您如何使用模板来专门化常量

Question

I want to return a different constant for each different type that is used for the templated class.

For example, when the class is

Stack< int > I want the constant EMPTY = -99

when the class is Stack< char > I want the constant EMPTY to be '\\0'.

So far I have been searching for the syntax for how to explicitly specialize variables and have no idea where to start looking for this.

A simple code example would be amazing, as this issue has been bugging me for a while.

What I have so far (without constants) is:

template<class T>
class Stack {
private:
       T* items_;
        //EMPTY constant here

public:

    Stack();

    ~Stack();

    void push(T value);

    T pop();
};

Answer 1

template<class T>
class Stack {
private:
       T* items_;
       static T EMPTY;

public:
    Stack();
    ~Stack();

    void push(T value);
    T pop();
};

extern template<> int Stack<int>::EMPTY;
extern template<> char Stack<char>::EMPTY;

Then in a cpp file:

template<> int Stack<int>::EMPTY = -99;
template<> char Stack<char>::EMPTY = '\0';

Note depending on usage this will limit the types you can instantiate your template with.

Answer 2

You might want to consider whether you want this to be part of your stack, or rather factor it out to a general traits class. Depending on what you do, it might be useful for other data structures.

template<typename T>
struct traits
{
    static T empty;
};
template<> int traits<int>::empty = 99;
template<> char traits<char>::empty = 0;


template<class T>
class Stack 
{
public:
    Stack()
    {
        T t = traits<T>::empty; 
    }
};

int main()
{
    Stack<int>(); 
    Stack<char>(); 
}