Overloading Memberfunction for a class with templates

Question

When I have a class with templates, how can I achieve, that dependent on the dataypes, I can calculate sth. with a certain memberfunction. I constructed a minimal example where the objective is to multiply two numbers, which can either be real or complex and than take as a result only the real part of it: Re{a*b}.

This is the header file, test.hpp:

ifndef __TEST_H__
#define __TEST_H__

template<typename T1, typename T2>
class A{
    public:
        A(T1 const & a_, T2 const & b_);

     double multiply_and_real();

    private:

    T1 const & a;
    T2 const & b;
};

#endif /* __TEST_H__ */

And this the cpp file, test.cpp:

#include "test.hpp"
#include <complex>
#include <cmath>

template<typename T1, typename T2>
A<T1,T2>::A(T1 const & a_, T2 const & b_):
    a(a_),
    b(b_)
    {}

template<typename T1, typename T2>
double A<double,double>::multiply_and_real(){

    return a*b;
}

template<typename T1, typename T2>
double A<std::complex<double>,double>::multiply_and_real(){

    return a.real()*b;
}

template<typename T1, typename T2>
double A<double,std::complex<double> >::multiply_and_real(){

    return a*b.real();
}

template<typename T1, typename T2>
double A<std::complex<double>,std::complex<double> >::multiply_and_real(){

    return a.real()*b.real();
}


template class A< double, double >;
template class A< std::complex<double>, double >;
template class A< double,std::complex<double> >;
template class A< std::complex<double>,std::complex<double> >;

How can I do it in the right way? With this implementation, I get errors messages of the kind:

error: prototype for ‘double A<double, double>::multiply_and_real()’ does not match any in class ‘A<double, double>’
 double A<double,double>::multiply_and_real(){

score 0 · Accepted Answer · edited May 23 '17 at 10:33

0

Since you're specializing use following syntax, notice template<> eveywhere

template<>
double A<double,double>::multiply_and_real(){

    return a*b;
}

template<>
double A<std::complex<double>,double>::multiply_and_real(){

    return a.real()*b;
}

template<>
double A<double,std::complex<double> >::multiply_and_real(){

    return a*b.real();
}

template<>
double A<std::complex<double>,std::complex<double> >::multiply_and_real(){

    return a.real()*b.real();
}

^{Also read this to avoid your future doubts ;}

edited May 23 '17 at 10:33

Community

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answered Nov 20 '14 at 19:26

P0W

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"multiple definition of `A::multiply_and_real()" – karlo86 Nov 20 '14 at 19:34
@karlo86 may be some multiple header inclusion problem. It shouldn't give any issue. See [this](http://rextester.com/NWXTJK38817) – P0W Nov 20 '14 at 19:38
@POW You are right. Bringing everything into one File works again. Do you see the issue here, having it in two files? – karlo86 Nov 20 '14 at 19:44
@POW Ok it works. It was just a mistake in the main file. Does this also work the same way, if let's say there is a third template for the class, but which doesn't play a role for the memberfunction multiply_and_real()? Do I have to write for the specializing: "template double A::multiply_and_real(){" – karlo86 Nov 20 '14 at 19:49

Overloading Memberfunction for a class with templates

1 Answers1