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How I do "cast" class references in Squirrel/Sqrat?
JonHodgson
#1 Posted : Thursday, May 19, 2016 10:08:31 AM(UTC)
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Hi,

I'm an experienced programmer, but a complete newbie to Squirrel and Sqrat.

I've successfully got a vm working, bound a class to it (using sqrat), and accessed a C++ created object of that class from a script.

Not a big deal I know! (though I did have to change sqratallocater.h to use the C++ 11 is_default_constructible to get it to compile on VS2013).

Now looking forwards I'm wondering how I'll achieve something, but it's such a standard use case I would hope it's pretty simple.

I have inherited classes, the usual sort of thing

Code:
class Bar

class Foo:public Bar

class Fun:public Bar


Which of course I can bind to Squirrel using Sqrat.

I also have functions along the lines of

Code:
Bar* getBar();


Which I assume I can also bind to squirrel (As I said, complete squirrel newbie here).

At which point I assume I'll be able to do something along the lines of

Code:
my_bar = getBar();


inside squirrel.

but often I will want to be able to identify the derived class and access the object as that class.

What in C++ would read

Code:
Foo* my_foo = dynamic_cast<Foo*>(my_bar);
if (my_foo)
{
    // Do Foo specific stuff here
}


How do I do the same sort of thing from a Squirrel script?
absence
#2 Posted : Friday, May 20, 2016 9:20:59 PM(UTC)
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By heart I am not sure if instanceof iterates through inheritance. If it does, this would suffice:
Code:

if (bar instanceof foo)
{

}


if it does not iterate, you still could do it yourself, like in this example:


Code:
class foo
{
[...]
  function getbase() { try { return base.getbase() ;} catch(x) { return base ; }}
}

if (bar.getbase() instanceof foo)
{

}


Be aware, in this special case the test will fail if bar actually is a foo instance itself, you would want to write

if ((bar instanceof foo) || (bar.getbase() instanceof foo))



Note this all might be bullshit, I am typing by heart without making sure I'm right :-)

Mephisto
#3 Posted : Sunday, May 22, 2016 2:47:27 PM(UTC)
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I would first start questioning why you would need to do this, since down-casting is typically an indication that your design could use some work.

Regardless, I'll attempt to answer your question assuming you know why you're doing what you're doing. I use SqPlus, so you'll have to translate to Sqrat.

One technique you could use is to create a static casting function for each inheritance object:

Code:

static Foo* Foo:SafeCastNative(NativeObject *pObj)
{
    return dynamic_cast<Foo*>(pObj);
}

static Foo* Foo:SafeCastScript(SquirrelObject sqObject)
{
    if (sqObject.GetType() == OT_INSTANCE)
    {
        return (Foo*)sqObject.GetInstanceUP(ClassType<Foo>::type());
    }
    return NULL;
}


Then bind the SafeCastScript:

Code:

RegisterGlobal(SafeCastScript, "CastToFoo");


In Squirrel, you'll do something like this (assuming you're working with a Bar object):
Code:

local Foo = CastToFoo(this);


But again, I really don't think you need this kind of thing. With native polymorphism set up correctly, and the right bindings to squirrel, you should be able to avoid using this scenario.
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