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digitalmars.D.learn - dynamic type casting in D

reply dave <garapata gmail.com> writes:
Hi,

sort of new to D programming, coming from C++. Basically the question is if
there is some sort of a way to dynamically cast a variable in D much like you
do in C++?

ex:

interface A {...}
class B {...}
class C : B {...}
class D : B, A {...}

function(B something) {
    A a = dynamic_cast<A>(something);
}

I can't seem to find a way to check if a variable has a particular class or
interface it inherits, would be nice.
May 30 2010
next sibling parent bearophile <bearophileHUGS lycos.com> writes:
dave:

 sort of new to D programming, coming from C++. Basically the question is if
 there is some sort of a way to dynamically cast a variable in D much like you
 do in C++?


D conflated all kinds of casts into the cast() syntax. So dynamic_cast can be
done with a cast().



 I can't seem to find a way to check if a variable has a particular class or
 interface it inherits, would be nice.


Cast it and if it is not null, then the dynamic cast is allowed. Or do you need
some other information?

Bye,
bearophile
May 30 2010
prev sibling parent reply Robert Clipsham <robert octarineparrot.com> writes:
On 30/05/10 19:46, dave wrote:

 Hi,

 sort of new to D programming, coming from C++. Basically the question is if
 there is some sort of a way to dynamically cast a variable in D much like you
 do in C++?

 ex:

 interface A {...}
 class B {...}
 class C : B {...}
 class D : B, A {...}

 function(B something) {
      A a = dynamic_cast<A>(something);
 }

 I can't seem to find a way to check if a variable has a particular class or
 interface it inherits, would be nice.


void func(B something)
{
	A a = cast(A)something;
}

Is what you're looking for I believe. As for your other request, take a 
look at std.traits, __traits, or if you're using D1 std.traits, 
tango.core.Traits, tango.core.RuntimeTraits.
May 30 2010
parent reply dave <garapata gmail.com> writes:
Awesome! I just tried it out, the cast expression is exactly what I was looking
for.

I'm currently using Tango and tried out its runtime traits api, this works as
expected:

if(implements(typeof(something).classinfo, A.classinfo))

Thanks for the info!
May 30 2010
parent Michal Minich <michal.minich gmail.com> writes:
On Sun, 30 May 2010 19:40:27 +0000, dave wrote:


 if(implements(typeof(something).classinfo, A.classinfo))


in simpler way:

In order to determine if an object o is an instance of a class B use a 
cast:

if (cast(B) o)
{
    // o is an instance of B
}
else
{
    // o is not an instance of B
}

or you can also create variable of type B inline:

if (auto b = cast(B) o) { use b ... } 

you can find more type information using 'is' expression
http://www.digitalmars.com/d/1.0/expression.html#IsExpression
May 30 2010