如何在模板 class 上实现运算符重载？

Question

I am having an issue trying to overload the addition operator on a template class for complex numbers.

In my.h I have forward declaration for both the class and the operator function function, and I specifically overloaded operator+<T> rather than just operator+ so that it works for multiple types. It compiles fine when I only include the.h file, but as soon as I try to declare MyComplex<int> it gives some weird errors that I'm having difficulty deciphering.

my_complex.h:

#ifndef _MY_COMPLEX_H
#define _MY_COMPLEX_H

using namespace std;

//forward declarations
template<typename T> class MyComplex;

template<typename T> MyComplex<T> operator+(const MyComplex<T>& cmplx1, const MyComplex<T>& cmplx2);

template<typename T>
class MyComplex{
        private:
                T _real;
                T _imaginary;
        public:
                MyComplex();
                MyComplex(T real, T img);
                T getReal();
                T getImaginary();
                void setReal(T real);
                void setImaginary(T img);

                friend MyComplex operator+<T>(const MyComplex& cmplx1, const MyComplex& cmplx2);
}
#endif

my_complex.cpp:

#include "my_complex.h"
...
template <typename T>
MyComplex<T> operator+(const MyComplex<T>& cmplx1, const MyComplex<T>& cmplx2){
        MyComplex<T> temp;
        temp._real = cmplx1._real + cmplx2._real;
        temp._imaginary = cmplx1._imaginary + cmplx2._imaginary;

        return temp;
}
...

main.cpp:

#include "my_complex.h"

int main(int argc, char* argv[]){
        MyComplex<int> cmplx1 = MyComplex<int>(3, 5);//error is here
        MyComplex<int> cmplx2 = MyComplex<int>(5, 7);

        cmplx1 = cmplx1 + cmplx2;

        return 0;

}

It compiles fine when I only include the.h file in main file, but as soon as I try to declare MyComplex<int> it gives some errors that I am having difficulty deciphering.

In file included from /usr/include/c++/7/bits/stl_algobase.h:67:0,
                     from /usr/include/c++/7/bits/char_traits.h:39,
                     from /usr/include/c++/7/ios:40,
                     from /usr/include/c++/7/ostream:38,
                     from /usr/include/c++/7/iostream:39,
                     from main.cpp:1:
    /usr/include/c++/7/bits/stl_iterator.h: In instantiation of ‘class std::reverse_iterator<int>’:
    /usr/include/c++/7/bits/stl_iterator.h:400:5:   required by substitution of ‘template<class _Iterator> std::reverse_iterator<_Iterator> std::operator+(typename std::reverse_iterator<_Iterator>::difference_type, const std::reverse_iterator<_Iterator>&) [with _Iterator = int]’
    my_complex.h:36:20:   required from ‘class MyComplex<int>’
    main.cpp:8:17:   required from here
    /usr/include/c++/7/bits/stl_iterator.h:101:11: error: no type named ‘iterator_category’ in ‘struct std::iterator_traits<int>’
         class reverse_iterator
               ^~~~~~~~~~~~~~~~
    /usr/include/c++/7/bits/stl_iterator.h:115:55: error: no type named ‘difference_type’ in ‘struct std::iterator_traits<int>’
           typedef typename __traits_type::difference_type difference_type;
                                                           ^~~~~~~~~~~~~~~
    /usr/include/c++/7/bits/stl_iterator.h:116:48: error: no type named ‘pointer’ in ‘struct std::iterator_traits<int>’
           typedef typename __traits_type::pointer  pointer;
                                                    ^~~~~~~
    /usr/include/c++/7/bits/stl_iterator.h:117:50: error: no type named ‘reference’ in ‘struct std::iterator_traits<int>’
           typedef typename __traits_type::reference  reference;
                                                      ^~~~~~~~~
    /usr/include/c++/7/bits/stl_iterator.h: In instantiation of ‘class std::move_iterator<int>’:
    /usr/include/c++/7/bits/stl_iterator.h:1201:5:   required by substitution of ‘template<class _Iterator> std::move_iterator<_IteratorL> std::operator+(typename std::move_iterator<_IteratorL>::difference_type, const std::move_iterator<_IteratorL>&) [with _Iterator = int]’
    my_complex.h:36:20:   required from ‘class MyComplex<int>’
    main.cpp:8:17:   required from here
    /usr/include/c++/7/bits/stl_iterator.h:1019:50: error: no type named ‘reference’ in ‘struct std::iterator_traits<int>’
           typedef typename __traits_type::reference  __base_ref;
                                                      ^~~~~~~~~~
    /usr/include/c++/7/bits/stl_iterator.h:1023:57: error: no type named ‘iterator_category’ in ‘struct std::iterator_traits<int>’
           typedef typename __traits_type::iterator_category iterator_category;
                                                             ^~~~~~~~~~~~~~~~~
    /usr/include/c++/7/bits/stl_iterator.h:1024:52: error: no type named ‘value_type’ in ‘struct std::iterator_traits<int>’
           typedef typename __traits_type::value_type   value_type;
                                                        ^~~~~~~~~~
    /usr/include/c++/7/bits/stl_iterator.h:1025:55: error: no type named ‘difference_type’ in ‘struct std::iterator_traits<int>’
           typedef typename __traits_type::difference_type difference_type;
                                                           ^~~~~~~~~~~~~~~
    /usr/include/c++/7/bits/stl_iterator.h:1032:24: error: no type named ‘reference’ in ‘struct std::iterator_traits<int>’
         __base_ref>::type  reference;
                            ^~~~~~~~~

Answer 1

You cannot define a templated method in a.cpp file, put it in the header and it should work. See https://isocpp.org/wiki/faq/templates#templates-defn-vs-decl

Furthermore, this

friend MyComplex operator+<T>(const MyComplex& cmplx1, const MyComplex& cmplx2);
// should match the forward declaration
friend MyComplex<T> operator+<>(const MyComplex<T>& cmplx1, const MyComplex<T>& cmplx2);

so you should add <T>

This actually does not explain your error, but the error message does not fit the files you gave, the.hpp has not 36 lines but that's where the error is, so debugging this is quite hard.