operator= Overload from a templated class

Question

I have a project will all my classes templated for int, double and float, getCoordinate return an object of the type CCoordinate.

 tempCoordinate = m_shapes.at(i)->getCoordinate(j);

Before I apply the templates it was working correctly. But then some errors appear.

From what I understand I need I'm missing and operator= overload to typecast the values in case for example that i have a float and I'm receiving an int, for example:

 CCoordinate<float> coorFloat; CCoordinate<int> coorInt = coorFloat

How can i create this on my class? what format does it need ? .

I was thinking that it should look like this, but apparently i'm mistaken.

//CCoordinate.h template<class T> class CCoordinate { //Code public: template<class U> template <class U> CCoordinate<T> operator= (const CCoordinate<U>& c1); } //CCoordinate.cpp template <class U > CCoordinate<U> CCoordinate<T>::operator= (const CCoordinate<U>& c1) { // some kind of casting ? }

My Errors:

19:06:43 **** Incremental Build of configuration Debug for project ShapesRefV2 **** Info: Internal Builder is used for build g++ -O0 -g3 -Wall -c -fmessage-length=0 -Werror=return-type -o "myCode\\CRectangle.o" "..\\myCode\\CRectangle.cpp" g++ -O0 -g3 -Wall -c -fmessage-length=0 -Werror=return-type -o "myCode\\CPlane.o" "..\\myCode\\CPlane.cpp" ..\myCode\CPlane.cpp: In instantiation of 'GraSys::CRectangle<T> GraSys::CPlane<T>::boundingBox(std::string, std::string) [with T = int; std::string = std::basic_string<char>]': ..\myCode\CPlane.cpp:165:24: required from here ..\myCode\CPlane.cpp:115:20: error: no match for 'operator=' (operand types are 'GraSys::CCoordinate<double>' and 'const GraSys::CCoordinate<int>') tempCoordinate = m_shapes.at(i)->getCoordinate(j); ^ ..\myCode\CPlane.cpp:115:20: note: candidate is: In file included from ..\myCode\CGraphicElement.h:14:0, from ..\myCode\CPlane.h:11, from ..\myCode\CPlane.cpp:9: ..\myCode\CCoordinate.h:17:7: note: GraSys::CCoordinate<double>& GraSys::CCoordinate<double>::operator=(const GraSys::CCoordinate<double>&) class CCoordinate ^ ..\myCode\CCoordinate.h:17:7: note: no known conversion for argument 1 from 'const GraSys::CCoordinate<int>' to 'const GraSys::CCoordinate<double>&' ..\myCode\CPlane.cpp: In instantiation of 'GraSys::CRectangle<T> GraSys::CPlane<T>::boundingBox(std::string, std::string) [with T = float; std::string = std::basic_string<char>]': ..\myCode\CPlane.cpp:166:24: required from here ..\myCode\CPlane.cpp:115:20: error: no match for 'operator=' (operand types are 'GraSys::CCoordinate<double>' and 'const GraSys::CCoordinate<float>') tempCoordinate = m_shapes.at(i)->getCoordinate(j); ^ ..\myCode\CPlane.cpp:115:20: note: candidate is: In file included from ..\myCode\CGraphicElement.h:14:0, from ..\myCode\CPlane.h:11, from ..\myCode\CPlane.cpp:9: ..\myCode\CCoordinate.h:17:7: note: GraSys::CCoordinate<double>& GraSys::CCoordinate<double>::operator=(const GraSys::CCoordinate<double>&) class CCoordinate ^ ..\myCode\CCoordinate.h:17:7: note: no known conversion for argument 1 from 'const GraSys::CCoordinate<float>' to 'const GraSys::CCoordinate<double>&' 19:06:44 Build Finished (took 674ms)

hint: operator = should return a reference to the class it lives in. — Johan Lundberg
– Johan Lundberg, Commented Dec 14, 2014 at 18:37

cdhowie · Accepted Answer · 2014-12-14 18:35:45Z

In the member declaration you have template <class U> too many times, and the member should return a reference to *this, so it needs to return CCordinate & (the <T> is implied if you omit it):

// Remove this vvvvvvvvvvvvvvvvvv template<class U> /* template <class U> */ CCoordinate & operator= (const CCoordinate<U>& c1); // ^- Return type changed to be a reference.

Since the member is a template and the class is a template, you have two levels of templates. You need to specify both levels when you implement the member.

It is also returning the wrong type (it returns CCoordinate<U> but you have declared it to return CCoordinate<T> in the class).

// You need the T template as well. // vvvvvvvvvvvvvvv template <class T> template <class U> CCoordinate<T> & CCoordinate<T>::operator= (const CCoordinate<U>& c1) // ^ ^- Added reference as per above. // \---- Changed to T; U makes no sense here and conflicts with your member // declaration in the class. { // Your logic to make the conversion. return *this; }

it compiles, but something is missing. undefined reference to `GraSys::CCoordinate<int>& GraSys::CCoordinate<int>::operator=<float>(GraSys::CCoordinate<float> const&)'
@IzonFreak Template implementations need to be in the header so the compiler can instantiate them when they are used. You probably put it in a .cpp file. You can't do that (unless you explicitly instantiate for all type parameters you need, but I doubt you are doing that and you don't want to anyway).

Pradhan · Accepted Answer · 2014-12-14 18:58:21Z

There are two issues with your attempt. The simpler issue is that the declaration syntax of your operator= has an extra template <class U>. It should look like this:

template<class U> CCoordinate<T> operator= (const CCoordinate<U>& c1);

However, even a correctly defined operator= will not allow you to write

CCoordinate<float> coorFloat; CCoordinate<int> coorInt = coorFloat;

This is because the second line above copy initializes coorInt. operator= is not considered for copy initialization - it only looks at user-defined conversions which only include non-explicit constructors and non-explicit conversion functions in this case.

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