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