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digitalmars.D.learn - desired side effect

reply JP <Joris.Putcuyps skynet.be> writes:
Hi

I'm trying to change the value of the locale variable 'var' in main() 
via a class.
Can someone please tell me what I'm doing wrong?

output:
=======
B.m_val.m_value=test
main var=test
B.m_val.m_value=test2
main var=test -> can't change this to 'test2'!

code:
=====
class A
{
	char[] m_value;
}


class B
{
	A m_val;

	char[] NewVal( char[] val )
	{
		m_val = new A;
		m_val.m_value = val;
		printf("B.m_val.m_value=%s\n", 	cast(char*)m_val.m_value);
		return m_val.m_value;
	}

	void Set( char[] val )
	{
		m_val.m_value = val;
		printf("B.m_val.m_value=%s\n", cast(char*)m_val.m_value);
	}
}


int main()
{
	auto b = new B;
	char[] var = b.NewVal("test");
	printf("main var=%s\n", cast(char*)var);
	b.Set("test2");
	printf("main var=%s\n", cast(char*)var);
	return 0;
}


thx

Joris
Dec 23 2005
next sibling parent Manfred Nowak <svv1999 hotmail.com> writes:
JP wrote:

[...]

 main var=test -> can't change this to 'test2'!


[...]

By design class B does not know anything about the variables one of 
its memebers is assigning to.

-manfred
Dec 23 2005
prev sibling parent reply BCS <BCS_member pathlink.com> writes:
JP wrote:


 class B
 {


[...]

     void Set( char[] val )
     {
         m_val.m_value = val;		//<<< this line
         printf("B.m_val.m_value=%s\n", cast(char*)m_val.m_value);
     }
 }
 
 


I think the problem is in the marked line.

What this line does is to change what m_val.m_value points to. Because 
m_val.m_value is a different pointer than main.var, changing it doesn't 
effect the array in main. To change var, you need to change it's 
contents, and as far as I know there is no way to do that without some 
sort of reference to main.var.

A number of solutions occur to me but which would work depends on what 
you are doing. Hope this helps.
Dec 23 2005
next sibling parent reply Chris Sauls <ibisbasenji gmail.com> writes:
BCS wrote:

 JP wrote:
 

 class B
 {


 
 [...]
 

     void Set( char[] val )
     {
         m_val.m_value = val;        //<<< this line
         printf("B.m_val.m_value=%s\n", cast(char*)m_val.m_value);
     }
 }


 I think the problem is in the marked line.
 
 What this line does is to change what m_val.m_value points to. Because 
 m_val.m_value is a different pointer than main.var, changing it doesn't 
 effect the array in main. To change var, you need to change it's 
 contents, and as far as I know there is no way to do that without some 
 sort of reference to main.var.
 
 A number of solutions occur to me but which would work depends on what 
 you are doing. Hope this helps.


I */think/* he could pull it off by using slice semantics.  I haven't tested
it, though.

-- Chris Sauls
Dec 23 2005
parent BCS <BCS_member pathlink.com> writes:
Chris Sauls wrote:

 BCS wrote:
 

 JP wrote:


 class B
 {



 [...]


     void Set( char[] val )
     {
         m_val.m_value = val;        //<<< this line
         printf("B.m_val.m_value=%s\n", cast(char*)m_val.m_value);
     }
 }


 I think the problem is in the marked line.

 What this line does is to change what m_val.m_value points to. Because 
 m_val.m_value is a different pointer than main.var, changing it 
 doesn't effect the array in main. To change var, you need to change 
 it's contents, and as far as I know there is no way to do that without 
 some sort of reference to main.var.

 A number of solutions occur to me but which would work depends on what 
 you are doing. Hope this helps.


 
 
 I */think/* he could pull it off by using slice semantics.  I haven't 
 tested it, though.
 
 -- Chris Sauls



yes, maybe but only if he doesn't have to extend the array. 
Concatenate/extend will sometimes move the data and change your array's ptr.
Dec 27 2005
prev sibling parent JP <Joris.Putcuyps skynet.be> writes:
BCS wrote:

 JP wrote:
 

 class B
 {


 
 [...]
 

     void Set( char[] val )
     {
         m_val.m_value = val;        //<<< this line
         printf("B.m_val.m_value=%s\n", cast(char*)m_val.m_value);
     }
 }


 I think the problem is in the marked line.
 
 What this line does is to change what m_val.m_value points to. Because 
 m_val.m_value is a different pointer than main.var, changing it doesn't 
 effect the array in main. To change var, you need to change it's 
 contents, and as far as I know there is no way to do that without some 
 sort of reference to main.var.
 
 A number of solutions occur to me but which would work depends on what 
 you are doing. Hope this helps.


Thanks for your reply.

I was writing a command line parser, but couldn't get a reference to the 
variables, which would have been neat:


	auto cmd = new Parser;

	char[] verbose = cmd.AddOption("verbose", "--verbose", "print more to 
stdout");
	char[] connectstring = cmd.AddArgument("connectstring", 
"username/password db");
	char[] database = cmd.AddArgument("table", "table to export");

	cmd.Parse(args);

	printf("connectstring=%s\n", cast(char*)connectstring);
Dec 24 2005