Hi,

I have the problem, that I want to always construct a struct with a parameter. To make this more comfortable (e.g. provide a default parameter I have a factory function to create this struct).

struct S {
  int i;
  this(int i_) { i = i_; }
}

S createS(int i=5) {
  return S(i);
}

My question now is:
Is there a way to enfore the creation of the struct with createS?
Or to put it in another way. Is it possible to forbid something like:
S s; or even auto s = S(1);? I read about the this(this) thing, but that
is only used when the struct is copied, as far as I understood.


(My struct has the nature to only work if it is not constructed with the default struct constructor).

thank in advance

christian

On 09/26/2011 10:55 PM, Christian Köstlin wrote:
> Hi,
>
> I have the problem, that I want to always construct a struct with a
> parameter. To make this more comfortable (e.g. provide a default
> parameter I have a factory function to create this struct).
>
> struct S {
> int i;
> this(int i_) { i = i_; }
> }
>
> S createS(int i=5) {
> return S(i);
> }
>
> My question now is:
> Is there a way to enfore the creation of the struct with createS?
> Or to put it in another way. Is it possible to forbid something like:
> S s; or even auto s = S(1);? I read about the this(this) thing, but that
> is only used when the struct is copied, as far as I understood.
>
>
> (My struct has the nature to only work if it is not constructed with the
> default struct constructor).
>
> thank in advance
>
> christian

Starting with DMD 2.055, this works.

struct S{
    this() @disable;
    this(int i_) { i = i_; }
}

You can hide the constructor from external code by using "private
this()". And you can disable the default constructor via @disable
this();
You may also wish to make your fields private as well.

You don't necessarily have to make createS() a free function, you can make it a static function inside of S, ala:

struct S {
    @disable this();
    static S create(int _i) { return S(_i); }
private:
    int i;
    this(int i_) { i = i_; }
}

create() will not occupy any space in any of the S instances. If that doesn't compile it's because I haven't had my coffee yet. :p

On 9/26/11, Andrej Mitrovic <andrej.mitrovich@gmail.com> wrote:
> create() will not occupy any space in any of the S instances.

Sorry for that stupid comment. struct methods don't occupy space in struct instances regardless if they're static or not.

On 9/26/11 23:18 , Andrej Mitrovic wrote:
> You can hide the constructor from external code by using "private
> this()". And you can disable the default constructor via @disable
> this();
> You may also wish to make your fields private as well.
>
> You don't necessarily have to make createS() a free function, you can
> make it a static function inside of S, ala:
>
> struct S {
>      @disable this();
>      static S create(int _i) { return S(_i); }
> private:
>      int i;
>      this(int i_) { i = i_; }
> }
>
> create() will not occupy any space in any of the S instances. If that
> doesn't compile it's because I haven't had my coffee yet. :p
very good ... exactly what I wanted :)
thanks a lot!

christian

On 09/26/2011 11:15 PM, Timon Gehr wrote:
> On 09/26/2011 10:55 PM, Christian Köstlin wrote:
>> Hi,
>>
>> I have the problem, that I want to always construct a struct with a
>> parameter. To make this more comfortable (e.g. provide a default
>> parameter I have a factory function to create this struct).
>>
>> struct S {
>> int i;
>> this(int i_) { i = i_; }
>> }
>>
>> S createS(int i=5) {
>> return S(i);
>> }
>>
>> My question now is:
>> Is there a way to enfore the creation of the struct with createS?
>> Or to put it in another way. Is it possible to forbid something like:
>> S s; or even auto s = S(1);? I read about the this(this) thing, but that
>> is only used when the struct is copied, as far as I understood.
>>
>>
>> (My struct has the nature to only work if it is not constructed with the
>> default struct constructor).
>>
>> thank in advance
>>
>> christian
>
> Starting with DMD 2.055, this works.
>
> struct S{
> this() @disable;
> this(int i_) { i = i_; }
> }
mhh ... strange ... i have dmd 2.055 64bit for linux and this compiles without error and outputs
l: 0
c: 0


import std.stdio : writeln;
struct S {
private:
  int[] data;
  @disable this(this);
  @disable this();

  this(size_t s) {
    data = new int[s];
  }
public:
  void p() {
    writeln("l: ", data.length);
    writeln("c: ", data.capacity);
  }
  static S create(size_t  s = 8) {
    return S(s);
  }
}
unittest {
  auto s = S();
  s.p();
}
int main(string[] args) {
  return 0;
}


shouldnt this lead to a compile error?


thanks in advance

christian