[陈宗权C++]C++第1天PM--C++常量_指针_new_d

参考：

重点：

1.成员函数指针

###正文：
//达内C++教程\03_标准C++编程_陈宗权_7day\标准C++编程_day01PM_常量_指针_new_delete_函数

#include<iostream>
#include<cstdlib>// C++，把.h改为c
//#include<stdlib.h>//C
#include<string>

using namespace std;

struct date {
    int year;
    int month;
    int day;
    void print() { std::cout <<year<<"-"<<month<<"-"<<day << std::endl; }
};

void f(double d) { std::cout << d << std::endl; }

//形参 void写与不写等价
void f1() { std::cout <<"hello" << std::endl; }

void f2(void) {}

void f2(double) {}//有类型无名字：哑元，为了新旧兼容


// date里int成员的地址int date::*p
// 结构变量.*成员指针，结构指针->*成员指针
void showmember(date a[], int n, int date::*p)
{
    for (int i = 0; i < n; i++) {
        //std::cout << a[i].*p << " ";
        //std::cout << *(a+i).*p << " ";
        std::cout << (a + i)->*p << " ";
    }
}

int main() {

    double d = 123.15;
    //E的地址不能修改
    double* const E = &d;
    std::cout <<*E << std::endl;
    //////////////////////////////////////////////////////////
    // C++会优化，对于常量的访问，不会直接去内存取数据，而C不会
    // 自己的代码写的不能改，但是自己改，这是不合理的。
    const int n = 100;
    // volatile：从内存里取数据
    volatile const int m = 200;
    int *p = (int*)&n;
    *p = 123;
    
    p = (int*)&m;
    *p = 456;
    std::cout <<"n: "<< n <<",m: "<< m << std::endl;
    ///////////////////////////////////////////////////////////
    //static_cast: 合理转换，数值类型之间的转换，或者指针与void*之间的转换
    //const_cast: 把常量转成变量,用于临时去掉const限制，转换语句本身，出了这个语句就没了
    //reinterpret_cast: 任意两种指针类型之间，指针和数值之间的转换，最危险的转换，一般不用
    //dynamic_cast:以后再说
    // C++ 编译可能有警告
    //int n = 123.67;
    int nn = static_cast<int>(12.45);
    int* pp = static_cast<int*>(malloc(sizeof(int)*10));
    free(pp);
    // 不合理的转换，很少使用
    const int k = nn;
    std::cout << "before: k" << k << std::endl;
    const_cast<int&>(k) = 789;
    std::cout << "after: k" <<k <<std::endl;
    nn = (int)12.34;
    // 把类型当函数一样
    nn = int(12.34);
    std::cout <<"nn: "<<nn << std::endl;
    //空的表示数值的0
    nn = int();
    std::cout << "nn: " << nn << std::endl;

    //int mm = 100;
    // 把初始值放在括号里,初始化一次，赋值可以多次
    int mm(300);
    std::cout << "mm: " << mm << std::endl;
    
    //会被认为是函数声明
    //int x();

    //没必要这么写!!!!!
    //int(y) =200;//int(f)类似强制类型转换
    //std::cout <<"y:" <<y<< std::endl;
    ///////////////////////////////////////////////////////////
    //new 类型 <===> (类型*)mallac(sizeof(类型));
    
    //以下两个等同
    string* ppp = static_cast<string*>(malloc(sizeof(string)));
    int* qqq = new int;


    //初始化999
    int* rrr = new int(999);
    
    
    int nnn;
    nnn = 10;
    //nothrow 不扔出来
    //数组没有理由初始化，也没办法初始化
    int *a = new (nothrow)int[nnn];
    // 申请空间失败，返回的是是空指针
    if (a == NULL)
    {
        cout <<n*sizeof(int) <<"申请空间失败";
    }
    else 
    {
        //用new初始化，不初始化，不能保证是0!!!!!!!!!!!!!!!!!!!!!!!!!
        for (int i = 0; i < n; i++) {
            std::cout << a[i] << " ";
        }

    }
    std::cout << "*qqq: " << *qqq<< std::endl;

    
    //没有new 不能delete
    //delete ppp;
    free(ppp);

    delete qqq; qqq = NULL;
    delete rrr; rrr = NULL;

    delete[] a; a = NULL;
    //////////////////////////////////////////////////////////////////


    date aaaa[5] = { {2020,1,27},{ 2021,1,27 },{ 2022,1,27 },{ 2023,1,27 },{ 2024,1,27 }};
    date dddd = { 2025,1,27 };
    std::cout <<"&dddd: "<< &dddd << std::endl;
    std::cout << "&dddd.year: " << &dddd.year << std::endl;
    std::cout << "&dddd.month: " << &dddd.month << std::endl;
    std::cout << "&dddd.day: " << &dddd.day << std::endl;

    //取相对地址
    std::cout <<"&date::year: "<< &date::year << std::endl;
    //函数地址和成员地址输出都会设置成true
    std::cout << &main << std::endl;
    std::cout << &f << std::endl;

    //联合，共用同一块内存空间。
    union {
        int nnnn;
        int date::*mp;
    };
    mp = &date::day;
    //nnnn == mp,即可得到mp的地址
    std::cout <<"nnnn: "<<nnnn<< std::endl;

    union {
        int iNum;
        int* pNum2;
    };
    int mmmmm=2;
    pNum2 = &mmmmm;
    std::cout <<"iNum: "<<hex<<iNum << std::endl;
    std::cout.unsetf(ios::hex);
    std::cout <<"pNum2: "<<pNum2 << std::endl;
    std::cout <<"pNum2 add: "<<&pNum2 << std::endl;
    std::cout << "iNum add: "<<&iNum << std::endl;


    // .* 当成运算符
    std::cout << "dddd.*mp: " << dddd.*mp << std::endl;

    mp = &date::year;
    std::cout << "dddd.*mp: " << dddd.*mp << std::endl;

    showmember(aaaa,5,&date::month);
    showmember(aaaa, 5, &date::year);
    showmember(aaaa, 5, &date::day);
    
    dddd.print();
    aaaa[3].print();

    //成员函数指针
    void(date::*pf)() = &date::print;
    (dddd.*pf)();
    ((aaaa+0)->*pf)();
    system("pause");



}