六十课 数组类模板

我们前面学习了函数模板、类模板了，我们现在有能力来开发一个数组类模板。但是还需要些知识点。

一、预备知识

1、模板参数可以是数值型参数

template 
<typename T,int N>
void func
{
   T a[N]; //使用模板参数定义局部数组
}

2、数值型参数的限制

• 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处理的单元，因此，在编译阶段必须准确无误的唯一确定。

二、有趣的面试题

用你觉得最高效的办法求1+2+3.....+N的值！

#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;
}

示例：数组模板类的实现

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

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

#include <iostream>
#include <string>
#include "Array.h"
#include "HeapArray.h"

using namespace std;

int main()
{
    Array<double, 5> ad;
    
    for(int i=0; i<ad.length(); i++)
    {
        ad[i] = i * i;
    }
    
    for(int i=0; i<ad.length(); i++)
    {
        cout << ad[i] << endl;
    }
    
    cout << endl;
    
    HeapArray<char>* pai = HeapArray<char>::NewInstance(10);
    
    if( pai != NULL )
    {
        HeapArray<char>& ai = pai->self();
        
        for(int i=0; i<ai.length(); i++)
        {
            ai[i] = i + 'a';
        }
        
        for(int i=0; i<ai.length(); i++)
        {
            cout << ai[i] << endl;
        }
    }
    
    delete pai;
    
    return 0;
}