C++类模板

待更！

 egg1

//
//  stacktp.hpp
//
//  Created by Min Zhu on 16/10/6.
//  Copyright © 2016年 Min Zhu. All rights reserved.
//

#ifndef stacktp_hpp
#define stacktp_hpp

#include <stdio.h>

template <class Type>
class Stack {
private:
    enum{MAX = 10};     // constant specific to class
    Type items[MAX];    // holds stack items
    int top;            // index for top stack item
    
public:
    Stack();
    bool isempty();
    bool isfull();
    bool push(const Type & item);   // add item to satck
    bool pop(Type & item);          // pop top into item
};

template <class Type>
Stack<Type>::Stack() {
    top = 0;
};

template <class Type>
bool Stack<Type>::isempty(){
    return top == 0;
}

template <class Type>
bool Stack<Type>::isfull(){
    return top == MAX;
};

template <class Type>
bool Stack<Type>::push(const Type & item){
    if(top < MAX){
        items[top++] = item;
        return true;
    }
    else
        return false;
}

template <class Type>
bool Stack<Type>::pop(Type & item){
    if(top > 0){
        item = items[--top];
        return true;
    }
    else
        return false;
}


#endif /* stacktp_hpp */

//
//  stacktp.cpp
//
//  Created by Min Zhu on 16/10/6.
//  Copyright © 2016年 Min Zhu. All rights reserved.
//

#include "stacktp.hpp"
#include <iostream>
#include <string>
#include <cctype>
using namespace std;

int main(){
    Stack<string> st;  // create an empty stack
    char ch;
    string po;
    cout << "Please enter A to add apurchase order,
" << "P to process a PO, or Q to quit.
";
    
    while(cin >> ch && toupper(ch) != 'Q'){
        while(cin.get() != '
')
            continue;
        if(!isalpha(ch)){
            cout << 'a';
            continue;
        }
        switch (ch) {
            case 'A':
            case 'a':
                cout << "Enter a PO number to add: ";
                cin >> po;
                if(st.isfull())
                    cout << "stack already full
";
                else
                    st.push(po);
                break;
            case 'P':
            case 'p':
                if(st.isempty())
                    cout << "stack already empty
";
                else{
                    st.pop(po);
                    cout << "PO #" << po << " popped
";
                    break;
                }
        }
        cout << "Please enter A to add a purchase order,
" << "P to process a PO, or Q to quit.
";
    }
    cout << "Bye
";
    return 0;
}

运行结果（Xcode）：

Please enter A to add apurchase order,
P to process a PO, or Q to quit.
A
Enter a PO number to add: red911porsche
Please enter A to add a purchase order,
P to process a PO, or Q to quit.
A
Enter a PO number to add: blueR8audi
Please enter A to add a purchase order,
P to process a PO, or Q to quit.
A
Enter a PO number to add: silver747boeing
Please enter A to add a purchase order,
P to process a PO, or Q to quit.
P
PO #silver747boeing popped
Please enter A to add a purchase order,
P to process a PO, or Q to quit.
P
PO #blueR8audi popped
Please enter A to add a purchase order,
P to process a PO, or Q to quit.
P
PO #red911porsche popped
Please enter A to add a purchase order,
P to process a PO, or Q to quit.
P
stack already empty
Please enter A to add a purchase order,
P to process a PO, or Q to quit.
Q
Bye
Program ended with exit code: 0

　　

深入探讨模板类

可以使用char指针替换上面程序中的string对象吗？毕竟，这种指针是处理c风格字符串的内置方式。答案是可以创建指针栈，不过必须对程序包进行重大修改。

1.不正确地使用指针栈

设计模板时应牢记一些教训，切忌盲目使用模板。以下列举3个试图对上面程序eg1进行修改，使之使用指针栈的简单（但有缺陷的）示例。它们都以完全正确的Stack<Type>模板为基础：Stack<char *> st;　

版本1，将eg1中的string po;替换为char * po;

目的是用char指针而不是string对象来接收键盘输入。这种方法是失败的：因为仅仅创建指针而没有创建用于保存输入字符串的空间（程序将通过编译，但在cin试图将输入保存在某些不合适的内存单元中时崩溃）

