I've been trying to create a templated interface, and can't figure out what I'm doing wrong. Here's the basic example:

interface I(T) {
  public T x();
}

class C(T) : I(T) {
  private T value;
  public this(T val) { this.value = val; }
  public T x() { return value; }
}

void main() {
  I!(char) c = new C!(char)('a');
  char value = c.x();
}

I've tried many many slight variations of this, to try to satisfy the compiler, but it always rejects the code, usually by saying something like "Interface I used as a type".

Of course! That's because interface I *is* a type.

What am I doing wrong?

--benji

Benji Smith a écrit :
> I've been trying to create a templated interface, and can't figure out what I'm doing wrong. Here's the basic example:
> 
> interface I(T) {
>   public T x();
> }
> 
> class C(T) : I(T) {

class C(T) : I!(T) {

Ary Borenszweig wrote:
> Benji Smith a écrit :
>> I've been trying to create a templated interface, and can't figure out what I'm doing wrong. Here's the basic example:
>>
>> interface I(T) {
>>   public T x();
>> }
>>
>> class C(T) : I(T) {
> 
> class C(T) : I!(T) {

Very nice! Thanks!

Now let's say I want to create an array of those interface instances:

interface I(T) {
  public T x();
}

class C(T) : I!(T) {
  private T value;
  public this(T val) { this.value = val; }
  public T x() { return value; }
}

void main() {
  I!(char)[] array = new I!(char)[];
  array ~ new C!(char)('a');
  array ~ new C!(char)('b');
  array ~ new C!(char)('c');
  char value = array[0].x();
}

Now the compiler is complaining about the line where I create the array. It says "Error: new can only create structs, dynamic arrays or class objects, not I[]'s"

Obviously, I'm trying to create a dynamic array of interface instances, so I'm not sure what I'm doing wrong.

Thanks!

--benji

On Sun, 24 Aug 2008 22:20:46 +0200, Benji Smith <dlanguage@benjismith.net> wrote:

> Ary Borenszweig wrote:
>> Benji Smith a écrit :
>>> I've been trying to create a templated interface, and can't figure out what I'm doing wrong. Here's the basic example:
>>>
>>> interface I(T) {
>>>   public T x();
>>> }
>>>
>>> class C(T) : I(T) {
>>  class C(T) : I!(T) {
>
> Very nice! Thanks!
>
> Now let's say I want to create an array of those interface instances:
>
> interface I(T) {
>    public T x();
> }
>
> class C(T) : I!(T) {
>    private T value;
>    public this(T val) { this.value = val; }
>    public T x() { return value; }
> }
>
> void main() {
>    I!(char)[] array = new I!(char)[];
>    array ~ new C!(char)('a');
>    array ~ new C!(char)('b');
>    array ~ new C!(char)('c');
>    char value = array[0].x();
> }
>
> Now the compiler is complaining about the line where I create the array. It says "Error: new can only create structs, dynamic arrays or class objects, not I[]'s"
>
> Obviously, I'm trying to create a dynamic array of interface instances, so I'm not sure what I'm doing wrong.
>
> Thanks!
>
> --benji

You do not need to initialize the the array with the "new I!(char)[]". Just use

	I!(char)[] array;
	array ~= new C!(char)('a');
	...
	char value = array[0].x();

-- 
Simen

Simen Kjaeraas wrote:
> You do not need to initialize the the array with the "new I!(char)[]". Just use
> 
>     I!(char)[] array;
>     array ~= new C!(char)('a');
>     ...
>     char value = array[0].x();

Gotcha.

Coming from Java, it's hard getting used to which types require a 'new' and which ones don't. I find the array types especially confusing.

Thanks for your help!

