On 12/02/2011 09:05 PM, Timon Gehr wrote:
On 12/02/2011 08:10 PM, Adam wrote:
A second possible use case:
class C(T): T{
// some declarations
}
Now you really want that template to be instantiable with T being
either
an abstract or a concrete class. Anything else is bound to become
extremely annoying.
Could you expand on this case a bit? I'm not sure I follow the point
one way or another.
This is an useful pattern. I don't have a very useful example at hand,
but this one should do. It does similar things that can be achieved with
traits in Scala for example.
import std.stdio;
abstract class Cell(T){
abstract void set(T value);
abstract const(T) get();
private:
T field;
}
class AddSetter(C: Cell!T,T): C{
override void set(T value){field = value;}
}
class AddGetter(C: Cell!T,T): C{
override const(T) get(){return field;}
}
class DoubleCell(C: Cell!T,T): C{
override void set(T value){super.set(2*value);}
}
class OneUpCell(C: Cell!T,T): C{
override void set(T value){super.set(value+1);}
}
class SetterLogger(C:Cell!T,T): C{
override void set(T value){
super.set(value);
writeln("cell has been set to '",value,"'!");
}
}
class GetterLogger(C:Cell!T,T): C{
override const(T) get(){
auto value = super.get();
writeln("'",value,"' has been retrieved!");
return value;
}
}
class ConcreteCell(T): AddGetter!(AddSetter!(Cell!T)){}
class OneUpDoubleSetter(T): OneUpCell!(DoubleCell!(AddSetter!(Cell!T))){}
class DoubleOneUpSetter(T): DoubleCell!(OneUpCell!(AddSetter!(Cell!T))){}
void main(){
Cell!string x;
x = new ConcreteCell!string;
x.set("hello");
writeln(x.get());
Cell!int y;
y = new SetterLogger!(ConcreteCell!int);
y.set(123); // prints: "cell has been set to '123'!
y = new GetterLogger!(DoubleCell!(ConcreteCell!int));
y.set(1234);
y.get(); // prints "'2468' has been retrieved!"
y = new AddGetter!(OneUpDoubleSetter!int);
y.set(100);
writeln(y.get()); // prints "202"
y = new AddGetter!(DoubleOneUpSetter!int);
y.set(100);
writeln(y.get()); // prints "201"
// ...
}
Oh, forgot to mention: This would not compile, if an explicit 'abstract'
declaration on template class definitions was required.
