Re: How to dynamically call class virtual method dynamically

[John Colvin]

On Tuesday, 1 April 2014 at 07:31:43 UTC, Frustrated wrote:
Basically in programming to interfaces I need to decide to call 
a virtual method of an object if it exists else call a final 
method in the interface:

interface A
{
    static final void foo() { ... }
}

class B : A
{
    void bar() { ... }     // optional
}

class C : B
{
    void bar() { ... }     // optional
}

void main()
{
    A a = new B;  // or new C;

    // if a.bar exists call it, else call foo
    // code should work independent of the classes. (there 
might be more)
}

The point of the code is simply to allow the class to implement 
bar optionally but provide default behavior with foo. I need a 
way to dynamically determine if bar exists and fall back on 
foo. This should be possible.

e.g., suppose

class B : A { }

then I would like to b.bar() to actually call A.foo() (since 
bar doesn't exist in b).

I guess the exist way would be to create an opDispatch and have 
it call foo if bar is passed. This works great and does 
everything I need it to except requires adding the code in the 
class which I can't have. Also I'm not sure how it would work 
with virtual methods.

I've tried using hasMember but since my object is cast to a 
type of Object it never works.

The traditional OO approach would probably be this:

import std.stdio;
class A
{
        static final void foo() { writeln("A.foo"); }
        void bar() { foo(); }
}
class B : A
{
        override void bar() { writeln("B.bar"); }     // optional
}
class C : A {}
void main()
{
        A a0 = new B;  // or new C;
        A a1 = new C;  // or new C;
        a0.bar();  //B.bar
        a1.bar();  //A.foo
}

Also, bear in mind that polymorphism is one-way: you can call 
base class methods through an inherited class reference, but not 
the other way around.

interface A {}
class B : A
{
    void bar() {}
}
void main()
{
    A a = new B;
    a.bar(); // Error: no property 'bar' for type 'A'
}