[Solved] [Moved] Can QList<T*> take subclasses of T?

[reactive]

Hi, I am having problems understanding why this doesn't compile.

I have 2 classes, A and B where B publicly inherits A.
The pseudocode is:

@
function(const QList<A*>& list){ ... }

QList<B*>* list = ...;
function(*list);
@

Error:

cannot convert parameter 1 from QList<T> to Aconst with T = B

I'd appreciate any suggestions. Thanks!

EDIT: please use @-tags for code highlighting, Gerolf

[vsorokin]

You confused in templates and classes.

B is subclass A
but QList<B*> is not subclass QList<A*>

i.e you can:
@QList<B*>* bList;
QList<A*>* aList;
...
A *a = bList->at(0);
aList->append(a);@

but you can't

@QList<B*>* bList;
QList<A*>* aList;
...
aList = bList;@

[goetz]

See "C++ FAQ 21.3":http://www.parashift.com/c++-faq-lite/proper-inheritance.html#faq-21.3 for an explanation and 21.1/21.2 for the background.

PS:
I moved this to the C++ guru forum, as it's a general C++ question, but not specifically Qt related.

[reactive]

Thanks for your help. I guess I was confused because Java allows this in their generic programming with List<B extends A>. I'm new to Templates but I figured it would work under the Liskov Substitution Principle, since all Bs are As and it's just a pointer (so the size of the object doesn't matter).

[goetz]

You have that List<B extends A> in C++ too, you can put all A subclass types into the list, but if you have B extending A you cannot assign a List<B> to a variable of type List<A>. Both are different cases.

What you've tried does not work in Java either, the following snippet yields an error:

@
public InheritanceTest() {
ArrayList<String> sl = new ArrayList<String>();
String x = "abc";
sl.add(x);

ArrayList<Object> ol = new ArrayList<Object>();
Integer z = new Integer(1);
ol.add(z);

ol = sl;

}

// Error on the last line:
// Type mismatch: cannot convert from ArrayList<String> to ArrayList<Object>
@