版本2，将string po;替换为char po[40];

这为输入的字符串分配了空间。另外，po的类型为char *，因此可以放在栈中，不过数组与pop()方法的假设相冲突：

template <class Type>
bool Stack<Type>::pop(Type & item){
    if(top > 0){
        item = items[--top];
        return true;
    }
    else
        return false;
}

首先，引用变量必须引用某种类型的左值，而不是数组名。其次，代码假设可以给item赋值，即使item能够引用数组，也不能为数组名赋值。故此法失效。

版本3，将string po;替换为char * po = new char[40];

这为输入的字符串分配了空间。另外，po的类型为变量，因此与pop()的代码兼容。然而问题是：只有一个pop变量，它总是指向相同的内存单元。在每当读取新字符串时，内存的内容都将发生改变，而每次执行压入操作时，加入到栈中的地址都相同。则对栈执行弹出操作时，得到的地址总是相同的，它总是指向读入的最后一个字符串。具体就是栈并未保存每一个新字符串，因而没有什么用途。

2.正确地使用指针栈

使用指针栈的方法之一是，让调用程序提供一个指针数组，其中每个指针指向不同的字符串。栈的任务是管理指针，而不是创建指针。

//stcktp1.h -- modified Stack template
#ifndef STCKTP1_H_
#define STCKTP1_H_

template <class Type>
class Stack{
private:
	enum {SIZE = 10};
	int stacksize;
	Type * items;
	int top;
public:
	explicit Stack(int ss = SIZE);
	Stack(const Stack & st);
	~Stack(){ 
		delete[] items;
	}
	bool isempty(){
		return top == 0;
	}
	bool isfull(){
		return top == stacksize;
	}
	bool push(const Type & item);	// add item to stack
	bool pop(Type & item);			// pop top into item
	Stack & operator=(const Stack & st);
};

template <class Type>
Stack<Type>::Stack(int ss) :stacksize(ss), top(0){
	items = new Type [stacksize];
}

template <class Type>
Stack<Type>::Stack(const Stack & st){
	stacksize = st.stacksize;
	top = st.top;
	items = new Type [stacksize];
	for (int i = 0; i < top; i++){
		items[i] = st.items[i];
	}
}

template <class Type>
bool Stack<Type>::push(const Type & item){
	if (top < stacksize){
		items[top++] = item;
		return true;
	}
	else
		return false;
}

template <class Type>
bool Stack<Type>::pop(Type & item){
	if (top > 0){
		item = items[--top];
		return true;
	}
	else
		return false;
}

template <class Type>
Stack<Type> & Stack<Type>::operator=(const Stack<Type> & st){
	if (this == &st)
		return *this;
	delete [] items;
	stacksize = st.stacksize;
	top = st.top;
	items = new Type [stacksize];
	for (int i = 0;i< top; i++) {
		items[i] = st.items[i];
	}
	return *this;
}

#endif

　　

#include <iostream>
#include <cstdlib>   // for rand().srand()
#include <ctime>     // for time()
#include "stcktp1.h"
using namespace std;

const int Num = 10;

int main(){
	srand(time(0));  //randomize rand()
	cout << "Please enter stack size:";
	int stacksize;
	cin >> stacksize;
	//create an empty stack with stacksize slots
	Stack<const char *> st(stacksize);

	//in basket
	const char * in[Num] = {
		" 1: Hank Gilgamesh", " 2: Kiki Ishatar",
		" 3: Betty Rocker", " 4: Ian Flagrant",
		" 5: Wolfgang Kibble", " 6: Portia koop",
		" 7: Joy Almondo", " 8: Xaverie Paprika",
		" 9: Juan Moore", "10: Misha Mache"
	};

	//out basket
	const char * out[Num];

	int processed = 0;
	int nextin = 0;
	while (processed < Num){
		if (st.isempty())
			st.push(in[nextin++]);
		else if (st.isfull())
			st.pop(out[processed++]);
		else if (rand() % 2 && nextin < Num)
			st.push(in[nextin++]);
		else
			st.pop(out[processed++]);
	}
	for (int i = 0; i < Num; i++){
		cout << out[i] << endl;
	}
	
	cout << "Bye
";
	return 0;
}