C++ templates - implementing outside the class - c++

Question

Consider the following decleration
template<class T, int N>
class Stack
{
public:
Stack() : T[N]{} {};
class iterator;
iterator insert(iterator it, const T &v);
private:
T[N];
};
template<class T, int N>
class Stack<T,N>::iterator
{
...
};
I want to implement Stack::insert outside the class, so I tried the following
template<class T, int N>
Stack::iterator Stack<T, N>::insert(Stack::iterator p, const T &v)
{
...
}
Now I get the following error
'Stack' is not a class, namespace, or enumeration
I tried to change to the following
template<class T, int N>
Stack<T, N>::iterator Stack<T, N>::insert(Stack::iterator p, const T &v)
{
...
}
and now the error changed to
Missing 'typename' prior to dependent type name 'Stack<T, N>::iterator'
I don't understand why I get this error and how to fix it, hope that someone can help

Answer 1

I am surprised that you are getting the error you said because you have a very obvious syntax error in the constructor in data member:
Stack() : T[N]{} {}; // T[N] isn't valid!
T[N]; // same!
If T = int, the above would translate to int[4] resulting in the error. What you probably want is:
Stack() : arr{} {}
T arr[N];
Assuming this is fixed, we finally get to the error:
Missing 'typename' prior to dependent type name 'Stack<T, N>::iterator'
What this is saying is that iterator is a type and you can't directly access it using :: operator. You have to write typename before Stack<T, N>::iterator so that the compiler knows you want to access the subtype inside Stack. Read the answerWhere and why do I have to put the “template” and “typename” keywords? for more details.
To fix this, change the function to:
template<class T, int N>
typename Stack<T, N>::iterator Stack<T, N>::insert(typename Stack::iterator p, const T &v)
//^^typename here ^^ and here
{
...
}
As you can probably feel that typing so much is quite hectic. Hence auto keyword was introduced to save the programmer from typing these out everytime he wants to use it:
Stack<int,4>::iterator i = s.insert(it, 3); // imagine typing this everytime
//use auto
auto it = s.insert(iterator, 3); //good