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const method not being used

Posted on 1997-06-25
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Last Modified: 2010-04-10
say I have:

template <class T>
class Foo
{
   public:
      T& operator * ();
      const T& operator * () const;
};

int main()
{
   Foo<int> fred;

   int &ri1 = *fred;
   const int &ri1 = *fred;
}

both lines call the non-const op *.  I thought I remeber const methods were suposed to be called if they could be.  
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Question by:strobert
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8 Comments
 
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Accepted Solution

by:
yonat earned 600 total points
ID: 1164131
The const method would have been called if fred was const:

const Foo<int> cfred;
Foo<int> fred;

const int &cr = *cfred; // const method called
int &r = *cfred; // error
int &r2 = *fred; // non-const method called

> const methods were suposed to be called if they could be.
Not exactly. If a methid is overloaded by constness, the const
method will be called for const objects, and the non-const
method otherwise.
In any case, the return value is never considered in overloading
resolution.

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Author Comment

by:strobert
ID: 1164132
is the abiltiy to overload by const spec'd anywhere? (I couldn't find it in stroustrup)

also, reason why I wanted this...(if you want to try a solution to this add a comment and I'll up the points)

I have a vector class.  I am creating an external iterating class
I would like the same itr class to be used for iterating on const  vector's and non-const vector's.  I was going to keep a state variable as to what the itr was contructed with (a Vector& or a const vector&) and in the T& op * throw an exception if we were itring on a const vector&...
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Expert Comment

by:yonat
ID: 1164133
> is the abiltiy to overload by const spec'd anywhere?

Check out the latest version of the ANSI C++ draft standard at
http://www.cygnus.com/misc/wp/dec96pub/over.html

Basically, it sais that a member function is like a regular
function with one extra argument: a pointer to the object (this)
[There are many differences, but this view help to understand
overloading]. This means that you can overload by the constness
of the this argument.

> I would like the same itr class to be used for iterating on
> const vector's and non-const vector's. I was going to keep a
> state variable as to what the itr was contructed with (a
> Vector& or a const vector&) and in the T& op * throw an
> exception if we were itring on a const vector&...

Your solution sounds OK, just keep in mind that the error will
only be discovered at runtime instead if at compile time.
Another solution is to have two classes: iter and const_iter.
Instead of constructing an iterator by
iter i(a_vector);
You can add a member function to vector that returns an iterator
to the beginning (or end) of the vector:

class vector
{
public:
    // ...
    iter begin();
    const_iter begin() const;
};

This way the user of vector will get the correct iterator class,
without having to worry about it.

> I have a vector class.

Why write your own vector instead of using STL?

You can get STL from
* http://www.ipmce.su/~fbp/stl/

And there are several short tutorials at:
* http://www.sgi.com/Technology/STL/stl_introduction.html
* http://www.objectplace.com/technology/articles/9508.shtml
* http://www.cs.brown.edu/people/jak/programming/stl-tutorial/
                          tutorial.html
* http://www.infosys.tuwien.ac.at/Research/Component/
                                  tutorial/prwmain.htm
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Author Comment

by:strobert
ID: 1164134
Run-time was fine with me (basically it is an assert like case so runtime is fine just like out of bounds on a bracket)

good thought with the begin...
I will probably either go with that or the two class (Itr and ConstItr) method.

Why I don't use the STL:
1) I actually have some philisophical disagreements with the way it does things.
2) wether I like it or not, I can't use it.  My dev platform is VC++ under windows and the STL implementation Microsquish ships makes it almost unusable.  Plus given the way the STL tends to abuse templates I wouldn't trust it under VC++ since there are a lot of broken things in VC++ templates.
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LVL 5

Expert Comment

by:yonat
ID: 1164135
> I actually have some philisophical disagreements with the way
> [STL] does things.

They don't use inheritence - true. But that gains a lot of
efficiency since all the little functions (operator++ etc) are
inlined. I think that STL is an eficient and usuable solution
to a wide variety of problems (but maybe not yours ...)

> My dev platform is VC++ under windows and the STL
> implementation Microsquish ships makes it almost unusable

The STL from http://www.ipmce.su/~fbp/stl/  works well with VC++
4.0, 4.1 and 4.2 . They say it is "partially compatible" with 5.0 .

Good luck!
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Author Comment

by:strobert
ID: 1164136
Inheritance is one of the things, but yes it is understanable not to use it for speed.  I think my biggest gripe is actually the naming conventions...maybe they will grow on me..

I'll give that stl version a look... I'll be impressed if it is still nice under VC++

thanks


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Author Comment

by:strobert
ID: 1164137
Adjusted points to 150
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Expert Comment

by:yonat
ID: 1164138
> I'll give that stl version a look... I'll be impressed if it is
> still nice under VC++

I just found out that there is an STL version adapted for VC5
available from:
http://www.sirius.com/~ouchida/

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