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digitalmars.D.learn - Question about template mixins as nested functions.

reply Gerald Stocker <gobstocker gmail.com> writes:
Hello,

I have a question about using a template mixin to add a nested function 
to a class method, when the nested function references class member 
variables. For example:

class SomeClass
{
     int memberVar;
     void doSomething()
     {
         int localVar;
         mixin HelperFunction;	// see below
         helperFunction();
     }
}

template HelperFunction()
{
     void helperFunction()
     {
         localVar++;	// This works fine
         memberVar++;	// Error: need 'this' to access member memberVar
     }
}

I'm not sure why the mixed-in nested function can't access member 
variables of the enclosing class (it works if I just write the nested 
function directly inside the class method). Is this a bug, or (more 
likely) have I not fully grasped the ins and outs of using template mixins?

Thanks,
Gerald Stocker
Feb 15 2007
parent reply Derek Parnell <derek nomail.afraid.org> writes:
On Fri, 16 Feb 2007 00:31:07 -0500, Gerald Stocker wrote:


 Hello,
 
 I have a question about using a template mixin to add a nested function 
 to a class method...


I have trouble with mixin templates too. I believe the effect you see is
because the mixin introduces its own scope and you have to qualify
reference that are outside the mixin.

However, the new mixin(Statement) might be useful to you ...

-------------
import std.stdio;
class SomeClass
{
     int memberVar;
     void doSomething()
     {
        int localVar;
        mixin(helperFunction);
        helperFunction();
     }
}

const char[] helperFunction = "
     void helperFunction()
     {
         localVar++;
         memberVar++;
     }
    "; 
void main()
{
    SomeClass x = new SomeClass();
    x.doSomething();
    writefln("memberVar = %s", x.memberVar);
    x.doSomething();
    writefln("memberVar = %s", x.memberVar);
}
--------------



-- 
Derek
(skype: derek.j.parnell)
Melbourne, Australia
"Justice for David Hicks!"
16/02/2007 4:49:50 PM
Feb 15 2007
parent Gerald Stocker <gobstocker gmail.com> writes:
Derek Parnell wrote:

 On Fri, 16 Feb 2007 00:31:07 -0500, Gerald Stocker wrote:
 

 Hello,

 I have a question about using a template mixin to add a nested function 
 to a class method...


 
 I have trouble with mixin templates too. I believe the effect you see is
 because the mixin introduces its own scope and you have to qualify
 reference that are outside the mixin.
 
 However, the new mixin(Statement) might be useful to you ...
 


It is, thanks! It turns out several of the things I was trying (and 
sometimes failing) to do with template mixins make more sense as 
statment mixins.

--Gerald
Feb 15 2007