Undefined reference error in C++ Link step [duplicate] - c++

Question

This question already has answers here:
Why can templates only be implemented in the header file?
(17 answers)
Closed 8 years ago.
I'm trying to make a simple bounds checked array in C++. I have declared a class in a header file, and defined the class in a separate source file. The compile step runs fine, but when I try to link the objects, I get the following error:
$ g++.exe -o a.exe src\main.o src\array.o
src\main.o: In function `main':
../src/main.cpp:7: undefined reference to `Array<int>::Array(int)'
../src/main.cpp:9: undefined reference to `Array<int>::operator[](int)'
../src/main.cpp:10: undefined reference t o `Array<int>::operator[](int)'
../src/main.cpp:11: undefined reference t o `Array<int>::operator[](int)'
../src/main.cpp:13: undefined reference t o `Array<int>::operator[](int)'
../src/main.cpp:7: undefined reference to `Array<int>::~Array()'
../src/main.cpp:7: undefined reference to `Array<int>::~Array()'
collect2.exe: error: ld returned 1 exit status
main.cpp
#include <iostream>
#include "array.hpp"
using namespace std;
int main() {
Array<int> x(10); // compiler error
x[0] = 1; // compiler error
x[1] = 2; // compiler error
x[2] = 3; // compiler error
cout << x[1] << endl; // compiler error
return 0;
}
array.hpp
#ifndef ARRAY_HPP_
#define ARRAY_HPP_
template <class T>
class Array{
private:
T* array;
int length_;
public:
Array(int);
~Array();
int Length();
int& operator[](int);
};
#endif /* ARRAY_HPP_ */
array.cpp
#include "array.hpp"
template <class T>
Array<T>::Array(int size) {
length_ = size;
array = new T[size];
}
template <class T>
Array<T>::~Array() {
delete[] array;
}
template <class T>
int Array<T>::Length() {
return length_;
}
template <class T>
int& Array<T>::operator[](const int index) {
if (index < 0 || index >= length_) {
throw 100;
}
return array[index];
}

Answer 1

Definitions of members of a template class shall be in the same header where the template class is defined itself.

Answer 2

Template classes must have all their code in the header file or they can only be used for types you explicitly instantiated in the cpp file.