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March 17, 2015 Defining a single opCast disables explicit cast to base interfaces | ||||
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The following program compiles fine: interface I {} class B : I {} class C : B { int i; } void main() { auto c = new C; auto i = cast(I)c; // compiles auto b = cast(B)c; // compiles } Let's add an unrelated opCast to C: class C : B { int i; int opCast(T : int)() { return i; } } Now the last two lines of main fail to compile: Error: template instance opCast!(I) does not match template declaration opCast(T : int)() Error: template instance opCast!(B) does not match template declaration opCast(T : int)() Is this per spec? (Actually, where is the spec? (Trick question. ;) ) There is a workaround: Add a catch-all opCast that forwards to the all-powerful std.conv.to: T opCast(T)() { import std.conv; return this.to!T; } Now it compiles and works as expected. However, the question remains... Thank you, Ali |
March 17, 2015 Re: Defining a single opCast disables explicit cast to base interfaces | ||||
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Posted in reply to Ali Çehreli | I forgot to mention that this discussion is carried over from the D.learn newsgroup: http://forum.dlang.org/thread/uwuvqurfqbetypdlwkdy@forum.dlang.org Ali |
March 17, 2015 Re: Defining a single opCast disables explicit cast to base interfaces | ||||
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Posted in reply to Ali Çehreli | On Tuesday, 17 March 2015 at 05:27:38 UTC, Ali Çehreli wrote:
> The following program compiles fine:
>
> interface I
> {}
>
> class B : I
> {}
>
> class C : B
> {
> int i;
> }
>
> void main()
> {
> auto c = new C;
>
> auto i = cast(I)c; // compiles
> auto b = cast(B)c; // compiles
> }
>
> Let's add an unrelated opCast to C:
>
> class C : B
> {
> int i;
>
> int opCast(T : int)()
> {
> return i;
> }
> }
>
> Now the last two lines of main fail to compile:
>
> Error: template instance opCast!(I) does not match template declaration opCast(T : int)()
> Error: template instance opCast!(B) does not match template declaration opCast(T : int)()
>
> Is this per spec? (Actually, where is the spec? (Trick question. ;) )
>
> There is a workaround: Add a catch-all opCast that forwards to the all-powerful std.conv.to:
>
> T opCast(T)()
> {
> import std.conv;
> return this.to!T;
> }
>
> Now it compiles and works as expected.
>
> However, the question remains...
>
> Thank you,
> Ali
There is also another trick: generally when something cannot be cast then it's often possible to cast it as a pointer to the cast type that is directly derefered. And it work...
---
T opCast(T)()
{
static if(is(T==int))
return i;
else
return *cast(T*) &this;
}
---
...ed until version 2.067 (now it's deprecated because of &this since this is already a ptr).
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March 18, 2015 Re: Defining a single opCast disables explicit cast to base interfaces | ||||
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Posted in reply to Ali Çehreli | On Monday, March 16, 2015 22:27:36 Ali Çehreli via Digitalmars-d wrote: > The following program compiles fine: > > interface I > {} > > class B : I > {} > > class C : B > { > int i; > } > > void main() > { > auto c = new C; > > auto i = cast(I)c; // compiles > auto b = cast(B)c; // compiles > } > > Let's add an unrelated opCast to C: > > class C : B > { > int i; > > int opCast(T : int)() > { > return i; > } > } > > Now the last two lines of main fail to compile: > > Error: template instance opCast!(I) does not match template declaration > opCast(T : int)() > Error: template instance opCast!(B) does not match template declaration > opCast(T : int)() > > Is this per spec? (Actually, where is the spec? (Trick question. ;) ) > > There is a workaround: Add a catch-all opCast that forwards to the all-powerful std.conv.to: > > T opCast(T)() > { > import std.conv; > return this.to!T; > } > > Now it compiles and works as expected. > > However, the question remains... Defining opCast destroys basically all built-in casts, which I think is a horrible idea. The problem that you're describing was reported a couple of years ago: https://issues.dlang.org/show_bug.cgi?id=9249 But it also affects stuff like shared, and a bug report for that was created four years ago: https://issues.dlang.org/show_bug.cgi?id=5747 And there may be other bug reports for similar issues, but basically, as it stands, declaring opCast borks the built-in casts, forcing you to redefine them all, which is horrible IMHO. But Kenji expressed concern about fixing it in #5747, and no one has stepped up to sort it out. - Jonathan M Davis |
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