Error while using a static data member template

I am trying understand the concept of static data member templates. And i came across the following example in a book:

class Collection {
    public: 
    
    template<typename T>
    static T zero = 0;
}; 

When i try to execute the program it gives the error that:

undefined reference to `Collection::zero<int>'

To solve the above error i tried added the following code in the above program but it still gives error:

template<typename T> T Collection::zero = 0; //even after adding this it still gives error

Error now says:

duplicate initialization of 'Collection::zero'

My question is that is this a mistake(typo) in this example of the book. If yes, then what is the problem and how can i solve it?

Yes this is a typo in the book. The problem is that you've specified an initializer for the static data member template even though it is not inline.

Solution

There are 2 ways to solve this problem both of which are given below.

Method 1: C++17

In C++17, you can make use of the keyword inline.

class Collection {
    public:
    
    
    template<typename T>
    inline static T zero = 0; //note the keyword inline here
}; 
//no need for out of class definition of static data member template
int main(){
    int x =Collection::zero<int>;
}

Method 2: C++14

In this case you need to remove the initializer 0 from the in-class declaration of the static data member template.

class Collection {
    public:
    
    template<typename T>
    static T zero ; //note initializer 0 removed from here since this is a declaration
}; 

template<typename T> T Collection::zero = 0;
int main(){
    int x =Collection::zero<int>;
}