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【CPP】类和对象

news 2025/8/2 16:52:48

1- Classes and Objects

Structures

  • A struct in C is a type consisting of a sequence of data members
  • Some functions/Statements are needed to operate the data members of an object of a struct type

在这里插入图片描述

不不小心操作错误，不小心越界

Classes

  • You should be very careful to manipulated the data members in a struct object
  • Can we improve struct to a better one ?
  • Yes, it is class ! We can put some member functions in it
class Student
{private:static size_t student_total; // declaration only//inline static size_t student_total = 0; //C++17, definition outside isn't neededchar * name;int born;bool male; void setName(const char * s){strncpy(name, s, sizeof(name));}
};

Student yu;
yu.setName("Yu");

firstclass.cpp

#include <iostream>
#include <cstring>class Student
{public:char name[4];int born;bool male; void setName(const char * s){strncpy(name, s, sizeof(name));}void setBorn(int b){born = b;}void setGender(bool isMale){male = isMale;}void printInfo(){std::cout << "Name: " << name << std::endl;std::cout << "Born in " << born << std::endl;std::cout << "Gender: " << (male ? "Male" : "Female") << std::endl;}
};int main()
{Student yu;yu.setName("Yu");yu.setBorn(2000);yu.setGender(true);yu.born = 2001; // it can also be manipulated directlyyu.printInfo();std::cout << "It's name is " << yu.name << std::endl; return 0;
}

Name: Yu
Born in 2001
Gender: Male
It's name is Yu

Access Specifiers

  • You can protect data members by access specifier private
  • Then data member can only be accessed by well designed member functions

access_attribute.cpp

#include <iostream>
#include <cstring>class Student
{private:char name[4];int born;bool male; public:void setName(const char * s){strncpy(name, s, sizeof(name));}void setBorn(int b){born = b;}void setGender(bool isMale){male = isMale;}void printInfo(){std::cout << "Name: " << name << std::endl;std::cout << "Born in " << born << std::endl;std::cout << "Gender: " << (male ? "Male" : "Female") << std::endl;}
};int main()
{Student yu;yu.setName("Yu");yu.setBorn(2000);yu.setGender(true);yu.born = 2001; // you cannot access a private memberyu.printInfo();return 0;
}

access-attribute.cpp:37:8: error: 'born' is a private member of 'Student'yu.born = 2001; // you cannot access a private member^
access-attribute.cpp:8:9: note: declared private hereint born;^

Member Functions

  • A member function can be defined inside or outside class
  • 如果在类内部实现函数则就是inline 函数

function.cpp

#include <iostream>
#include <cstring>class Student
{private:char name[4];int born;bool male; public:void setName(const char * s){strncpy(name, s, sizeof(name));}void setBorn(int b){born = b;}// the declarations, the definitions are out of the classvoid setGender(bool isMale);void printInfo();
};void Student::setGender(bool isMale)
{male = isMale;
}
void Student::printInfo()
{std::cout << "Name: " << name << std::endl;std::cout << "Born in " << born << std::endl;std::cout << "Gender: " << (male ? "Male" : "Female") << std::endl;
}int main()
{Student yu;yu.setName("Yu");yu.setBorn(2000);yu.setGender(true);yu.printInfo();return 0;
}

Name: Yu
Born in 2000
Gender: Male

File Structures

  • The source code can be placed into multiple files

student.hpp

#pragma once#include <cstring>
class Student
{private:char name[4];int born;bool male; public:void setName(const char * s){strncpy(name, s, sizeof(name));}void setBorn(int b){born = b;}// the declarations, the definitions are out of the classvoid setGender(bool isMale);void printInfo();
};

student.cpp

#include <iostream>
#include "student.hpp"void Student::setGender(bool isMale)
{male = isMale;
}
void Student::printInfo()
{std::cout << "Name: " << name << std::endl;std::cout << "Born in " << born << std::endl;std::cout << "Gender: " << (male ? "Male" : "Female") << std::endl;
}

如果include <> 从编译器路径查找，如果是include "" 从编译器和当前目录找

main.cpp

#include "student.hpp"int main()
{Student yu;yu.setName("Yu");yu.setBorn(2000);yu.setGender(true);yu.printInfo();return 0;
}

CMakeList.txt

cmake_minimum_required(VERSION 3.12)project(persondemo)ADD_EXECUTABLE(persondemo main.cpp student.cpp)
cd multi-files
mkdir build
cd build
cmake ..
make
./persondemo

