I'm trying to construct a struct template where one of the template parameters is passed by reference (as option)

struct S(T, U)
{
   void opCall(T t, U u) { }
}

alias X = S!(T, U);
alias RX = S!(T, ref U); //not working

The objective is  to call X(t, u) or RX(t, u), but in second case I want "u" to be passed by reference.

The only solution I found is to pass U as a pointer or as a class.

On Mon, 23 Mar 2015 22:01:03 +0000, rumbu wrote:

> I'm trying to construct a struct template where one of the template parameters is passed by reference (as option)
> 
> struct S(T, U)
> {
>     void opCall(T t, U u) { }
> }
> 
> alias X = S!(T, U);
> alias RX = S!(T, ref U); //not working
> 
> The objective is  to call X(t, u) or RX(t, u), but in second case I want
> "u" to be passed by reference.
> 
> The only solution I found is to pass U as a pointer or as a class.

`ref` is not a part of a type, so you can't do it like this. you have to have two different templates for that. or try to use `auto ref`, it may help.

On Monday, 23 March 2015 at 22:13:37 UTC, ketmar wrote:
> On Mon, 23 Mar 2015 22:01:03 +0000, rumbu wrote:
>
>> I'm trying to construct a struct template where one of the template
>> parameters is passed by reference (as option)
>> 
>> struct S(T, U)
>> {
>>     void opCall(T t, U u) { }
>> }
>> 
>> alias X = S!(T, U);
>> alias RX = S!(T, ref U); //not working
>> 
>> The objective is  to call X(t, u) or RX(t, u), but in second case I want
>> "u" to be passed by reference.
>> 
>> The only solution I found is to pass U as a pointer or as a class.
>
> `ref` is not a part of a type, so you can't do it like this. you have to
> have two different templates for that. or try to use `auto ref`, it may
> help.

I have only one template and I think it's impossible to define specializations since it's supposed that a random member of A... will be passed sometime by reference:

struct MulticastDelegate(ReturnType, A...)
{
  //...
  ReturnType opCall(A args) { //... }
}

Here it's what I have right now:

alias SomeDelegateWithInt = MulticastDelegate!(bool, char, int);
alias SomeDelegateWithRefInt = MulticastDelegate!(bool, char, int*);

SomeDelegateWithInt d1;
SomeDelegateWithRefInt d2;
...

int value;
bool b = d1('a', value);
bool b = d2('b', &value);

It's working, but I don't like the &value thing, ref would be better.

On Mon, 23 Mar 2015 22:39:27 +0000, rumbu wrote:

> It's working, but I don't like the &value thing, ref would be better.

alas.

you can write a complex mixin that will generate the appropriate templates for you, though, so you'll be able to do something like this:

mixin(MulticastDelegate!(`withRefInt`, `bool`, `char`, `ref int`));

but it will be error-prone and ugly. so the best thing is to stick with pointers.

On Monday, 23 March 2015 at 22:39:28 UTC, rumbu wrote:

> I have only one template and I think it's impossible to define specializations since it's supposed that a random member of A... will be passed sometime by reference:

auto ref?
http://dlang.org/template.html#auto-ref-parameters

On Tue, 24 Mar 2015 00:54:12 +0000, weaselcat wrote:

> On Monday, 23 March 2015 at 22:39:28 UTC, rumbu wrote:
> 
>> I have only one template and I think it's impossible to define specializations since it's supposed that a random member of A... will be passed sometime by reference:
> 
> auto ref? http://dlang.org/template.html#auto-ref-parameters

nope, that is not what OP wants. OP's wish is impossible, due to `ref` not being a part of a type.