Re: floating point number wrapper

From:
Alex Shulgin <alex.shulgin@gmail.com>
Newsgroups:
comp.lang.c++.moderated
Date:
Tue, 3 Jun 2008 09:39:01 CST
Message-ID:
<7074640b-14e1-4d5d-bfe9-c4a6aeb764c3@b1g2000hsg.googlegroups.com>
On Jun 3, 6:00 am, "Hicham Mouline" <hic...@mouline.org> wrote:

hello,

I am trying to define a double wrapper that wraps only >=0.0 doubles,

class double_constant_wrapper {
public:
  explicit spot_rate_constant(double r);
  double operator()() const;
  double& operator=(double r);
private:
 double r_;

};

inline double_constant_wrapper::double_constant_wrapper(double r)
: r_(r)
{
  if (r<0.0)
    throw exception("number cannot be <0");

}

inline double double_constant_wrapper::operator() const
{
  return r_;

}

inline double& double_constant_wrapper::operator=(double r)
{
  return r_=r;

}

If the user attempts to construct a double_constant_wrapper with a <0.0
double, she will get an exception.
Do the members signatures seem ok?


No, the corrected compilable version is below. ;)

#include <stdexcept>
#include <iostream>
#include <ostream>

class double_constant_wrapper {
public:
  explicit double_constant_wrapper(double r);
  double operator()() const;
  double& operator=(double r);
private:
 double r_;
};

inline double_constant_wrapper::double_constant_wrapper(double r)
    : r_(r)
{
  if (r<0.0)
      throw std::runtime_error("number cannot be <0");
}

inline double double_constant_wrapper::operator()() const
{
  return r_;
}

inline double& double_constant_wrapper::operator=(double r)
{
  return r_=r;
}

int
main()
{
    double_constant_wrapper r(0);
    (r=1)=-1;
    std::cout << r() << std::endl;
}

And this demonstrates that returning double& in operator=() isn't a
very bright idea with respect to maintaining the class invariant
(r_>=0).

--
Regards,
Alex

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