On 5/27/2011 7:08 PM, Steven Schveighoffer wrote:
On Fri, 27 May 2011 02:34:34 -0400, Matthew Ong <[email protected]> wrote:
Hi All,
Currently within D, to make use of a parent class method you have to do:
class Parent{
void methodA(int x){...}
}
class Child : Parent{
// I understand that it has to do with preventing accidental hijacking
alias Parent.methodA methodA;
void methodA(long x){...}
}
void main(string[]){
Child obj=new Child();
obj.methodA(1); // expecting to call Child.methodA but calling
Parent.methodA;
}
and also from this URL.
http://www.digitalmars.com/d/2.0/function.html
If, through implicit conversions to the base class, those other
functions do get called, an std.HiddenFuncError exception is raised.
-----------------------------------------------------------------------
That is to prevent silently changing the program's behavior. b.foo(1)
could happily be a call to B.foo(long) today. Imagine one of the base
classes changed and now there is A.foo(int). Then our b.foo(1) would
silently start calling that new function. That would cause a tough bug.
Ali // a Better explanation than the document for the current syntax.
------------------------------------------------------------------------
However, there is a foreseeable problem coming when a program grow.
How about when the inheritance tree becomes deeper than 4? And more
and more overloaded functions are in different classes? That does not
meant the calling class/method has a sense if it is calling from Child
or Parent. Because, those 2 classes source code might not be available
for coder. How does the coder knows about that?
Does it mean we have to do more alias at child class at the bottom?
Harder to issues solve in the child class at the bottom of the tree.
It seem to me that the entire purpose is just to protect auto
promotion matching method signature in the base to avoid function
hijacking.
How about doing this another way? Just a suggestion if you like to
avoid parent function from accidental hijack but still needs to be
public. New keywords are needed: nooverload and inheritall
class Parent{
nooverload void methodA(int x){...} // entirely deny this name to be
overloaded.
}
// this would have avoided the aliasing all over child class and still
allow child class to see any >public< method of the parent.
class Child: inheritall Parent{ // auto inheriting all parent methods
except private ones. As per usual also for package/protected...
void methodA(long x){...} // compilation error. because nooverload is
used at Parent
void methodA(string x){...} // compilation error. because nooverload
... etc
void methodB(){
methodA(123); // No error now, and the entire hijacking is avoided.
}
}
void main(string[] args){
Child obj=new Child();
obj.methodB(); // no problem
obj.methodA(123); // no accidental hijacking...Always use parent class.
}
Reverse sequence as Ali has shown can also be avoided because if
someone does that by adding 'new' methodA in parent where child
already has methodA overloaded already without knowledge. Show up in
compilation exception for such cases with -w flag on.
How about that? Possible solution?
I don't think it will work that well. Consider how function hijacking
happens. For instance, the parent class author may not even know his
code is being overridden, and he may simply not mark his base function
as nooverload. Let's say that the child is inheriting all the parent's
methods because he wanted a different method (an already existing one),
and the author of the parent class adds methodA (without the nooverload
attribute) after the child is already written. That's an unintentional
hijack. The problem is the child is relying on the parent to cooperate
in preventing hijacking, instead of controlling whether its functions
can be hijacked or not.
In the current solution, the system warns me or throws an error if a
function becomes hijacked. If this happens, I can examine the code and
add an alias if needed. It happens rarely for me. Do you have cases
where you have to "alias all over the place"? Maybe you are not doing
something correctly, you shouldn't need this feature all the time.
Note that drastic proposals like this are very unlikely to be accepted.
Especially for something that has been in use and not really complained
about for years. You need to present a very compelling argument,
including real examples is helpful. Also, if there's any way to get rid
of adding a keyword, you have a much better shot of success. No keywords
have been added to the language for a long time.
-Steve
Hi Steve,
Please note that the proposal is not to remove the existing function
hijacking detection but as an alternative to the existing aliasing.
>Consider how function hijacking happens. For instance, the parent
>class author may not even know his code is being overridden, and he
>may simply not mark his base function as nooverload.
From OO stand point, overloading is NOT overriding.
Please do not mix up the two. Fundamentally different.
http://users.soe.ucsc.edu/~charlie/book/notes/chap7/sld012.htm
>parent class adds methodA (without the nooverload attribute)
If that happens, still flag as function hijacking using existing
detection. Please note that I did NOT ask for the removal of using
aliasing on existing source code for inherited overloaded function.
BTW, default D documentation is Not too friendly for inheritance tree
navigation. Unlike in java.
However, new keywords can be added to the compiler. So that future code
can be written without spending many brain cycle on looking up such
alias. Let the compiler do the hard work of AST searching rather than
a manual process. We have quad core now a days, even in asia.
Have you systemetic go over the proposal I posted and gave your counter
arguement? How about the fact that currently there are such aliasing all
over the child class. From what I understand D inheritance is Not
automatic, if I am wrong do let me know.
Do you have cases where you have to "alias all over the place"?
news://news.digitalmars.com:119/[email protected]
http://hg.dsource.org/projects/dwt2/file/d00e8db0a568/base/src/java/io/ByteArrayInputStream.d
>Maybe you are not doing something correctly, you shouldn't need this
feature all the time.
Not me, others that has coded the dwt and I suspect other code in
dsource where they tries to mimic Java Library and perhaps C# also.
>instead of controlling whether its functions can be hijacked or not.
Why NOT? If a problem can be prevented easily as an modifier keyword.
Hijacking is not a feature, it is a problem correct???
>No keywords have been added to the language for a long time.
Perhaps there is no-one that seen different from angle?
Most source code development process needs to look at at least 7
different dimensions. Inheritance down the tree, up the tree, interfaces
and its implementation, changes over time, testing, cpu cycle, memory
creation cycle. That is just single threaded model.
That is a tasks burdensome enough for most person.
--
Matthew Ong
email: [email protected]
