specialized member function takes precedence over generic template member function

[bluekite2000]

Here is the compilable code, along w/ the error

#include<iostream>
#include<complex>
typedef std::complex<float> ComplexSingle;
using namespace std;
template<typename T> class X
{
private:
T number;
public:
X(T value)
{
number=value;
}
template<typename Other> X(Other Y)
{
assign(Y);
}
template<typename Other> void assign(Other Y);


T return_number()
{
return number;
}
};

template<typename T>
template<typename Other> void X<T>::assign( Other Y)
{
number=Y.return_number();
}

template<> template<typename ComplexSingle> void
X<float>::assign(ComplexSingle Y)
{
number=norm(Y.return_number());
}
int main (void)
{
//this works fine
ComplexSingle a(2,3);
X<ComplexSingle> A(a);
X<float> B(A);


//error
//In member function `void X<T>::assign(Other) [with Other =
//X<double>, T = float]':
//ex1.cc:16: instantiated from `X<T>::X(Other) [with Other =
// X<double>, T = float]'
//ex1.cc:45: instantiated from here
//ex1.cc:35: error: no matching function for call to `norm(double)'

X<double> C(3.3);
X<float> D(C);
return 0;
}

 

[Victor Bazarov]

Here is the compilable code, along w/ the error

#include<complex>

template<typename T> class X
{
T number;
public:
[..]
T return_number()
{
return number;
}
};

template<> template<typename ComplexSingle> void
X<float>::assign(ComplexSingle Y)
{
number=norm(Y.return_number());

'norm' is a template declared for all complex<T>. When you call it like
that with 'Y' as 'X<double>', it looks around and cannot find any 'norm'
for 'double', then it sees that there is 'norm<T>(const complex<T>&)',
but since the argument you give is 'double', it can't understand what 'T'
should be. This is not one of contexts from which 'T' can be deduced.

What are you trying to achieve, anyway? I can only recommend defining
your own 'norm':

double norm(double a) {
return fabs(a); // or whatever
}

}

V