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

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

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.

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

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);

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.