21-使用指向对象的指针

12.5　使用指向对象的指针

C++程序经常使用指向对象的指针，因此，这里来练习一下。程序清单12.6使用数组索引值来跟踪最短的字符串和按字母顺序排在最前面的字符串。另一种方法是使用指针指向这些类别的开始位置，程序清单12.7使用两个指向String的指针实现了这种方法。最初，shortest指针指向数组中的第一个对象。每当程序找到比指向的字符串更短的对象时，就把shortest重新设置为指向该对象。同样，first指针跟踪按字母顺序排在最前面的字符串。这两个指针并不创建新的对象，而只是指向已有的对象。因此，这些指针并不要求使用new来分配内存。

除此之外，程序清单12.7中的程序还使用一个指针来跟踪新对象：

String * favorite = new String(sayings[choice]);

这里指针favorite指向new创建的未被命名对象。这种特殊的语法意味着使用对象saying [choice]来初始化新的String对象，这将调用复制构造函数，因为复制构造函数（const String &）的参数类型与初始化值（saying [choice]）匹配。程序使用srand()、rand()和time()随机选择一个值。

程序清单12.7　sayings2.cpp

// sayings2.cpp -- using pointers to objects
// compile with string1.cpp
#include <iostream>
#include <cstdlib> // (or stdlib.h) for rand(), srand()
#include <ctime>   // (or time.h) for time()
#include "string1.h"
const int ArSize = 10;
const int MaxLen = 81;
int main()
{
    using namespace std;
    String name;
    cout <<"Hi, what's your name?\n ";
    cin >> name;
    cout << name << ", please enter up to " << ArSize
         << " short sayings <empty line to quit>:\n";
    String sayings[ArSize];
    char temp[MaxLen];            // temporary string storage
    int i;
    for (i = 0; i < ArSize; i++)
    {
        cout << i+1 << ": ";
        cin.get(temp, MaxLen);
        while (cin && cin.get() != '\n')
            continue;
        if (!cin || temp[0] == '\0') // empty line?
            break; // i not incremented
        else
            sayings[i] = temp; // overloaded assignment
    }
    int total = i; // total # of lines read
    if (total > 0)
    {
        cout << "Here are your sayings:\n";
        for (i = 0; i < total; i++)
            cout << sayings[i] << "\n";
    // use pointers to keep track of shortest, first strings
        String * shortest = &sayings[0]; // initialize to first object
        String * first = &sayings[0];
        for (i = 1; i < total; i++)
        {
            if (sayings[i].length() < shortest->length())
                shortest = &sayings[i];
            if (sayings[i] < *first) // compare values
                first = &sayings[i]; // assign address
        }
        cout << "Shortest saying:\n" << * shortest << endl;
        cout << "First alphabetically:\n" << * first << endl;
        srand(time(0));
        int choice = rand() % total; // pick index at random
    // use new to create, initialize new String object
        String * favorite = new String(sayings[choice]);
        cout << "My favorite saying:\n" << *favorite << endl;
        delete favorite;
    }
    else
        cout << "Not much to say, eh?\n";
    cout << "Bye.\n";
    return 0;
}

使用new初始化对象

通常，如果Class_name是类，value的类型为Type_name，则下面的语句：

Class_name * pclass = new Class_name(value);

将调用如下构造函数：

Class_name(Type_name);

这里可能还有一些琐碎的转换，例如：

Class_name(const Type_name &);

另外，如果不存在二义性，则将发生由原型匹配导致的转换（如从int到double）。下面的初始化方式将调用默认构造函数：

Class_name * ptr = new Class_name;

下面是程序清单12.7中程序的运行情况：

Hi, what's your name?
>> Kirt Rood
Kirt Rood, please enter up to 10 short sayings <empty line to quit>:
1: a friend in need is a friend indeed
2: neither a borrower nor a lender be
3: a stitch in time saves nine
4: a niche in time saves stine
5: it takes a crook to catch a crook
6: cold hands, warm heart
7:
Here are your sayings:
a friend in need is a friend indeed
neither a borrower nor a lender be
a stitch in time saves nine
a niche in time saves stine
it takes a crook to catch a crook
cold hands, warm heart
Shortest saying:
cold hands, warm heart
First alphabetically:
a friend in need is a friend indeed
My favorite saying:
a stitch in time saves nine
Bye

由于该程序随机选择用户输入的格言，因此即使输入相同，显示的结果也可能不同。