c++ copy constructor between different templates

Question

I have a template class A. What I want to do is to write a copy constructor between A with template T1 and A with template T2. How do I solve this? Here is some example code from the struct header:

template <typename T>
struct A {
 A(const A<T>& other); // copy from same type T
 template <typename T2>
 A(const A<T2>& other); // copy from different type T2
}

How would I go about to implement this in a .cpp file?

EDIT: What I really meant was: "How would I go about to implement this outside the class definition?" (a syntax problem)

• 463035818_is_not_an_ai: templates go in header files
• Myone: of course! What I meant was "How would I go about to implement this outside the class definition?"

Answer 1

The syntax for defining a template member function of a template class is not immediately obvious. You need to have two template parameter lists; one for the class and one for the function:

template <typename T> //class template parameters
template <typename T2> //function template parameters
A<T>::A (const A<T2>& other) {
 //...
}

Note that templates need to be implemented in the header file, so you can't put this in the .cpp.

Answer 2

You cannot define template function in .cpp file. But in header, outside of class definition you can just

template<typename T>
template<typename T2>
A<T>::A(const A<T2&>& other)
{
 // implementation
}

Answer 3

You could make that other constructor a template itself:

template< class T > class A
{
public:
 A()
 {
 }
 A(const A<T> & other)
 {
 cout << "T" << endl;
 }
 template<class T1> A(const A<T1> & other)
 {
 cout << "T1" << endl;
 }
};
int main()
{
 A<int> a1;
 A<int> a2 = a1; // prints "T"
 A<double> a3 = a1; // prints "T1"
 return 0;
}