C++ Primer Plus 学习笔记第十六章 string类，智能指针模板类

红太狼 2022-01-15 10:55 252阅读 0赞

使用string类

string类对每个关系运算符都做了重载 并且都以三种方式被重载：

string 与string

string与c-string

c-string 与 string

用心， 赞！

size()和length()都能够返回字符串中的字符数 效果一样

find()

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3NvdWx3eWI_size_16_color_FFFFFF_t_70][]

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3NvdWx3eWI_size_16_color_FFFFFF_t_70 1][]

string类的内存分配原理

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3NvdWx3eWI_size_16_color_FFFFFF_t_70 2][]

程序示例

#include <iostream>
    #include <string>
    
    int main()
    {
      using namespace std;
      string empty;
      string small = "bit";
      string larger = "Elephants are a girl's best friend";
      cout << "Sizes:\n";
      cout << "\tempty: " << empty.size() << endl;
      cout << "\tsmall: " << small.size() << endl;
      cout << "\tlarger: " << larger.size() << endl;
      cout << "Capacities:\n";
    // 查看string对象分配的内存大小
      cout << "\tempty: " << empty.capacity() << endl;
      cout << "\tsmall: " << small.capacity() << endl;
      cout << "\tlarger: " << larger.capacity() << endl;
    // 手动设置string对象的内存大小
      empty.reserve(50);
      cout << "Capacity after empty.reserve(50): "
           << empty.capacity() << endl;
    
      return 0;
    }

运行结果

![20190613094714790.png][]

通过c\_str()函数能够将string转换成c风格字符串 返回指向该字符串的指针。

string种类

![20190613095151694.png][]

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3NvdWx3eWI_size_16_color_FFFFFF_t_70 3][]

只能指针模板类

有3种

auto\_ptr

unique\_ptr

shared\_ptr

![20190613095701683.png][]

智能指针模板包含在memory头文件中

示例：

#include <iostream>
    #include <string>
    #include <memory>
    
    class Report
    {
      private:
        std::string str;
      public:
        Report(const std::string s) : str(s)
          { std::cout << "Object created!\n";}
        ~Report() {std::cout << "Object deleted!\n";}
        void comment() const {std::cout << str << "\n";}
    };
    
    int main()
    {
      // 我这里的编译器表示木有这货 估计被移除了
      // {
      //   std::auto_ptr<Report> ps (new Report("using auto_ptr"));
      //   ps->comment();
      // }
      {
        std::shared_ptr<Report> ps (new Report("using shared_ptr"));
        ps->comment();
      }
      {
        std::unique_ptr<Report> ps (new Report ("using unique_ptr"));
        ps->comment();
      }
      return 0;
    }

![20190613104934866.png][]

不要把非堆内存丢到智能指针中去

shared\_ptr：策略

![20190613111151735.png][]

示例：

#include <iostream>
    #include <string>
    #include <memory>
    
    int main()
    {
      using namespace std;
      shared_ptr<string> films[5] = 
      {
        shared_ptr<string> (new string("Fowl Balls")),
        shared_ptr<string> (new string("Duck Walks")),
        shared_ptr<string> (new string("Chicken Runs")),
        shared_ptr<string> (new string("Turkey Errors")),
        shared_ptr<string> (new string("Goose Eggs"))
      };
      shared_ptr<string> pwin;
      pwin = films[2];
      cout << "The nominees for best avian baseball film are\n";
      for (int i = 0; i < 5; i++)
        cout << *films[i] << endl;
      cout << "The winner is " <<*pwin << "!\n";
      cin.get();
      return 0;
    }

运行结果

![20190613111233205.png][]

![2019061311134466.png][]

unique\_ptr

![20190613112348213.png][]

两个智能指针指向同一个对象的时候 unique\_ptr会在编译的时候就报错 而不是在运行阶段

但unique\_ptr允许悬挂指针的存在

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3NvdWx3eWI_size_16_color_FFFFFF_t_70 4][]

![20190613112639287.png][]

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3NvdWx3eWI_size_16_color_FFFFFF_t_70 5][]

感觉unique\_ptr很费劲 不用悬挂指针吧 用 move()

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3NvdWx3eWI_size_16_color_FFFFFF_t_70 6][]

![20190613113259176.png][]

new\[\] 的话 使用unique\_ptr<>

new 的话 使用 shared\_ptr<>, unique\_ptr<>

不是new的话 那就再见

如何选择智能指针

建议用shared\_ptr<>的场景

![20190613113719414.png][]

建议使用uniqure\_ptr<>的场景

![2019061311440845.png][]

uniqre\_ptr可以转换成shared\_ptr

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3NvdWx3eWI_size_16_color_FFFFFF_t_70 7][]

完结

[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3NvdWx3eWI_size_16_color_FFFFFF_t_70]: /images/20220115/7008a742b3e74c24b7a543decbbdd2f1.png
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[20190613095701683.png]: /images/20220115/d5fd0f44a24d4a8d8dc7215c9e8a83f4.png
[20190613104934866.png]: /images/20220115/d57abcc4d9b642da8eb59787f9860c2d.png
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[20190613111233205.png]: /images/20220115/48ab7a36930845f9a7114dbbbbba523f.png
[2019061311134466.png]: /images/20220115/ade277513f924147a571cef7ea91084b.png
[20190613112348213.png]: /images/20220115/abffc7ef45ce41b199960bb7dff7fdc7.png
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[20190613112639287.png]: /images/20220115/95cf9d8a28604606b09070db7fb1c6de.png
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[20190613113259176.png]: /images/20220115/590c1c7b9cb448c08c959b595d58c24b.png
[20190613113719414.png]: /images/20220115/3ce7429728454aa1b74e584fc8563099.png
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