c++oop分析二：

动态内存和类

stringBad类包含一个字符串指针和一个字符串长度的值。使用new 和delete来动态开发该类：

// strngbad.h -- flawed string class definition#define _CRT_SECURE_NO_WARNINGS //加入放弃安全检查#include <iostream>#ifndef STRNGBAD_H_#define STRNGBAD_H_class StringBad{private:    char * str;                // pointer to string    int len;                   // length of string    static int num_strings;    // number of objectspublic:    StringBad(const char * s); // constructor    StringBad();               // default constructor    ~StringBad();              // destructor// friend function    friend std::ostream & operator<<(std::ostream & os,                        const StringBad & st);};#endif

// strngbad.cpp -- StringBad class methods#define _CRT_SECURE_NO_WARNINGS#include <cstring>                    // string.h for some#include "strngbad.h"using std::cout;// initializing static class memberint StringBad::num_strings = 0;// class methods// construct StringBad from C stringStringBad::StringBad(const char * s){    len = std::strlen(s);             // set size    str = new char[len + 1];          // allot storage    std::strcpy(str, s);              // initialize pointer    num_strings++;                    // set object count    cout << num_strings << ": \"" << str         << "\" object created\n";    // For Your Information}StringBad::StringBad()                // default constructor{    len = 4;    str = new char[4];    std::strcpy(str, "C++");          // default string    num_strings++;    cout << num_strings << ": \"" << str         << "\" default object created\n";  // FYI}StringBad::~StringBad()               // necessary destructor{    cout << "\"" << str << "\" object deleted, ";    // FYI    --num_strings;                    // required    cout << num_strings << " left\n"; // FYI    delete [] str;                    // required}std::ostream & operator<<(std::ostream & os, const StringBad & st){    os << st.str;    return os; }

// vegnews.cpp -- using new and delete with classes// compile with strngbad.cpp#include <iostream>using std::cout;#include "strngbad.h"void callme1(StringBad &);  // pass by referencevoid callme2(StringBad);    // pass by valueint main(){    using std::endl;    {        cout << "Starting an inner block.\n";        StringBad headline1("Celery Stalks at Midnight");        StringBad headline2("Lettuce Prey");        StringBad sports("Spinach Leaves Bowl for Dollars");        cout << "headline1: " << headline1 << endl;        cout << "headline2: " << headline2 << endl;        cout << "sports: " << sports << endl;        callme1(headline1);        cout << "headline1: " << headline1 << endl;        callme2(headline2);        cout << "headline2: " << headline2 << endl;        cout << "Initialize one object to another:\n";        StringBad sailor = sports;        cout << "sailor: " << sailor << endl;        cout << "Assign one object to another:\n";        StringBad knot;        knot = headline1;        cout << "knot: " << knot << endl;         cout << "Exiting the block.\n";    }    cout << "End of main()\n";    // std::cin.get();    return 0;}void callme1(StringBad & rsb){    cout << "String passed by reference:\n";    cout << "    \"" << rsb << "\"\n";}void callme2(StringBad sb){    cout << "String passed by value:\n";    cout << "    \"" << sb << "\"\n";}

nums_string被创建为静态类变量，所有对象共享同一个静态成员。
构造函数：
StringBad::StringBad(const char * s)
{
len = std::strlen(s); // set size
str = new char[len + 1]; // allot storage
std::strcpy(str, s); // initialize pointer
num_strings++; // set object count
cout << num_strings << “: \”” << str
<< “\” object created\n”; // For Your Information
}
析构函数：
StringBad::~StringBad() // necessary destructor
{
cout << “\”” << str << “\” object deleted, “; // FYI
–num_strings; // required
cout << num_strings << ” left\n”; // FYI
delete [] str; // required
}
一般的在构造函数里面使用new来分配内存，必须在相应的析构函数中用delete来释放内存。
下面分析程序运行情况：

Starting an inner block.1: "Celery Stalks at Midnight" object created2: "Lettuce Prey" object created3: "Spinach Leaves Bowl for Dollars" object createdheadline1: Celery Stalks at Midnightheadline2: Lettuce Preysports: Spinach Leaves Bowl for DollarsString passed by reference:    "Celery Stalks at Midnight"headline1: Celery Stalks at MidnightString passed by value:    "Lettuce Prey""Lettuce Prey" object deleted, 2 leftheadline2: 葺葺葺葺葺葺葺葺菅Initialize one object to another:sailor: Spinach Leaves Bowl for DollarsAssign one object to another:3: "C++" default object createdknot: Celery Stalks at MidnightExiting the block."Celery Stalks at Midnight" object deleted, 2 left"Spinach Leaves Bowl for Dollars" object deleted, 1 left"葺葺葺葺葺葺葺葺葺葺葺葺葺葺葺葺葺葺葺葺" object deleted, 0 left

*结果分析：
程序明显出现乱码，就是复制构造函数惹的祸，即
在使用下列语句初始化对象的时候：
StringBad sailor = sports;
knot = headline1;
编译器自动生成复制构造函数为对象创建一个副本，但此函数并不知道要更新静态变量
nums_string。*

解决：
定义一个显示的复制构造函数来解决问题：也就是说进行深度复制，复制构造函数应当复制字符串并将副本的地址给str成员，而不仅仅是复制字符串的地址。

String::String(const String & st){    num_strings++;             // handle static member update    len = st.len;              // same length    str = new char [len + 1];  // allot space    std::strcpy(str, st.str);  // copy string to new location}

关于赋值运算符

c++允许类对象赋值，这是通过自动为类重载赋值运算符实现的。
例如：
StringBad &StringBad::operator=(const StringBad &)

下面解决赋值的问题：
（1）由于目标对象可能引用了以前的数据，所以函数使用delete[]来释放这些数据。
（2）函数应该避免对象赋给自身；
（3）函数返回一个指向调用对象的引用

String & String::operator=(const String & st){    if (this == &st)        return *this;    delete [] str;    len = st.len;    str = new char[len + 1];    std::strcpy(str, st.str);    return *this;}