浙大 C++ 课程学习笔记1 - 类的基础

浙大 C++

网易云课堂-C++

本机环境

> g++ -v     

Apple clang version 14.0.3 (clang-1403.0.22.14.1)
Target: x86_64-apple-darwin22.5.0
Thread model: posix
InstalledDir: /Library/Developer/CommandLineTools/usr/bin

第一个 C++ 程序

touch main.cpp

#include <iostream>

using namespace std;

int main(void) {
    cout << "Hello world! I am " <<  18 << " today!" << endl;
    return 0;
}

g++ main.cpp

./a.out

What is an object

• Object = Entity
• Objects = Atrributes + Services
  • Data
  • Operations

1. in C language

#include <stdio.h>

typedef struct point3d{
    float x;
    float y;
    float z;
}Point3;

void Point3Print(const Point3 *p);

int main(void) {
    Point3 a;
    a.x = 1;
    a.y = 2;
    a.z = 3;
    Point3Print(&a);
    return 0;
}

void Point3Print(const Point3 *p){
    printf("(%f,%f,%f)",p->x,p->y,p->z);
}

2. in c++

#include <iostream>

using namespace std;

class Point3{
private:
    float x;
    float y;
    float z;
public:
    Point3(float x, float y, float z);
    void Point3Print();
};

Point3::Point3(float x, float y, float z){
    this->x=x;
    this->y=y;
    this->z=z;
}

void Point3::Point3Print(){
    cout << "x=" << x << ",y=" << y << ",z=" << z << endl;
}

int main(void) {
    Point3 a(1, 2, 3);
    a.Point3Print();
    return 0;
}

头文件

1. Declarations vs Definitions (声明 vs 定义)

• A .cpp file is a compile unit
• Only declaraions are allowed to be in .h
  • extern variables
  • function prototypes
  • class/struct declaration

2. #include

• #include "xxx" : first search in the current directory,then the directories delared somewhere
• #include <xxx> : search in the speified directories

3. 条件编译

标准头文件结构

#ifndef _MY_H_
#define _MY_H_

void hello();

#endif //_MY_H_

4. extern int gl;

仅是告诉编译器在某个地方有一个变量叫 gl ；具体定义这个变量的地方暂时可以不知道。

成员变量

class A{
public:
    int i;
    void f();
};

void A::f(){
    cout << i;
}

int main(){
    A a;
    a.i = 10;
    a.f();
}

构造和析构

class A{
public:
    A();
    ~A();
    void f();
};

A::A(){
    cout << "init" << endl;
}

A::~A(){
    cout << "destory" << endl;
}

void A::f(){
    cout << "a f() is run" << endl;
}

int main(void) {
    cout << "main begin" << endl;
    {
        cout << "local begin" << endl;
        A a;
        a.f();
        cout << "local end" << endl;
    }
    cout << "main end" << endl;
    return 0;
}

对象初始化

1. struct

struct Y
{
    float f;
    int a;
    Y(int a);
};

Y::Y(int a){}

int main(void) {
    Y y1[] = {Y(1), Y(2), Y(3)};
    Y y2[2] = {Y(11)}; // error
    return 0;
}

2. class

class Y
{
public:
    Y(int a);
};

Y::Y(int i){}

int main(void) {
    Y y1[] = {Y(1), Y(2), Y(3)};
    Y y2[2] = {Y(11)}; // error
    return 0;
}

fix

class Y
{
public:
    Y();
    Y(int a);
};

Y::Y(int i){}
Y::Y(){}

int main(void) {
    Y y1[] = {Y(1), Y(2), Y(3)};
    Y y2[2] = {Y(11)}; // compile pass
    return 0;
}

new & delete

1. dynamic memory allocation

• new
  • new int;
  • new A;
  • new int[10];
• delete
  • delete p;
  • delete[] arr;

//
int * i = new int;
delete i;
// 
int * psome = new int[10];
delete[] psome;

// 
A *a = new A();
delete a;
A *r = new A[10];
delete[] r;

访问限制

1. 关键字

• private
  • no one can access that member except inside funcion members of that type.
  • 这个类的成员函数有访问权限
• public
• protected

class A{
private:
    int i;
public:
    void set(int i){this->i = i;}
    void g(A* a){cout << a->i << endl;} // private 的权限限制仅仅在编译时刻
};

int main(void) {
    A a;
    a.set(100);
    
    A b;
    b.g(&a);
    return 0;
}

2. friend

• 你可以声明别人是你的朋友，然后它就可访问你的私有变量了
• 可以访问朋友的私有变量

struct X; // Declaration

struct Y
{
    void f(X*);
};

struct X{ // Definition
private:
    int i;
public:
    void init();
    friend void g(X*,int); // Global friend
    friend void Y::f(X*); // Struct member friend
    friend struct Z; // Entire struct is a friend
    friend void h();
};

void X::init(){
    i = 333;
}

void g(X* x,int i){
    x->i = i; // 在 X 中被定义为 friend ，可以访问私有变量
}

void Y::f(X* x){
    // 在 X 中被定义为 friend ，可以访问私有变量
    cout << x->i << endl;
}

struct Z{

};

void h(){
    X x;
    x.i = 200; // 在 X 中被定义为 friend ，可以访问私有变量
    cout << x.i << endl;
}

void m(){
    X x;
    // x.i = 200; error 没有定义为 friend ,不能直接访问私有变量
}

int main(void) {
    X x;
    x.init();
    // cout << x.i; error 不能直接访问私有变量

    Y y;
    y.f(&x);

    g(&x, 100);
    y.f(&x);
    
    h();
    return 0;
}

3. class vs. struct

in c++ , struct is class, but

• class default to private
• struct default to public
• 首选 class

初始化列表

• 早于构造函数的执行

1. 案例

class A{
private:
    int i, j;
public:
    A():i(100),j(200){ cout << "A init" << endl;}
    void f() { cout << "A::i=" << i << " j=" << j << endl;};
};

2. Initialization vs. assignment

class Student{
private:
    string name;
public:
    Student(string);
    void printName() { cout << "Student::name=" << name << endl;};
};

// assignment
// inside constructor
// string must have a default constructor
// Student::Student(string s){
//     name = s;
// }

// initialization 
// before constructor
// 推荐用法
Student::Student(string s):name(s){
}

int main(void) {
    Student a("zhangsan");
    a.printName();
    return 0;
}