digitalmars.D.learn - How to prevent direct public creation of a struct?
- Nick Sabalausky (37/37) May 03 2012 I want to do something like this:
- Brad Anderson (8/53) May 03 2012 I would have thought adding a private this(string str)
- Nick Sabalausky (38/96) May 03 2012 Hmm, interestingly, it works if the template calls through an intermedia...
- Justin Whear (8/13) May 03 2012 The @disable annotation can do this, I believe:
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
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
"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: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 :)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
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