On Tuesday, 11 November 2014 at 15:31:07 UTC, Kagamin wrote:
http://forum.dlang.org/post/[email protected]
c:\D\import\dfl\button.d(381): Error: function
dfl.button.Button.text cannot have
e an in contract when overriden function
dfl.control.Control.text
does not have an in contract
This is the line 381:
override void text(Dstring txt) // setter
in
{
if(txt.length)
assert(!this.image, "Button image with text not supported");
}
body
{
super.text = txt;
}
But if a button can't display image with text by design, it's
its contract, that text can't be set with image, while the base
class has no such restriction. What does it mean, DbC is
incompatible with inheritance?
Contracts are a tool for enforcing the Liskov substitution
principle(http://en.wikipedia.org/wiki/Liskov_substitution_principle).
What you are trying to do violates the LSP because I can't set
the text of a Control if what I have in hand happens to be it's
subclass, Button.
Now, I'm not saying that you shouldn't do that assert - I think
the best-practices work better as rules of thumb than as sacred
religious rules - but since contracts are modeled after the LSP
they are probably not the right tool for this job...
