Re: Help with operator overloading

From:
"Victor Bazarov" <v.Abazarov@comAcast.net>
Newsgroups:
comp.lang.c++
Date:
Mon, 26 Mar 2007 21:23:32 -0400
Message-ID:
<it2dnejHGtuU75XbnZ2dnUVZ_t-mnZ2d@comcast.com>
Zilla wrote:

On Mar 23, 8:37 am, "Zilla" <zill...@bellsouth.net> wrote:

On Mar 22, 5:33 pm, "Victor Bazarov" <v.Abaza...@comAcast.net> wrote:

Zillawrote:

On Mar 22, 4:06 pm, "Victor Bazarov" <v.Abaza...@comAcast.net>
wrote:

[..]

    class D1 : public A<D1> { // derived one
    public:
        // constructors are not inherited, so you need one here
        D1(int v) : A<D1>(v) {}
    };

    class D2 : public A<D2> { // derived two
    public:
        // constructors are not inherited, so you need one here
        D2(int v) : A<D2>(v) {}
    };
[..]

Would that work? Use it as a starting point.

V


Thanks, so far it worked for these test cases. It's scary how the
compiler knows how to implement the "=" operation. I'll code it so
as NOT to leave it to chance.

int main()
{
   D1 d33(1);
   D1 d22(1);
   D1 d11=d33;
   cout << "d11.value: " << d11.getVal() << endl;
   d11=1;
   cout << "d11.value: " << d11.getVal() << endl;
   d11 = d11 + 1;
   cout << "d11.value: " << d11.getVal() << endl;
   d11 = 1 + d11;
   cout << "d11.value: " << d11.getVal() << endl;
   d11 = d22 + d11;
   cout << "d11.value: " << d11.getVal() << endl;
   return 0;
}


Just to bring it to your attention: it is important to not get the
types mixed up when copy-pasting your code. The class 'D1' is
defined as deriving from 'A<D1>' and 'D2' is from 'A<D2>'. This
is known as CRTP, IIRC. If when you define some 'D3' you make it
derive from 'A<D2>', the stuff can start going wrong and it might
be difficult to find. Or maybe not... Good luck!

V
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Thanks. Yes I noticed this right off. I'm playing with this code now,
and even trying out the straight-forward use of the template, ie,
without the class declarations inheritting the template.

class D1 {
   blah()

};

class D2 {
   blah()

};

int main()
{
   A<D1> d1;
   A<D2> d2;
   d1=blah...

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Ok, I was having problems with your example, so I went back to basics.
I stripped it down to the following code for operator=() only. I can't
even get it to compile...

g++ -c timers.cpp

timers.cpp: In function `int main()':
timers.cpp:35: no match for `tRCD & = double'
timers.cpp:30: candidates are: class tRCD & tRCD::operator =(const
tRCD &)

Why isn't the Timer<T>::operator=(...) being seen?

#include <iostream>

////////////////////////////////////////////////////////
// Base class that defines ALL the operations
template<class T>
class Timer {
public:
  Timer();
  T& operator=(const double v);

  double _min;
};

template<class T>
Timer<T>::Timer() : _min(0)
{
}

template<class T>
T& Timer<T>::operator=(const double v)
{
  // do something
}

class tRCD : public Timer<tRCD> {
public:
  tRCD() : Timer<tRCD>() {}
  ~tRCD() {}
};

int main()
{
  tRCD t1;
  t1=3.0;
  cout << "t1.min: " << t1._min << endl;
}


The assignment operator from the base class is hidden by the
assignment operator from the derived class. You need to add
a line with 'using' in it to the 'tRCD' class.

The next thing is to decide what your Timer<T>::operator=
is going to return. I am not sure how you're going to handle
that, since you can't really return *this (the usual value
returned from the assignment op).

V
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