Thread overview
How do I make a template function that only accepts instances of a specific class?
Jan 18
Jack
Jan 18
Jack
Jan 18
Jack
January 18
isInstanceOf from std.traits seems to not work with class the way I need to. I'd like to make a template function accepts only class of a specified class type

class A { }
class B : A { }
class C : A { }


import std.traits : isInstanceOf;

int f(T)(in A[int] arr)
if(isInstanceOf!(A, T)) // doesn't work for class from what I saw
{
  foreach(c; arr) {
     if((cast(A)c) !is null) {
     }
  }
  // ...
}

int f(T)(in A[int] arr)
if(cast(T) !is null) // run time this is ok but not at compile time
{
  // ...
}

January 18
On Monday, 18 January 2021 at 18:40:37 UTC, Jack wrote:
> isInstanceOf from std.traits seems to not work with class the way I need to. I'd like to make a template function accepts only class of a specified class type
>
> class A { }
> class B : A { }
> class C : A { }
>
> int f(T)(in A[int] arr)

Use

if(is(T : A))

the syntax there is similar to the class declaration itself.

isInstanceOf is for checking instances of templates rather than classes.
January 18
On Monday, 18 January 2021 at 19:02:10 UTC, Adam D. Ruppe wrote:
> On Monday, 18 January 2021 at 18:40:37 UTC, Jack wrote:
>> isInstanceOf from std.traits seems to not work with class the way I need to. I'd like to make a template function accepts only class of a specified class type
>>
>> class A { }
>> class B : A { }
>> class C : A { }
>>
>> int f(T)(in A[int] arr)
>
> Use
>
> if(is(T : A))
>
> the syntax there is similar to the class declaration itself.

is that sytax derived from there?

> isInstanceOf is for checking instances of templates rather than classes.

I see

Thanks!



January 18
On Monday, 18 January 2021 at 19:34:52 UTC, Jack wrote:
> is that sytax derived from there?

sort of. it is the type pattern matching "is expression" described here:

https://dlang.org/spec/expression.html#IsExpression

The is(A:B) thing technically means "if A is implicitly convertible to B" which of course works for base classes and interfaces well but that's not all it does, like is(int : long) passes too.

I just remember the syntax thanks to its similarity to type declarations.
January 18
On Monday, 18 January 2021 at 19:54:04 UTC, Adam D. Ruppe wrote:
> On Monday, 18 January 2021 at 19:34:52 UTC, Jack wrote:
>> is that sytax derived from there?
>
> sort of. it is the type pattern matching "is expression" described here:
>
> https://dlang.org/spec/expression.html#IsExpression

I read std.traits and some related stuff but missed reading is expression doc lol

>
> The is(A:B) thing technically means "if A is implicitly convertible to B" which of course works for base classes and interfaces well but that's not all it does, like is(int : long) passes too.

C# uses something like that too but I totally missed to look for the equivalent in D, which would be pretty much same thing

> I just remember the syntax thanks to its similarity to type declarations.

good way to remember too