www.digitalmars.com         C & C++   DMDScript  

digitalmars.D.learn - Template constructor in a non-template struct.

reply "ChrisG" <joe hotmail.com> writes:
Hi, I couldn't find information about problem I'm having. This 
builds:

enum { Option1, Option2 }

struct boring {
   this(int Opt = Option1)(int arg1, int arg2) { ... }
}

However, I can't figure out how to instantiate that constructor. 
When I call it like this:

auto a = boring(1, 2);

The compiler doesn't seem the template constructor, only the 
non-template constructors which take a different set of 
arguments. When I call it like this:

auto a = boring!(Option1)(1, 2);

It complains that my struct isn't a template struct, which makes 
sense; however, I don't really understand how I'd differentiate a 
constructor template from a class/struct template.

Any ideas?

-Chris
Feb 08 2015
parent reply "bearophile" <bearophileHUGS lycos.com> writes:
ChrisG:


 I don't really understand how I'd differentiate a constructor 
 template from a class/struct template.


One solution is to use a template struct (struct/class names 
start with an upper case in D, while typed enum members usually 
start with a lower case):


enum E { option1, option2 }

struct Boring(E Opt = E.option1) {
     this(int arg1, int arg2) {}
}

void main() {
     auto a = Boring!(E.option2)(1, 2);
}


If you want to instantiate the constructor template without type 
inference, then this seems to work, but it's not idiomatic D:


enum E { option1, option2 }

struct Boring {
     this(E Opt = E.option1)(int arg1, int arg2) {}
}

void main() {
     auto a = Boring().__ctor!(E.option2)(1, 2);
}


Bye,
bearophile
Feb 08 2015
parent reply "ChrisG" <joe hotmail.com> writes:
Thanks bearophile. Your first suggestion about making the struct 
a template is something I considered. However, because of all the 
code I've already written that approach would force me to use 
inheritance or convert a ton of functions to templates. Ick.

The __ctor syntax looks like the answer to my question. If my 
struct is used by anyone else though, I'm not sure it would be OK 
to assume they would even be aware of that syntax. hmmm... 
Thinking I'll just make it a runtime parameter for now.

Thank you.

-Chris

On Sunday, 8 February 2015 at 21:58:40 UTC, bearophile wrote:

 ChrisG:


 I don't really understand how I'd differentiate a constructor 
 template from a class/struct template.


 One solution is to use a template struct (struct/class names 
 start with an upper case in D, while typed enum members usually 
 start with a lower case):


 enum E { option1, option2 }

 struct Boring(E Opt = E.option1) {
     this(int arg1, int arg2) {}
 }

 void main() {
     auto a = Boring!(E.option2)(1, 2);
 }


 If you want to instantiate the constructor template without 
 type inference, then this seems to work, but it's not idiomatic 
 D:


 enum E { option1, option2 }

 struct Boring {
     this(E Opt = E.option1)(int arg1, int arg2) {}
 }

 void main() {
     auto a = Boring().__ctor!(E.option2)(1, 2);
 }


 Bye,
 bearophile
Feb 08 2015
parent reply "Marc =?UTF-8?B?U2Now7x0eiI=?= <schuetzm gmx.net> writes:
On Sunday, 8 February 2015 at 22:19:35 UTC, ChrisG wrote:

 Thanks bearophile. Your first suggestion about making the 
 struct a template is something I considered. However, because 
 of all the code I've already written that approach would force 
 me to use inheritance or convert a ton of functions to 
 templates. Ick.

 The __ctor syntax looks like the answer to my question. If my 
 struct is used by anyone else though, I'm not sure it would be 
 OK to assume they would even be aware of that syntax. hmmm... 
 Thinking I'll just make it a runtime parameter for now.


You could also hide the ugly __ctor() call behind a nicely named 
factory method.
Feb 09 2015
parent "ChrisG" <joe hotmail.com> writes:
On Monday, 9 February 2015 at 09:52:13 UTC, Marc Schütz wrote:

 You could also hide the ugly __ctor() call behind a nicely 
 named factory method.


Yes, that's pretty much what I ended up doing. Added a couple 
static member functions like:

static Boring CreateWithOption1(args);
static Boring CreateWithOption2(args);

Then made the struct constructor private. Not too bad for now. 
Thanks for your help.
Feb 14 2015