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digitalmars.D.learn - How to prevent direct public creation of a struct?

reply "Nick Sabalausky" <SeeWebsiteToContactMe semitwist.com> writes:
I want to do something like this:

------------------------------------------
module moduleFoo;

// Only allow certain values of "str"
template foo(string str)
{
    static assert(str == "a" || str == "b" || str == "c");
    immutable foo = Foo(str);
}

struct Foo
{
    // Only "a", "b", and "c" should be allowed,
    // checked at compile-time via "template foo".
    // Also, not modifyable.
    private string _str;
     property string str()
    {
        return _str;
    }
}
------------------------------------------

I want struct Foo itself to be public, but I want to *force* all code 
outside moduleFoo to *create* Foo via the "foo" template. Copying should be 
allowed though. Ie:

------------------------------------------
auto a = foo!"a"; // ok
a.str = "b"; // Error: str is a read-only property
auto z = foo!"z"; // Error: fails static assert
auto a2 = a; // Copy it: ok

auto b = Foo("b"); // Error: I want to *force* the usage of "template foo"

Foo x; // Kinda ambivalent about this: I'd prefer if it were disallowed, but 
I don't think that's possible. If it's indeed impossible, I know I can just 
declare Foo.str as (string str = "a";) so that's not a big problem.
------------------------------------------

The *key* thing here that I'm not sure how to do is: How do I disallow 
this?:

auto b = Foo("b"); // Error: I want to *force* the usage of "template foo"
May 03 2012
next sibling parent reply "Brad Anderson" <eco gnuk.net> writes:
On Thursday, 3 May 2012 at 21:36:53 UTC, Nick Sabalausky wrote:

 I want to do something like this:

 ------------------------------------------
 module moduleFoo;

 // Only allow certain values of "str"
 template foo(string str)
 {
     static assert(str == "a" || str == "b" || str == "c");
     immutable foo = Foo(str);
 }

 struct Foo
 {
     // Only "a", "b", and "c" should be allowed,
     // checked at compile-time via "template foo".
     // Also, not modifyable.
     private string _str;
      property string str()
     {
         return _str;
     }
 }
 ------------------------------------------

 I want struct Foo itself to be public, but I want to *force* 
 all code
 outside moduleFoo to *create* Foo via the "foo" template. 
 Copying should be
 allowed though. Ie:

 ------------------------------------------
 auto a = foo!"a"; // ok
 a.str = "b"; // Error: str is a read-only property
 auto z = foo!"z"; // Error: fails static assert
 auto a2 = a; // Copy it: ok

 auto b = Foo("b"); // Error: I want to *force* the usage of 
 "template foo"

 Foo x; // Kinda ambivalent about this: I'd prefer if it were 
 disallowed, but
 I don't think that's possible. If it's indeed impossible, I 
 know I can just
 declare Foo.str as (string str = "a";) so that's not a big 
 problem.
 ------------------------------------------

 The *key* thing here that I'm not sure how to do is: How do I 
 disallow
 this?:

 auto b = Foo("b"); // Error: I want to *force* the usage of 
 "template foo"


I would have thought adding a private this(string str) 
constructor would work but then foo can't be called from a 
separate module ("privcons.d(5): Error: struct privcons.Foo 
member this is not accessible" which is the same but desirable 
error it gives if you do "auto b = Foo("b");").  I thought all 
module members had access to private members of the same module 
but I guess that's not the case with template functions.
May 03 2012
parent "Nick Sabalausky" <SeeWebsiteToContactMe semitwist.com> writes:
"Brad Anderson" <eco gnuk.net> wrote in message 
news:jhsccvjskiqqzzqbdugx forum.dlang.org...

 On Thursday, 3 May 2012 at 21:36:53 UTC, Nick Sabalausky wrote:

 I want to do something like this:

 ------------------------------------------
 module moduleFoo;

 // Only allow certain values of "str"
 template foo(string str)
 {
     static assert(str == "a" || str == "b" || str == "c");
     immutable foo = Foo(str);
 }

 struct Foo
 {
     // Only "a", "b", and "c" should be allowed,
     // checked at compile-time via "template foo".
     // Also, not modifyable.
     private string _str;
      property string str()
     {
         return _str;
     }
 }
 ------------------------------------------

 I want struct Foo itself to be public, but I want to *force* all code
 outside moduleFoo to *create* Foo via the "foo" template. Copying should 
 be
 allowed though. Ie:

 ------------------------------------------
 auto a = foo!"a"; // ok
 a.str = "b"; // Error: str is a read-only property
 auto z = foo!"z"; // Error: fails static assert
 auto a2 = a; // Copy it: ok

 auto b = Foo("b"); // Error: I want to *force* the usage of "template 
 foo"

 Foo x; // Kinda ambivalent about this: I'd prefer if it were disallowed, 
 but
 I don't think that's possible. If it's indeed impossible, I know I can 
 just
 declare Foo.str as (string str = "a";) so that's not a big problem.
 ------------------------------------------

 The *key* thing here that I'm not sure how to do is: How do I disallow
 this?:

 auto b = Foo("b"); // Error: I want to *force* the usage of "template 
 foo"


 I would have thought adding a private this(string str) constructor would 
 work but then foo can't be called from a separate module ("privcons.d(5): 
 Error: struct privcons.Foo member this is not accessible" which is the 
 same but desirable error it gives if you do "auto b = Foo("b");").  I 
 thought all module members had access to private members of the same 
 module but I guess that's not the case with template functions.


Hmm, interestingly, it works if the template calls through an intermediary 
private function:

http://d.puremagic.com/issues/show_bug.cgi?id=8028

I don't know if I'm relying on a bug or working around a bug, but this seems 
to work, *and* disallows "Foo b;" which is nice:

---------------------------------------------------
template foo(string str)
{
    static assert(["a", "b", "c"].find(str), "Invalid str: '"~str~"'");
    immutable foo = _foo(name);
}

private Foo _foo(string str)
{
    return Foo(str);
}

struct Foo
{
    immutable string str;

    private this(string str)
    {
        this.str = str;
    }

     disable this();
}

---------------------------------------------------
// In a serparate module:

Foo a = foo!"a";  // Ok
auto a2 = a;  // Ok

a.str = "b";  // Error: can only initialize const member name inside 
constructor

auto z = foo!"z";  // Error: static assert  "Invalid str: 'z'"  instantiated 
from here: foo!("z")

auto b = Foo("b");  // Error: struct test2.Foo member this is not accessible

Foo x;  // Error: variable test1.main.x initializer required for type Foo
---------------------------------------------------

So that's awesome, everything as I wanted :)
May 03 2012
prev sibling parent Justin Whear <justin economicmodeling.com> writes:
On Thu, 03 May 2012 17:37:47 -0400, Nick Sabalausky wrote:

 The *key* thing here that I'm not sure how to do is: How do I disallow
 this?:
 
 auto b = Foo("b"); // Error: I want to *force* the usage of "template
 foo"


The  disable annotation can do this, I believe:
struct Foo
{
	 disable this();   // Cannot use default constructor
}

But this disables it for code within the module as well as outside, so 
you may wish to also add a private constructor.
May 03 2012