Name: Yu
Born in 2000
Gender: Male

2-Constructors and Destructors

Constructors

  • Different from struct in C, a constructor will be invoked when creating an object of a class

(1) struct in C: allocate memory
(2) class in C++: allocate memory & invoke a constructor

  • But, No constructor is defined explicitly in previous examples

(1) the compiler wil generate one with empty body

如果没有人为定义构造函数，则自动会有一个空的构造函数

  • The same name with the class
  • Have no return value

class Student
{private:char name[4];int born;bool male; public:Student(){name[0] = 0;born = 0;male = false;cout << "Constructor: Person()" << endl;}}Student(const char * initName, int initBorn, bool isMale){setName(initName);born = initBorn;male = isMale;cout << "Constructor: Person(const char, int , bool)" << endl;}
}
  • The members can also be initialized as follows
    Student(const char * initName): born(0), male(true){setName(initName);cout << "Constructor: Person(const char*)" << endl;}

把成员变量born 初始化为0 ， 把male 初始化为true

constructor.cpp

#include <iostream>
#include <cstring>using namespace std;class Student
{private:char name[4];int born;bool male; public:Student(){name[0] = 0;born = 0;male = false;cout << "Constructor: Person()" << endl;}Student(const char * initName): born(0), male(true){setName(initName);cout << "Constructor: Person(const char*)" << endl;}Student(const char * initName, int initBorn, bool isMale){setName(initName);born = initBorn;male = isMale;cout << "Constructor: Person(const char, int , bool)" << endl;}void setName(const char * s){strncpy(name, s, sizeof(name));}void setBorn(int b){born = b;}// the declarations, the definitions are out of the classvoid setGender(bool isMale);void printInfo();
};void Student::setGender(bool isMale)
{male = isMale;
}
void Student::printInfo()
{std::cout << "Name: " << name << std::endl;std::cout << "Born in " << born << std::endl;std::cout << "Gender: " << (male ? "Male" : "Female") << std::endl;
}int main()
{Student yu;yu.printInfo();yu.setName("Yu");yu.setBorn(2000);yu.setGender(true);yu.printInfo();Student li("li");li.printInfo();Student xue = Student("XueQikun", 1962, true);//a question: what will happen since "XueQikun" has 4+ characters?xue.printInfo();Student * zhou =  new Student("Zhou", 1991, false);zhou->printInfo();delete zhou;return 0;
}

Constructor: Person()
Name: 
Born in 0
Gender: Female
Name: Yu
Born in 2000
Gender: Male
Constructor: Person(const char*)
Name: li
Born in 0
Gender: Male
Constructor: Person(const char, int , bool)
Name: XueQ�
Born in 1962
Gender: Male
Constructor: Person(const char, int , bool)
Name: Zhou�
Born in 1991
Gender: Female

Destructors

  • The destructor will be invoked when the object is destroyed
  • Be formed from the class name preceded by a tilde(~)
  • Have no return value, no parameters
    ~Student(){cout << "To destroy object: " << name << endl;delete [] name;}

析构函数只能有一个
析构函数常做的事情：释放内存，关闭文件，断掉网络etc

destructor.cpp

#include <iostream>
#include <cstring>using namespace std;class Student
{private:char * name;int born;bool male; public:Student(){name = new char[1024]{0};born = 0;male = false;cout << "Constructor: Person()" << endl;}Student(const char * initName, int initBorn, bool isMale){name =  new char[1024];setName(initName);born = initBorn;male = isMale;cout << "Constructor: Person(const char, int , bool)" << endl;}~Student(){cout << "To destroy object: " << name << endl;delete [] name;}void setName(const char * s){strncpy(name, s, 1024);}void setBorn(int b){born = b;}// the declarations, the definitions are out of the classvoid setGender(bool isMale);void printInfo();
};void Student::setGender(bool isMale)
{male = isMale;
}
void Student::printInfo()
{std::cout << "Name: " << name << std::endl;std::cout << "Born in " << born << std::endl;std::cout << "Gender: " << (male ? "Male" : "Female") << std::endl;
}int main()
{{Student yu;yu.printInfo();yu.setName("Yu");yu.setBorn(2000);yu.setGender(true);yu.printInfo();}Student xue = Student("XueQikun", 1962, true);xue.printInfo();Student * zhou =  new Student("Zhou", 1991, false);zhou->printInfo();delete zhou;return 0;
}

g++ destructor.cpp --std=c++11

Constructor: Person()
Name: 
Born in 0
Gender: Female
Name: Yu
Born in 2000
Gender: Male
To destroy object: Yu
Constructor: Person(const char, int , bool)
Name: XueQikun
Born in 1962
Gender: Male
Constructor: Person(const char, int , bool)
Name: Zhou
Born in 1991
Gender: Female
To destroy object: Zhou
To destroy object: XueQikun

