Hi,
I'm reading "C++ Coding Standards" by Herb Sutter.
On page 67 there's an example which I don't understand:
---------------------------------
class Base{// ...
virtual void Foo(int);
virtual void Foo(int, int);
void Foo(int, int, int);
};
class Derived : public Base { // ...
virtual void Foo(int); // overrides Base::Foo(int), but hides the others
};
Derived d;
d.Foo(1); // ok
d.Foo(1,2); // error
d.Foo(1,2,3); // error
-----------------------------------
I don't understand why the functions "virtual void Foo(int, int)" and
"void Foo(int, int, int)" are hidden by Derived::Foo(int) ?
My opinion was that a derived class is always inheriting all functions
of a base class and can additionally override them for a specific purpose
like Foo(int).
To bring the other Base::Foo overloads into scope Sutter is using the
statement
using Base::Foo;
within the Derive class.
Thank you for your answers.
Greeting,
Chris 4 1788
Christian Christmann wrote: Hi,
I'm reading "C++ Coding Standards" by Herb Sutter. On page 67 there's an example which I don't understand: --------------------------------- class Base{// ... virtual void Foo(int); virtual void Foo(int, int); void Foo(int, int, int); };
class Derived : public Base { // ... virtual void Foo(int); // overrides Base::Foo(int), but hides the others };
Derived d; d.Foo(1); // ok d.Foo(1,2); // error d.Foo(1,2,3); // error -----------------------------------
I don't understand why the functions "virtual void Foo(int, int)" and "void Foo(int, int, int)" are hidden by Derived::Foo(int) ? My opinion was that a derived class is always inheriting all functions of a base class and can additionally override them for a specific purpose like Foo(int).
It is a direct consequence of how the compiler looks up a function.
First it searches the class hierarchy for a class which has that function.
In your specific example, when you write
d.foo(1,2);
the compiler looks up the type of d. It is Derived. Thus it looks into class
Derived, if there are one or more functions called Foo(). If there are, proceed
to the next step. If there are no, then look up the base class and try there.
Note: The number and type of the arguments is irrelevant in this step. The compiler
searches for the functions just by looking at their names!
The next step is to decide which function to choose based on the number of arguments
and their types in the set of found functions of the previous step.
In your specific case, the set of those functions contains only 1 function, since Derived
contains only 1 function called 'Foo'. The compiler sees, that Foo takes only one argument
but the call specifies 2 parameters. Thus the error message.
Note: If no argument match can be made, the compiler stops! It doesn't look for other
functions in the base class(es).
That's called hiding: A function in a derived class hides all functions with the same
name in the base class(es). To bring the other Base::Foo overloads into scope Sutter is using the statement
using Base::Foo;
within the Derive class.
That's one way to work around function hiding.
--
Karl Heinz Buchegger kb******@gascad.at
* Christian Christmann: I'm reading "C++ Coding Standards" by Herb Sutter. On page 67 there's an example which I don't understand: --------------------------------- class Base{// ... virtual void Foo(int); virtual void Foo(int, int); void Foo(int, int, int); };
class Derived : public Base { // ... virtual void Foo(int); // overrides Base::Foo(int), but hides the others };
Derived d; d.Foo(1); // ok d.Foo(1,2); // error d.Foo(1,2,3); // error -----------------------------------
I don't understand why the functions "virtual void Foo(int, int)" and "void Foo(int, int, int)" are hidden by Derived::Foo(int) ?
It's a more-or-less arbitrary language design decision; there are arguments
for, and arguments against.
The hiding ensures that calls to Foo in Derived won't inadvertently become
bound to inherited Foo implementations; in particular, with the hiding in
effect, which implementation a call to Foo in Derived is bound to cannot be
changed by adding new functions to Base (oops, I added a Base function and
suddenly Derived stopped working), part of the "fragile base class" problem.
This gives control to the Derived programmer, but it also means that the
"Derived is a kind of Base" rule is broken. I think the AKO rule is _much_
more important than "fragile base class", and the hiding is at best only a
very partial solution to that, and nobody finds the hiding very intuitive
(well, nobody I know). But then that's how it is with a lot of things.
--
A: Because it messes up the order in which people normally read text.
Q: Why is it such a bad thing?
A: Top-posting.
Q: What is the most annoying thing on usenet and in e-mail?
Christian Christmann wrote: I don't understand why the functions "virtual void Foo(int, int)" and "void Foo(int, int, int)" are hidden by Derived::Foo(int) ? My opinion was that a derived class is always inheriting all functions of a base class and can additionally override them for a specific purpose like Foo(int).
I'm afraid I don't know why it was done this way, but it is indeed the
case. Section 13.2/1 has this example:
class B{
public:
int f(int);
};
class D: public B{
public:
int f(char*);
};
Here D::f(char*) hides B::f(int) rather than overloading it.
void h(D* pd)
{
pd->f(1); // error: D::f(char*) hides B::f(int)
pd->B::f(1); // OK
pd->f("Ben"); // OK, calls D::f
}
- end example.
Paragraph 10.2/2 is probably relevant, but I couldn't quite decipher it.
