> wrote:

 i've done that but i get this error:

 Error: function base.Value.Value.toString cannot override a
 non-virtual function
 Error: function base.Value.Value.toString override only 
 applies
 to class member functions


 If it's a struct then don't put "override".


 Just to provide a bit more info:

 Classes all derive from Object, which defines toString. Hence, 
 you need to override it to define your own.

 Structs don't have a parent (or any inheritance at all) and 
 hence you don't override anything, you just define the method.


i'm realy sorry... it's my mistake...

i have a struct not an object. I have someting like this when i 
get the error:

struct Value {
	string strVal;

         this(string val) {
		strVal = val;
	}

         override string toString() {
		return strVal;
	}
}
Apr 18 2013
next sibling parent David <d dav1d.de> writes:
Just drop the override:


 struct Value {
     string strVal;
 
         this(string val) {
         strVal = val;
     }
 
         override string toString() {
         return strVal;
     }
 }



struct Value {
    string strVal;

        this(string val) {
        strVal = val;
    }

    string toString() {
        return strVal;
    }
}
Apr 18 2013
prev sibling parent reply "John Colvin" <john.loughran.colvin gmail.com> writes:
On Thursday, 18 April 2013 at 18:46:09 UTC, gedaiu wrote:

 i have a struct not an object.


There's a slight nomenclature clash here:

Object is the base class in D. Therefore one could say that an 
object is an instatiation of Object and therefore a class.

However, by a wider definition of the word, a struct could also 
be said to be an object.
Apr 18 2013
parent reply =?UTF-8?B?QWxpIMOHZWhyZWxp?= <acehreli yahoo.com> writes:
On 04/18/2013 12:37 PM, John Colvin wrote:


 However, by a wider definition of the word, a struct could also be said
 to be an object.


You are missing some words there. :) Not a struct itself, but instances 
of it are said to be objects.

Ali
Apr 18 2013
parent reply "gedaiu" <szabobogdan yahoo.com> writes:
Ok, i understand now the diference...

my question is how I should solve this problem?

Thanks,
Bogdan

On Thursday, 18 April 2013 at 20:57:23 UTC, Ali Çehreli wrote:

 On 04/18/2013 12:37 PM, John Colvin wrote:


 However, by a wider definition of the word, a struct could


 also be said

 to be an object.


 You are missing some words there. :) Not a struct itself, but 
 instances of it are said to be objects.

 Ali
Apr 19 2013
parent =?UTF-8?B?QWxpIMOHZWhyZWxp?= <acehreli yahoo.com> writes:
On 04/19/2013 01:35 PM, gedaiu wrote:


 Ok, i understand now the diference...

 my question is how I should solve this problem?


The keyword 'override' cannot be used with structs. You have at least 
two good options with structs:

1) Define a const toString() member function that returns the string 
representation of the object, conveniently produced by std.string.format:

import std.stdio;
import std.string;

struct TimeOfDay
{
     int hour;
     int minute;

     string toString() const
     {
         return format("%02s:%02s", hour, minute);
     }
}

void main()
{
     auto t = TimeOfDay(10, 20);
     writeln(t);
}

2) As the previous method is unnecessarily inefficient especially when 
the members are structs as well, define the toString() overload that 
takes a sink delegate. std.format.formattedWrite is smart enough to take 
advantage of it and avoids multiple string instances. Everything gets 
appended to the same output string:

import std.stdio;
import std.format;

struct TimeOfDay
{
     int hour;
     int minute;

     void toString(scope void delegate(const(char)[]) output) const
     {
         formattedWrite(output, "%02s:%02s", hour, minute);
     }
}

struct Duration
{
     TimeOfDay begin;
     TimeOfDay end;

     void toString(scope void delegate(const(char)[]) output) const
     {
         formattedWrite(output, "from %s to %s", begin, end);
     }
}

void main()
{
     auto d = Duration(TimeOfDay(10, 20), TimeOfDay(11, 22));
     writeln(d);
}

Ali
Apr 19 2013