人工手动调用析构函数 delete zhou，作用域结束跳出也会自动调用析构函数
如果对于new 的对象不进行手动删除delete 则作用域结束也不会动态调用析构函数，造成内存泄漏

    Student * class1 = new Student[3]{{"Tom", 2000, true},{"Bob", 2001, true},{"Amy", 2002, false},};
  • What is the different between the following two lines?
delete class1;
delete [] class1;

array.cpp

#include <iostream>
#include <cstring>using namespace std;class Student
{private:char * name;int born;bool male; public:Student(){name = new char[1024]{0};born = 0;male = false;cout << "Constructor: Person()" << endl;}Student(const char * initName, int initBorn, bool isMale){name =  new char[1024];setName(initName);born = initBorn;male = isMale;cout << "Constructor: Person(const char, int , bool)" << endl;}~Student(){cout << "To destroy object: " << name << endl;delete [] name;}void setName(const char * s){strncpy(name, s, 1024);}void setBorn(int b){born = b;}// the declarations, the definitions are out of the classvoid setGender(bool isMale);void printInfo();
};void Student::setGender(bool isMale)
{male = isMale;
}
void Student::printInfo()
{std::cout << "Name: " << name << std::endl;std::cout << "Born in " << born << std::endl;std::cout << "Gender: " << (male ? "Male" : "Female") << std::endl;
}int main()
{Student * class1 = new Student[3]{{"Tom", 2000, true},{"Bob", 2001, true},{"Amy", 2002, false},};class1[1].printInfo();delete class1;delete []class1;return 0;
}

Constructor: Person(const char, int , bool)
Constructor: Person(const char, int , bool)
Constructor: Person(const char, int , bool)
Name: Bob
Born in 2001
Gender: Male
To destroy object: Tom

数组调用析构函数delete class1 , 只会调用第一个对象的析构函数，后面的对象不会被调用

数组调用析构函数 delete [] class1，则会调用全部对象的析构函数

Constructor: Person(const char, int , bool)
Constructor: Person(const char, int , bool)
Constructor: Person(const char, int , bool)
Name: Bob
Born in 2001
Gender: Male
To destroy object: Amy
To destroy object: Bob
To destroy object: Tom

3-this pointer

Why is this needed

  • How does a member function know which name?

在这里插入图片描述

this Pointer

  • All methods in a function have a this pointer
  • It is set to the address of the object that invokes the method

在这里插入图片描述

this.cpp

#include <iostream>
#include <cstring>using namespace std;class Student
{private:char * name;int born;bool male; public:Student(){name = new char[1024]{0};born = 0;male = false;cout << "Constructor: Person()" << endl;}Student(const char * name, int born, bool male){this->name =  new char[1024];this->setName(name);this->born = born;this->male = male;cout << "Constructor: Person(const char, int , bool)" << endl;}~Student(){cout << "To destroy object: " << name << endl;delete [] name;}void setName(const char * name){strncpy(this->name, name, 1024);}void setBorn(int born){this->born = born;}// the declarations, the definitions are out of the classvoid setGender(bool isMale);void printInfo();
};void Student::setGender(bool isMale)
{male = isMale;
}
void Student::printInfo()
{std::cout << "Name: " << name << std::endl;std::cout << "Born in " << born << std::endl;std::cout << "Gender: " << (male ? "Male" : "Female") << std::endl;
}int main()
{Student * class1 = new Student[3]{{"Tom", 2000, true},{"Bob", 2001, true},{"Amy", 2002, false},};class1[1].printInfo();delete []class1;return 0;
}

Constructor: Person(const char, int , bool)
Constructor: Person(const char, int , bool)
Constructor: Person(const char, int , bool)
Name: Bob
Born in 2001
Gender: Male
To destroy object: Amy
To destroy object: Bob
To destroy object: Tom