--benji

Simen Kjaeraas wrote:

> On Sun, 24 Aug 2008 22:20:46 +0200, Benji Smith <dlanguage@benjismith.net> wrote:
> 
>> Ary Borenszweig wrote:
>>> Benji Smith a écrit :
>>>> I've been trying to create a templated interface, and can't figure out what I'm doing wrong. Here's the basic example:
>>>>
>>>> interface I(T) {
>>>>   public T x();
>>>> }
>>>>
>>>> class C(T) : I(T) {
>>>  class C(T) : I!(T) {
>>
>> Very nice! Thanks!
>>
>> Now let's say I want to create an array of those interface instances:
>>
>> interface I(T) {
>>    public T x();
>> }
>>
>> class C(T) : I!(T) {
>>    private T value;
>>    public this(T val) { this.value = val; }
>>    public T x() { return value; }
>> }
>>
>> void main() {
>>    I!(char)[] array = new I!(char)[];
>>    array ~ new C!(char)('a');
>>    array ~ new C!(char)('b');
>>    array ~ new C!(char)('c');
>>    char value = array[0].x();
>> }
>>
>> Now the compiler is complaining about the line where I create the array. It says "Error: new can only create structs, dynamic arrays or class objects, not I[]'s"
>>
>> Obviously, I'm trying to create a dynamic array of interface instances, so I'm not sure what I'm doing wrong.
>>
>> Thanks!
>>
>> --benji
> 
> You do not need to initialize the the array with the "new I!(char)[]".
> Just use
> 
> I!(char)[] array;
> array ~= new C!(char)('a');
> ...
> char value = array[0].x();
> 

whoa, that's a bad idea!

You can not implicitly cast back and forth between interfaces and classes that implement those interfaces; the underlying pointer is not the same. There was a big discussion about this fairly recently on one of the D mailing lists...

"Benji Smith" <dlanguage@benjismith.net> wrote in message news:g8sih3$7h4$1@digitalmars.com...
> Simen Kjaeraas wrote:
>> You do not need to initialize the the array with the "new I!(char)[]". Just use
>>
>>     I!(char)[] array;
>>     array ~= new C!(char)('a');
>>     ...
>>     char value = array[0].x();
>
> Gotcha.
>
> Coming from Java, it's hard getting used to which types require a 'new' and which ones don't. I find the array types especially confusing.
>
> Thanks for your help!
>
> --benji

Well arrays work _almost_ the same way as in Java, the only difference being that they can change their size.

You can either do

auto array = new I!(char)[3];
array[0] = ..
array[1] = ..
array[2] = ..

Or just declare it and append stuff.  The former will be faster since you'll only do 1 memory allocation instead of n (although the implementation does over-allocate for arrays so that appending is faster, you may not want to depend on that for performance-critical code).

"Jason House" <jason.james.house@gmail.com> wrote in message news:g8sioq$6ul$1@digitalmars.com...

> whoa, that's a bad idea!
>
> You can not implicitly cast back and forth between interfaces and classes that implement those interfaces; the underlying pointer is not the same. There was a big discussion about this fairly recently on one of the D mailing lists...

No, what he's doing is OK.  The trouble only occurs when you try to cast *the array* up or down the inheritance hierarchy.

Jarrett Billingsley wrote:
> "Jason House" <jason.james.house@gmail.com> wrote in message news:g8sioq$6ul$1@digitalmars.com...
> 
>> whoa, that's a bad idea!
>>
>> You can not implicitly cast back and forth between interfaces and classes
>> that implement those interfaces; the underlying pointer is not the same.
>> There was a big discussion about this fairly recently on one of the D
>> mailing lists...
> 
> No, what he's doing is OK.  The trouble only occurs when you try to cast *the array* up or down the inheritance hierarchy. 

Yeah, with the code I'm currently working on, once the objects are in the array, I never again care what the specific class is for each instance, since I'll only be calling the interface methods.

--benji

Jarrett Billingsley wrote:
> Well arrays work _almost_ the same way as in Java, the only difference being that they can change their size.
> 
> You can either do
> 
> auto array = new I!(char)[3];
> array[0] = ..
> array[1] = ..
> array[2] = ..
> 
> Or just declare it and append stuff.  The former will be faster since you'll only do 1 memory allocation instead of n (although the implementation does over-allocate for arrays so that appending is faster, you may not want to depend on that for performance-critical code). 

Huh. I thought that, by putting an integer literal into the array declaration, it'd be declared as a static array rather than a dynamic array.

--benji