C++笔记（十七）类和动态内存分配

实现一个String类：

//learnc170.h

/* c++自动提供下面的成员函数 1：默认构造函数 2：默认析构函数 3：复制构造函数 4：赋值运算符 5：地址运算符 在构造函数中使用new时应注意： 1：在析构函数中使用delete 2：new和delete必须兼容，new对应delete，new[] 对应delete[] 3：如果有多个构造函数，必须以相同的方式使用new(要么都使用new,要么都使用new[]). */
#ifndef LEARNC_170
#define LEARNC_170

#include<iostream>

class String
{
private:
    char *str;
    int len;
    static int num_setting;//静态变量
public:
    String();
    String(const char *s);
    String(const String & st);//复制构造函数 调用时机，赋值或者返回对象的时候。
    ~String();
    friend std::ostream & operator<<(std::ostream &os, const String &st);
    String& operator=(const String &st);//赋值运算符重载
    static int howMany() {//静态函数，只能访问静态变量
        return num_setting;
    }
};

#endif // !LEARNC_170

//learnc171.cpp
#define _CRT_SECURE_NO_WARNINGS

#include<cstring>
#include "learnc170.h"

using std::cout;
using std::endl;


int String::num_setting = 0;

String::String(const char *s) {
    len = strlen(s);
    str = new char[len+1];
    std::strcpy(str, s);
    num_setting++;

    cout << num_setting << ":\"" << str << "\" object created String(const char *s)" << endl;
}

String::String() {
    len = 4;
    str = new char[4];
    std::strcpy(str, "C++");
    num_setting++;
    cout << num_setting << ":\"" << str << "\" object created String()" << endl;
}

String::String(const String & st) {
    len = strlen(st.str);
    str = new char[len + 1];
    std::strcpy(str, st.str);
    num_setting++;
    cout << num_setting << ":\"" << str << "\" object created String(const String & st)" << endl;
}


String::~String() {
    cout << ":\"" << str << "\" object deleted ~String()" << endl;
    num_setting--;
    cout << num_setting << " lefted" << endl;
    delete[] str;
}

std::ostream & operator<<(std::ostream & os, const String & st) {
    os << st.str;
    return os;
}

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

//learnc172.cpp
#include<iostream>
#include "learnc170.h"

using std::cout;
using std::endl;

const String& call(const String &st);


int main() {

    {
        String str1 = "str1";//调用构造函数
        String str2 = String("str2");//调用构造函数
        String str3("str3");//调用构造函数

        cout << "str1:" << str1 << endl;
        cout << "str2:" << str2 << endl;
        cout << "str3:" << str3 << endl;
        cout << "----------------------" << endl;
        String str = call(str1);//引用传参,返回引用
        cout << "str:" << str << endl;

        cout << "**************************" << endl;
        String str4 = str1;//调用复制构造函数，如果没有则调用默认复制构造函数
        cout << "str4:" << str4 << endl;
        cout << "++++++++++++++++++++++++++" << endl;
        String str5;//调用默认构造函数 C++
        str5 = str1;//调用赋值运算符重载函数
        cout << "str5:" << str5 << endl;

        cout << String::howMany() << endl;
        cout << "exiting the block" << endl;
    }

    cout << "end of main" << endl;

    return 0;
}


const String& call(const String &st) {
    cout << "String pass by refrence :" << st << endl;
    return st;
}

输出结果如下：

1:"str1" object created   String(const char *s)
2:"str2" object created   String(const char *s)
3:"str3" object created   String(const char *s)
str1:str1
str2:str2
str3:str3
----------------------
String pass by refrence :str1
4:"str1" object created  String(const String & st)
str:str1
**************************
5:"str1" object created  String(const String & st)
str4:str1
++++++++++++++++++++++++++
6:"C++" object created  String()
operator=
:"C++" object deleted
str5:str1
6
exiting the block
:"str1" object deleted  ~String()
5 lefted
:"str1" object deleted  ~String()
4 lefted
:"str1" object deleted  ~String()
3 lefted
:"str3" object deleted  ~String()
2 lefted
:"str2" object deleted  ~String()
1 lefted
:"str1" object deleted  ~String()
0 lefted
end of main