Regards,
Jacques.
"Christian Christmann" <pl*****@yahoo.de> wrote in message
news:42***********************@newsread4.arcor-online.net... Hi,
I'm reading "C++ Coding Standards" by Herb Sutter. On page 67 there's an example which I don't understand: --------------------------------- class Base{// ... virtual void Foo(int); virtual void Foo(int, int); void Foo(int, int, int); };
class Derived : public Base { // ... virtual void Foo(int); // overrides Base::Foo(int), but hides the others };
Derived d; d.Foo(1); // ok d.Foo(1,2); // error d.Foo(1,2,3); // error -----------------------------------
I don't understand why the functions "virtual void Foo(int, int)" and "void Foo(int, int, int)" are hidden by Derived::Foo(int) ? My opinion was that a derived class is always inheriting all functions of a base class and can additionally override them for a specific purpose like Foo(int).
To bring the other Base::Foo overloads into scope Sutter is using the statement
using Base::Foo;
within the Derive class.
Thank you for your answers.
Greeting, Chris
Regarding the issue of function hiding it does not really matter whether the
function is declared virtual or not. See section 23.7 of the FAQ.
HTH
Chris This thread has been closed and replies have been disabled. Please start a new discussion. Similar topics
by: Jacek Dziedzic |
last post by:
Is it valid to use a "using namespace foo" (as opposed to
using foo::bar which I'm sure is legal) within a class
declaration? My compiler rejects it, but I've been told it's
valid.
Can anyone...
|
by: beliavsky |
last post by:
Many of my C++ programs have the line
using namespace std;
but the "Accelerated C++" book of Koenig and Moo has many examples
where the library names are included one at a time, for example
...
|
by: Douglas |
last post by:
**** Post for FREE via your newsreader at post.usenet.com ****
Hello,
The following code does not compile if line 3 is uncommented "using
namespace std".
I do not understand it. Could...
|
by: john.burton.email |
last post by:
I've done some extensive searching and can't seem to find an answer to
this -
Is it correct to using "using" with templates, for example:
using std::vector;
Or do I need to specify the type...
|
by: Jacky Yuk |
last post by:
Hi all,
I am new to c++ but using c for long time. Recently, I created a MFC
GUI project by VC/C++ 6.0. Everything was fine until I wanted to use
"template":
template <typename T>
class...
|
by: Andreas Müller |
last post by:
Hi all,
I have a class SwitchCursor, that implements IDisposable. Is it legal
to use an object that is not assigned to a reference inside a using
statement like this:
using(new...
|
by: Geri Reshef |
last post by:
Many times I find code examples in the internet which don't have the "using" statements needed to run them, and without it the compilation fails.
Is there a way to find the correct "using" statement...
|
by: Pep |
last post by:
Is it best to include the code "using namespace std;" in the source or
should each keyword in the std namespace be qualified by the namespace tag,
such as
std::cout << "using std namespace" <<...
|
by: Steve Pope |
last post by:
Compiling the following works on my system:
file main.cpp:
#include <iostream>
namespace space {
int foo;
}
|
by: samjnaa |
last post by:
Please check for sanity and approve for posting at python-dev.
In Visual Basic there is the keyword "with" which allows an object-
name to be declared as governing the following statements. For...
|
by: DolphinDB |
last post by:
Tired of spending countless mintues downsampling your data? Look no further!
In this article, you’ll learn how to efficiently downsample 6.48 billion high-frequency records to 61 million...
|
by: isladogs |
last post by:
The next Access Europe meeting will be on Wednesday 6 Mar 2024 starting at 18:00 UK time (6PM UTC) and finishing at about 19:15 (7.15PM).
In this month's session, we are pleased to welcome back...
|
by: isladogs |
last post by:
The next Access Europe meeting will be on Wednesday 6 Mar 2024 starting at 18:00 UK time (6PM UTC) and finishing at about 19:15 (7.15PM).
In this month's session, we are pleased to welcome back...
|
by: ArrayDB |
last post by:
The error message I've encountered is; ERROR:root:Error generating model response: exception: access violation writing 0x0000000000005140, which seems to be indicative of an access violation...
|
by: PapaRatzi |
last post by:
Hello,
I am teaching myself MS Access forms design and Visual Basic. I've created a table to capture a list of Top 30 singles and forms to capture new entries. The final step is a form (unbound)...
|
by: CloudSolutions |
last post by:
Introduction:
For many beginners and individual users, requiring a credit card and email registration may pose a barrier when starting to use cloud servers. However, some cloud server providers now...
|
by: Defcon1945 |
last post by:
I'm trying to learn Python using Pycharm but import shutil doesn't work
|
by: Shællîpôpï 09 |
last post by:
If u are using a keypad phone, how do u turn on JavaScript, to access features like WhatsApp, Facebook, Instagram....
|
by: isladogs |
last post by:
The next Access Europe User Group meeting will be on Wednesday 3 Apr 2024 starting at 18:00 UK time (6PM UTC+1) and finishing by 19:30 (7.30PM).
In this session, we are pleased to welcome former...
| |