On 01/04/2015 07:37 AM, Suliman wrote:
> how to write code above without passing this as parameter...
For reference, here is your code:
http://www.everfall.com/paste/id.php?a5pp73ns1e4k
There is nothing fundamentally wrong in a class constructor passing
'this' to an object that it constructs:
class A
{
B b;
this() {
b = new B(this); // Makes its B
}
}
However, keep in mind that 'this' is not an A until we leave the
constructor. For that reason, the B's constructor must not use the A
reference that it receives.
In that code, apparently A owns a B for its purposes. Presumably, that
is why A's constructor is creating a B object. Commonly, it is either
not the case or even if so, we want to provide a B from "above" so that
A becomes testable. (Search for "parametrize from above" by "kevlin
henney"; or search for "dependency injection".)
So, usually the following idiom is prefered:
class A
{
B b;
this(B b) {
this.b = b; // Takes its B as a parameter
}
}
Well, it works fine until you need both of the objects to refer to each
other:
class A
{
B b;
this(B b) {
this.b = b;
}
}
class B
{
A a;
this(A a) {
this.a = a;
}
}
It is impossible to construct two objects at the same, which refer to
each other:
auto a = new A(/* where is my B? */); // Oops!
auto b = new B(a);
So, A must not have an invariant that requires non-null B and that it
must have a property to set the B later on:
auto a = new A(null);
// Here, A must be happy with null B
auto b = new B(a);
a.b = b; // Finally, we have two happy objects
So, your code is just fine because there is no general solution anyway.
Ali
