Re: <complex> : no match for 'operator*' // conversion operator double()

From:

Tom Widmer <tom_usenet@hotmail.com>

Newsgroups:

comp.lang.c++

Date:

Fri, 23 Jun 2006 10:58:38 +0100

Message-ID:

<e7gdof$nb2$1@nntp.aioe.org>

Arvid Requate wrote:

Hello,

I'd like to understand why the following code does not compile. It
looks like a strangeness in connection with overload resolution for the
<complex> header:
The conversion operator double() of class B is called for the member
complex::operator*=(double) as expected, but not for operator*(complex,
double).

The effect is, that the template matching (or overload resolution)
fails. Error message:
complex_double_conversion.cpp: In function 'int main()':
complex_double_conversion.cpp:19: error: no match for 'operator*' in 'z
* x'

Tested with g++ 4.0.2 20050901 (prerelease) on (SUSE Linux) and others.
Do <you> know a reason/remedy for this behaviour? Thanks in advance.

The operator* function you want to call is a non-member as follows:
template<class T>
complex<T> operator*(const complex<T>& lhs, const T& rhs);

Template argument deduction fails because it requires that the arguments
are exact matches for the deduced parameter types (with a few
exceptions), but B is not similar enough to double const& (user defined
conversions aren't considered).

The easiest fix is to write a suitable function (in the same namespace
as B so that it is found by ADL), something like:

complex<double> operator*(const complex<double>& lhs, const B& rhs)
{
return lhs * static_cast<double>(rhs);
}

Tom

//------------------------complex_double_conversion.cpp-------------------------
#include<complex>

class B {
  double v;
  public:
  operator double() const { return v; } // conversion operator
  B(double _v) : v(_v) {}
};

int main() {
  std::complex<double> z(0,1);
  B x(0.5);

  // next line works due to complex<_Tp>&
complex<_Tp>::operator*=(const _Tp&)
  // the conversion operator of class B is used
  z*=x;

  // the next line does not compile
  std::complex<double> y( z*x );

  // only with cast: z*((double) x)
  //
  // although in <complex> there is
  // template<typename _Tp>
  // inline complex<_Tp> operator*(const complex<_Tp>& __x, const
_Tp& __y)
  // { return complex<_Tp> (__x) *= __y; }
  //
  return 0;
}
//-----------------------------------------------------------------------------------------