Problem with c++ template constructor - c++

Question

code:
#include<iostream>
using namespace std;
template<class T, int N> class point {
T coordinate[N];
public:
point(const point<T,N>&);
const double& operator[](int i) const {
return coordinate[i];
}
};
template<class T, int N> point<T,N>::point(const point<T,N>&p)
{
for(int i=0;i<N;i++)
coordinate[i]=p.coordinate[i];
};
int main() {
point<int,2> P2;
point<double,3> P3;
cout<<P2[0]<<P3[1];
return 0;
}
output:
prog.cpp: In function ‘int main()’:
prog.cpp:17: error: no matching function for call to ‘point<int, 2>::point()’
prog.cpp:11: note: candidates are: point<T, N>::point(const point<T, N>&) [with T =
int, int N = 2]
prog.cpp:18: error: no matching function for call to ‘point<double, 3>::point()’
prog.cpp:11: note: candidates are: point<T, N>::point(const point<T, N>&) [with T =
double, int N = 3]
prog.cpp: In member function ‘const double& point<T, N>::operator[](int) const [with
T = int, int N = 2]’:
prog.cpp:19: instantiated from here
prog.cpp:8: warning: returning reference to temporary
Please help me sort out the faults.

Answer 1

The compiler-generated default constructor is not being provided because you have created a constructor of your own. Therefore when you create P2 with no arguments to its constructor, you need to define a default constructor for it to compile.

Answer 2

When you declare a variable something like,
point<int,2> P2;
It uses default constructor; it can be used in 2 scenarios:
You haven't declared ANY
constructor in your class body. Thus
compiler will generate a default one
automatically and you can use it.
You declare/define a default
constructor explicitly (be it empty,
if you don't do anything)
Since here you don't do anything: just declare an empty default constructor:
template<class T, int N> class point {
//...
public:
point() {} // <-- default constructor
};
This will clear your errors.
Also there is an Important Warning:
prog.cpp:8: warning: returning reference to temporary
That is because of your operator [].
Change the line,
const double& operator[](int i) const
To,
const T& operator[](int i) const // for <int, N> you should return 'int' not 'double'

Answer 3

The problem is that with these two lines
point<int,2> P2;
point<double,3> P3;
you are attempting to create two 'point' object via the default parameterless constructor.
However, this constructor is not automatically generated unless you do not specify any others. Implementing the default constructor will solve you problem