C++ template

#include <iostream>
using namespace std;

template <typename T>
T square(T a)
{
    return a * a;
}

int main(void)
{
    cout << square<int>(3) << endl;     // 可以显式告诉编译器是什么类型，也可以让编译器推断
    cout << square(3) << endl;
    cout << square<char>(9.3) << endl;    //  结果是Q ，81的ASCII码
    cout << square<int>(9.3) << endl;     // 结果81
    cout << square(5.5) << endl;         // 30.25 编译器自己推断为double
}

#include <iostream>
using namespace std;

template <typename T>
T square(T a)
{
    return a * a;
}

template <typename T>
class win
{
private:
    T arr[10];
    int size;
public:
    win():size(0) {cout << "creater" << endl; }
    ~win(){};
    int  getSize()const{ return size; }
    T get(int i)const{ return arr[i];}
    void push(T x) { arr[size] = x; size++;}
    void print()const;

};

template <typename T>
void win<T>::print()const{ 
    for (int i = 0; i < size; i++) {
        cout << arr[i] << endl;
    }
}


template <typename T>
win<T> operator*(const win<T> &left, const win<T> &right)
{
    win<T> ret;
    for( int i = 0 ; i < right.getSize(); i++) {
        ret.push(left.get(i) * right.get(i));
    }
    return ret;
}

int main(void)
{
    win<int> a;
    a.push(1);
    a.push(2);
    a.push(3);
    a.push(4);
    a.push(5);
    a.push(6);
    a.print();
    a = square(a);
    a.print();
}

#include <iostream>
using namespace std;

struct Student {
    int id;       
    float score;    
};

template <class T>
class Store {
private:
    T item; 
    bool haveValue;  
public:
    Store();
    T &getElem();  
    void putElem(const T &x);
};

template <class T>
Store<T>::Store(): haveValue(false) { }

template <class T>
T &Store<T>::getElem() {
    if (!haveValue) {
        cout << "No item present!" << endl;
        exit(1);
    }
    return item;
}

template <class T>
void Store<T>::putElem(const T &x) {
    haveValue = true;
    item = x;           
}

int main() {
    Store<int> s1, s2;
    s1.putElem(3);
    s2.putElem(-7);
    cout << s1.getElem() << "  " << s2.getElem() << endl;

    Student g = { 1000, 60.6 };
    Store<Student> s3;
    s3.putElem(g);
    cout << "The student‘s id is " << s3.getElem().id << endl;
    cout << "The student‘s score is " << s3.getElem().score << endl;

    Store<double> d;
    cout << "Retrieving object D... ";
    cout << d.getElem() << endl; //d未初始化，执行函数D.getElement()时导致程序终止
    return 0;
}