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digitalmars.D.learn - How do I make a template function that only accepts instances of a

reply Jack <jckj33 gmail.com> writes:
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
{
   // ...
}
Jan 18 2021
parent reply Adam D. Ruppe <destructionator gmail.com> writes:
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.
Jan 18 2021
parent reply Jack <jckj33 gmail.com> writes:
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!
Jan 18 2021
parent reply Adam D. Ruppe <destructionator gmail.com> writes:
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.
Jan 18 2021
parent Jack <jckj33 gmail.com> writes:
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.



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
Jan 18 2021