class Foo
{
    static Foo makeFoo()
    {
        Foo f = new Foo();
        return f;
    }
}

void main() {
    Foo f = Foo.makeFoo();
}

For a code like this. I'd like all users of the class to be forced to create instances using the static method makeFoo. I want to disallow "new Foo()". But I don't know if it's possible to disable the constructor, while still making it available in the makeFoo static method.

@disable this doesn't work, private this(); doesn't work either.

On Wednesday, 23 January 2019 at 19:26:37 UTC, JN wrote:
> class Foo
> {
>     static Foo makeFoo()
>     {
>         Foo f = new Foo();
>         return f;
>     }
> }
>
> void main() {
>     Foo f = Foo.makeFoo();
> }
>
> For a code like this. I'd like all users of the class to be forced to create instances using the static method makeFoo. I want to disallow "new Foo()". But I don't know if it's possible to disable the constructor, while still making it available in the makeFoo static method.
>
> @disable this doesn't work, private this(); doesn't work either.

private should work, if the class and the main are in different modules, no?

On Wednesday, 23 January 2019 at 19:41:44 UTC, Alex wrote:
> On Wednesday, 23 January 2019 at 19:26:37 UTC, JN wrote:
>> class Foo
>> {
>>     static Foo makeFoo()
>>     {
>>         Foo f = new Foo();
>>         return f;
>>     }
>> }
>>
>> void main() {
>>     Foo f = Foo.makeFoo();
>> }
>>
>> For a code like this. I'd like all users of the class to be forced to create instances using the static method makeFoo. I want to disallow "new Foo()". But I don't know if it's possible to disable the constructor, while still making it available in the makeFoo static method.
>>
>> @disable this doesn't work, private this(); doesn't work either.
>
> private should work, if the class and the main are in different modules, no?

I expected that too, but it doesn't even work in the same module.

class Foo
{
    private this();

    static Foo makeFoo()
    {
        Foo f = new Foo();
        return f;
    }
}

void main() {
}

fails with:

onlineapp.o:onlineapp.d:_D9onlineapp3Foo7__ClassZ: error: undefined reference to '_D9onlineapp3Foo6__ctorMFZCQzQr'
onlineapp.d:7: error: undefined reference to '_D9onlineapp3Foo6__ctorMFZCQzQr'
collect2: error: ld returned 1 exit status
Error: linker exited with status 1

I don't understand why is this a linker problem. My understanding is that for some reason static methods don't have access to the private constructor (they're not considered same module?). But even though, it should error with something like "Foo.makeFoo() cannot access private Foo.this()" rather than fail at linking.

You are declaring the constructor, but not defining it, i.e. you're telling the compiler that it's in some other compilation unit.

The compiler won't complain, but the linker will.

If you replace:

> private this();

with:

> private this() {}

it should work.

A.

On 1/24/19 1:48 PM, JN wrote:
> 
> I expected that too, but it doesn't even work in the same module.
> 
> class Foo
> {
>      private this();
> 
>      static Foo makeFoo()
>      {
>          Foo f = new Foo();
>          return f;
>      }
> }
> 
> void main() {
> }
> 
> fails with:
> 
> onlineapp.o:onlineapp.d:_D9onlineapp3Foo7__ClassZ: error: undefined reference to '_D9onlineapp3Foo6__ctorMFZCQzQr'
> onlineapp.d:7: error: undefined reference to '_D9onlineapp3Foo6__ctorMFZCQzQr'
> collect2: error: ld returned 1 exit status
> Error: linker exited with status 1
> 
> I don't understand why is this a linker problem. My understanding is that for some reason static methods don't have access to the private constructor (they're not considered same module?). But even though, it should error with something like "Foo.makeFoo() cannot access private Foo.this()" rather than fail at linking.

On Thursday, 24 January 2019 at 12:52:47 UTC, Arafel wrote:
> You are declaring the constructor, but not defining it, i.e. you're telling the compiler that it's in some other compilation unit.
>
> The compiler won't complain, but the linker will.
>
> If you replace:
>
>> [...]
>
> with:
>
>> [...]
>
> it should work.
>
> A.
>
> On 1/24/19 1:48 PM, JN wrote:
>> [...]

Doh. Of course. I feel so dumb. I just had it at @disable this();, then replaced @disable with private without thinking to add {}

On Thursday, 24 January 2019 at 12:58:15 UTC, JN wrote:

>
> Doh. Of course. I feel so dumb. I just had it at @disable this();, then replaced @disable with private without thinking to add {}

Give me a nickel for every time I've made an edit like that...!