4- const and static Members

const Variables

  • statements for constants

在这里插入图片描述

C++不推荐用 宏

const Members

  • const member variables behavior similar with normal const variables
  • const member functions promise not to modify member variables
class Student
{private:const int BMI = 24;public:Student(){BMI = 25;//can it be modified?int getBorn() const{born++; //Can it be modified?return born;}
};

常量函数，const 放在后面，不然跟前面的const int相冲突。不可以修改成员变量，born 是不可以被修改的，保证不修改函数里的变量

const.cpp

#include <iostream>
#include <cstring>using namespace std;class Student
{private:const int BMI = 24;char * name;int born;bool male; public:Student(){name = new char[1024]{0};born = 0;male = false;// BMI = 25;//can it be modified?cout << "Constructor: Person()" << endl;}Student(const char * name, int born, bool male){this->name =  new char[1024];setName(name);this->born = born;this->male = male;cout << "Constructor: Person(const char, int , bool)" << endl;}~Student(){cout << "To destroy object: " << name << endl;delete [] name;}void setName(const char * name){strncpy(this->name, name, 1024);}void setBorn(int born){this->born = born;}int getBorn() const{//born++; //Can it be modified?return born;}// the declarations, the definitions are out of the classvoid setGender(bool isMale);void printInfo();
};void Student::setGender(bool isMale)
{male = isMale;
}
void Student::printInfo()
{std::cout << "Name: " << name << std::endl;std::cout << "Born in " << born << std::endl;std::cout << "Gender: " << (male ? "Male" : "Female") << std::endl;
}int main()
{Student yu("Yu", 2000, true);cout << "yu.getBorn() = " << yu.getBorn() << endl;return 0;
}

Constructor: Person(const char, int , bool)
yu.getBorn() = 2000
To destroy object: Yu

static members

  • static members are not bound to class instances
class Student
{private:static size_t student_total; // declaration onlypublic:Student(){student_total++;}~Student(){student_total--;}static size_t getTotal() {return student_total;}
};// definition it here
size_t Student::student_total = 0;

静态成员不绑定在类对象上，只有一个

static.cpp

#include <iostream>
#include <cstring>using namespace std;class Student
{private:static size_t student_total; // declaration only//inline static size_t student_total = 0; //C++17, definition outside isn't neededchar * name;int born;bool male; public:Student(){student_total++;name = new char[1024]{0};born = 0;male = false;cout << "Constructor: Person(): student_total = " << student_total << endl;}Student(const char * initName, int initBorn, bool isMale){student_total++;name =  new char[1024];setName(initName);born = initBorn;male = isMale;cout << "Constructor: Person(const char, int , bool): student_total = " << student_total << endl;}~Student(){student_total--;cout << "To destroy object: " << name ;cout << ". Then " << student_total << " students are left" << endl;delete [] name;}void setName(const char * s){strncpy(name, s, 1024);}void setBorn(int b){born = b;}static size_t getTotal() {return student_total;}// the declarations, the definitions are out sof the classvoid setGender(bool isMale);void printInfo();
};void Student::setGender(bool isMale)
{male = isMale;
}
void Student::printInfo()
{std::cout << "Name: " << name << std::endl;std::cout << "Born in " << born << std::endl;std::cout << "Gender: " << (male ? "Male" : "Female") << std::endl;
}size_t Student::student_total = 0; // definition it hereint main()
{cout << "---We have " << Student::getTotal() << " students---" << endl;Student * class1 = new Student[3]{{"Tom", 2000, true},{"Bob", 2001, true},{"Amy", 2002, false},};cout << "---We have " << Student::getTotal() << " students---" << endl;Student yu("Yu", 2000, true);cout << "---We have " << Student::getTotal() << " students---" << endl;class1[1].printInfo();delete []class1;cout << "---We have " << Student::getTotal() << " students---" << endl;return 0;
}

---We have 0 students---
Constructor: Person(const char, int , bool): student_total = 1
Constructor: Person(const char, int , bool): student_total = 2
Constructor: Person(const char, int , bool): student_total = 3
---We have 3 students---
Constructor: Person(const char, int , bool): student_total = 4
---We have 4 students---
Name: Bob
Born in 2001
Gender: Male
To destroy object: Amy. Then 3 students are left
To destroy object: Bob. Then 2 students are left
To destroy object: Tom. Then 1 students are left
---We have 1 students---
To destroy object: Yu. Then 0 students are left

静态函数里面不可以修改非静态数据

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