朝花夕拾－－C++再学习

一，寻址

通过变量名
通过内存地址

取地址运算符&，用来获得变量的地址．

void getAddress() {int var;cout<<"var address is: "<<&var<<endl;
}

输出结果是：var address is: 0x7ffcf4dd4e14

为何这里是12位的16进制呢，换成2进制就是48位，难道我的机器不是64位机吗？

二，对齐

内存对齐，文件对齐

程序在编译链接后会被分割成一个个区块，而区块在内存和文件中按照一定的规律对齐．

//32位系统内存对齐值是：1000H == 4KB
//64位系统内存对齐值是：2000H == 8KB
//文件对齐值是：200H

操作系统处理内存时内存页是4KB

https://blog.csdn.net/zyq522376829/article/details/46357691

三，指针

既然指针变量存放的是一个地址，而32位系统的地址都是4个字节，为什么不用一个整型变量来存储而要发明指针呢？

如何根据指针变量来获取指针所存地址对应的值？－－Deference(解引用)，即在指针变量前加*号．

如std::cout<<*aPtr<<endl;

因此1，变量a和a的指针aPtr，代表同一个值，即a == *aPtr;

因此2，*aPtr = -100，将会修改a的值．

(1) *的两种用途：

1. 用于创建指针

int *mPtr = &myInt;

2. 对指针进行解引用

*mPtr = 123456;

(2) 指针与数组

数组的名字就是一个指针，它指向数组的基地址(也就是第一个元素的地址)．因此，下面两句等价：

int *arrPtr1 = &mArray[0];
int *arrPtr2 = mArray;

指针式遍历数组：intPtr++;

void travesalArray(){const unsigned short SIZE = 5;int intArr[SIZE] = {1,2,3,4,5};char charArr[SIZE] = {'F','i','s','C','o'};int *intPtr = intArr;char *charPtr = charArr;cout<<"intager array output: "<<endl;for(int i=0; i<SIZE; i++){cout<<*intPtr<<" at "<<reinterpret_cast<unsigned long>(intPtr)<<endl;intPtr++;}cout<<"character array output: "<<endl;for(int i = 0; i<SIZE; i++){cout<<*charPtr<<" at "<<reinterpret_cast<unsigned long>(charPtr)<<endl;charPtr++;}
}

输出结果如下：

intager array output:
1 at 140729290089744
2 at 140729290089748
3 at 140729290089752
4 at 140729290089756
5 at 140729290089760
character array output:
F at 140729290089776
i at 140729290089777
s at 140729290089778
C at 140729290089779
o at 140729290089780

四，结构体

(1) 结构体创建和使用

void tryStruct() {struct Student {string name;int age;char gender;};//第一种方式，赋值one by oneStudent stu1;stu1.name = "Sessy";stu1.age = 19;stu1.gender = 'F';cout<<"student1 info: "<<endl;cout<<"+name: "<<stu1.name<<endl;cout<<"+age: "<<stu1.age<<endl;cout<<"+gender: "<<stu1.gender<<endl;cout<<"-----------------------"<<endl;//第二种方式，like 数组Student stu2 = {"John", 18, 'M'};cout<<"student2 info: "<<endl;cout<<"+name: "<<stu1.name<<endl;cout<<"+age: "<<stu1.age<<endl;cout<<"+gender: "<<stu1.gender<<endl;
}

上图可以看到用"结构体变量.结构体成员"的方式来访问结构体成员；

(2) 用指针访问结构体成员

 Student *pStu = &stu2;cout<<"get struct info with pointer: "<<endl;cout<<"+name:"<<pStu->name<<endl;cout<<"+age:"<<pStu->age<<endl;cout<<"+gender:"<<pStu->gender<<endl;

五，传值，传址，传引用

(1) 传值

默认情况下，参数只能以值传递的方式给函数，也就是说被传递到函数的只是变量的值，不是变量本身．

int main() {int aa = 11, bb = 22;trySwapByValue(aa, bb);cout<<"aa == "<<aa<<", bb == "<<bb<<endl;return 0;
}void trySwapByValue(int a, int b) {int temp = a;b = a;a = temp;
}

aa, bb将自己的值传给函数的参数a, b之后就和aa, bb没关系了．所以trySwapByValue里面再热闹，也跟aa, bb无关．

(2) 传地址

int main() {int aa = 11, bb = 22;trySwapByAddress(&aa, &bb);cout<<"aa == "<<aa<<", bb == "<<bb<<endl;return 0;
}void trySwapByAddress(int *a, int *b) {int temp = *b; //将b的地址暂存到temp*b = *a;       //指针b指向a的地址*a = temp;     //指针a指向b的地址
}

输出结果：aa == 22, bb == 11
成功交换了aa和bb

(3) 传引用

int main() {int aa = 11, bb = 22;trySwapByReference(aa, bb);cout<<"aa == "<<aa<<", bb == "<<bb<<endl;return 0;
}//为何称为传引用呢？－－可能因为变量名被称为引用？
void trySwapByReference(int &a, int &b) {int temp = b;b = a;a = temp;
}

输出结果：aa == 22, bb == 11
成功交换了aa和bb

六，对象

小demo一个：

#include <iostream>
using namespace std;#define FULL_GAS 81
class Car {public:string color;string engine;unsigned int gas_tank;unsigned int wheel;Car(void);void setColor(string color);void setEngine(string engine);void setWheel(unsigned int w);void fillTank(int liter);int running(void);void warning(void);
};//constructor
Car::Car(void) {color = "Red";engine = "V8";wheel = 4;gas_tank = FULL_GAS;
}void Car::setColor(string col) {color = col;
}void Car::setEngine(string eng) {engine = eng;
}void Car::setWheel(unsigned int w) {wheel = w;
}void Car::fillTank(int liter) {gas_tank += liter;
}int Car::running(void) {cout<<"I'm running at a high speed"<<endl;gas_tank--;cout<<"gas left: "<<100*gas_tank/FULL_GAS<<"%"<<endl;return gas_tank;
}void Car::warning(void) {cout<<"WARNING!"<<endl;cout<<100*gas_tank/FULL_GAS<<"% oil in tank"<<endl;
}int main() {char i;Car mCar;mCar.setColor("Yellow");mCar.setEngine("V8");mCar.setWheel(4);mCar.gas_tank = FULL_GAS;while(mCar.running()) {if(mCar.running() < 10) {mCar.warning();cout<<"Need to add oil?(Y/N)"<<endl;cin>>i;if('Y' == i || 'y' == i){mCar.fillTank(FULL_GAS);}}}return 0;
}