1. 预备知识

模板参数可以是数值型参数（非类型参数）

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数值型模板参数的限制
- 变量不能作为模板参数
- 浮点数不能作为模板参数
- 类对象不能作为模板参数
本质：模板参数是在编译阶段被处理的单元，因此，在编译阶段必须准确无误的 唯一确定

2. 问题

最高效的方法求 1 + 2 + 3 + … + N 的值

2.1 编程实验：数值型模板参数

#include <iostream>
#include <string>

using namespace std;

template
< typename T, int N >
void func()
{
    T a[N] = {0};
    
    for(int i=0; i<N; i++)
    {
        a[i] = i;
    }
    
    for(int i=0; i<N; i++)
    {
        cout << a[i] << endl;
    }
}

template
< int N >
class Sum
{
public:
    static const int VALUE = Sum<N-1>::VALUE + N;
};

template
< >
class Sum < 1 >
{
public:
    static const int VALUE = 1;
};


int main()
{
    cout << "1 + 2 + 3 + ... + 10 = " << Sum<10>::VALUE << endl;
    cout << "1 + 2 + 3 + ... + 100 = " << Sum<100>::VALUE << endl;
    
    return 0;
}

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2.2 编程实验：数组模板类

// array.h
#ifndef _ARRAY_H_
#define _ARRAY_H_

template
<typename T, int N>
class Array {
    T m_array[N];
public:
    int length();
    bool set(int index, T value);
    bool get(int index, T& value);
    T& operator[] (int index);
    T operator[] (int index) const;
    virtual ~Array();
};

template
<typename T, int N>
int Array<T, N>::length() {
    return N;
}

template
<typename T, int N>
bool Array<T, N>::set(int index, T value) {
    bool ret = (0 <= index) && (index < N);
    if (ret) {
        m_array[index] = value;
    }

    return ret;
}

template
<typename T, int N>
bool Array<T, N>::get(int index, T& value) {
    bool ret = (0 <= index) && ( index < N);
    if (ret) {
        value = m_array[index];
    }

    return ret;
}

template
<typename T, int N>
T& Array<T, N>::operator[] (int index) {
    return m_array[index];
}

template
<typename T, int N>
T Array<T, N>::operator[] (int index) const {
    return m_array[index];
}

template
<typename T, int N>
Array<T, N>::~Array() {

}

#endif

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2.3 编程实验：堆数组模板类

// HeapArray.h
#ifndef _HEAPARRAY_H_
#define _HEAPARRAY_H_

template
< typename T >
class HeapArray {
private:
    int m_length;
    T* m_pointer;
    
    HeapArray(int len);
    HeapArray(const HeapArray<T>& obj);
    bool construct();
public:
    static HeapArray<T>* NewInstance(int length); 
    int length();
    bool get(int index, T& value);
    bool set(int index ,T value);
    T& operator [] (int index);
    T operator [] (int index) const;
    HeapArray<T>& self();
    ~HeapArray();
};

template
< typename T >
HeapArray<T>::HeapArray(int len) {
    m_length = len;
}

template
< typename T >
bool HeapArray<T>::construct() {
    m_pointer = new T[m_length];
    
    return m_pointer != NULL;
}

template
< typename T >
HeapArray<T>* HeapArray<T>::NewInstance(int length)  {
    HeapArray<T>* ret = new HeapArray<T>(length);
    
    if( !(ret && ret->construct())) {
        delete ret;
        ret = 0;
    }
    
    return ret;
}

template
< typename T >
int HeapArray<T>::length() {
    return m_length;
}

template
< typename T >
bool HeapArray<T>::get(int index, T& value) {
    bool ret = (0 <= index) && (index < length());
    
    if( ret ) {
        value = m_pointer[index];
    }
    
    return ret;
}

template
< typename T >
bool HeapArray<T>::set(int index, T value) {
    bool ret = (0 <= index) && (index < length());
    
    if( ret ) {
        m_pointer[index] = value;
    }
    
    return ret;
}

template
< typename T >
T& HeapArray<T>::operator [] (int index) {
    return m_pointer[index];
}

template
< typename T >
T HeapArray<T>::operator [] (int index) const {
    return m_pointer[index];
}

template
< typename T >
HeapArray<T>& HeapArray<T>::self() {
    return *this;
}

template
< typename T >
HeapArray<T>::~HeapArray() {
    delete[]m_pointer;
}

#endif