C++简易

一.内存

1、栈区（stack）：由编译器自动分配释放，存放函数的参数值，局部变量的值等。其操作方式类似于数据结构中的栈。 
2、堆区（heap）：动态内存分配例如malloc，一般由程序员分配释放， 若程序员不释放，程序结束时可能由OS回收 。注意它与数据结构中的堆是两回事，分配方式类似于链表。 
3、全局区（静态区，static）：全局变量和静态变量的存储是放在一块的，初始化的全局变量和静态变量在一块区域， 未初始化的全局变量和未初始化的静态变量在相邻的另一块区域。 - 程序结束后有系统释放 
4、文字常量区：常量字符串就是放在这里的。 程序结束后由系统释放 
5、程序代码区：存放函数体的二进制代码。

二.命名空间

防止命名冲突

#include<iostream>
using namespace std;

namespace mfc
{
    int inflag;
    void g(int a){cout << a << endl;}
}

namespace owl
{
    int inflag;
    void g(int b){cout << b << endl;}
}

int main()
{
    mfc::inflag = 1;
    owl::inflag = 2;
    cout << mfc::inflag << endl;//1
    cout << owl::inflag << endl;//2

    mfc::g(1);//1
    owl::g(2);//2

    return 0;
}

#include<iostream>
using namespace std;

namespace mfc
{
    int inflag;
    void g(int a){cout << a << endl;}
}

namespace owl
{
    int inflag;
    void g(int b){cout << b << endl;}
}
using mfc::inflag;
int main()
{
    inflag = 1;
    owl::inflag = 2;
    cout << inflag << endl;//1
    cout << owl::inflag << endl;//2
    return 0;
}

#include<iostream>
using namespace std;

namespace mfc
{
    int inflag;
    void g(int a){cout << a << endl;}
}

namespace owl
{
    int inflag;
    void g(int b){cout << b << endl;}
}
using namespace mfc;

int main()
{
    inflag = 1;
    owl::inflag = 2;
    cout << inflag << endl;//1
    cout << owl::inflag << endl;//2
    g(1);//1
    return 0;
}

三.操作符

#include<iostream>
#include <string>
#include <iomanip>//操作符
using namespace std;

int main()
{
    double a = 3.1415;
    double b = 2.2222;
    string str;

    //cout << right右对齐，cout << left左对齐，默认右对齐
    cout << setw(8) << a << endl;//空格空格3.1415，设置字段位数
    cout << setfill('c') << setw(8) << a << endl;//cc3.1415，填充字符
    cout << setw(8) << a << endl;//cc3.1415
    cout << setprecision(2) << a << endl;//3.1，设置字段精度
    cout << setprecision(2) << fixed << a << endl;//3.14，小数点后精度
    cout << showpoint << 2.2 << endl;//2.20，强制显示小数点及尾部0，可以用noshowpoint取消
    cout << scientific << 33.33 << endl;//3.33e+001

    cout << hex << 16 << endl; // 10
    cout << oct << 8 << endl;  // 10
    cout << dec << 0x10 << endl; // 16

    return 0;
}

四.文件

#include<iostream>
#include<fstream>
using namespace std;

int main()
{
    ifstream infile;
    ofstream outfile;

    int num;
    infile.open("E:\1.txt");
    outfile.open("E:\2.txt", ios::app);
    /*
    ios::out输出到文件删除原有内容
    ios::binary以二进制格式打开文件
    ios::app输出到文件保留原有内容
    */

    if (! infile)//测试文件是否成功打开
    {
        cerr << "Error" << endl;
    }
    while (infile >> num)
    {
        cout << num << endl;
        outfile << "Num = " << num << endl;
        /*
        Num = 1
        Num = 2
        Num = 3
        Num = 4
        Num = 5
        */
    }

    infile.close();
    outfile.close();


    return 0;
}

#include<iostream>
#include<fstream>
using namespace std;

int main()
{
    ifstream infile;
    ofstream outfile;

    int num;
    infile.open("E:\1.txt");
    outfile.open("E:\2.txt", ios::app);
    /*
    ios::out输出到文件删除原有内容
    ios::binary以二进制格式打开文件
    ios::app输出到文件保留原有内容
    */

    if (!infile)//测试文件是否成功打开
    {
        cerr << "Error" << endl;
    }
    while (infile >> num)
    {
        cout << num << endl;
        outfile << "Num = " << num << endl;
        /*
        Num = 1
        Num = 2
        Num = 3
        Num = 4
        Num = 5
        */
    }

    long location = outfile.tellp();//当前指针
    location = 10L;
    outfile.seekp(location);//移动到第十个字节处
    location = outfile.tellp();//10
    outfile.seekp(location, ios::beg);//从头数，移动到第十个字节处
    location = outfile.tellp();//10
    outfile.seekp(location, ios::cur);//从当前，移动到第十个字节处
    location = outfile.tellp();//20
    outfile.seekp(5L, ios::end);//从尾数，移动到第十个字节处
    location = outfile.tellp();//5


    location = infile.tellg();//当前指针
    location = 10L;
    infile.seekg(location);//移动到第十个字节处
    location = infile.tellg();//-1
    infile.seekg(location, ios::beg);//从头数，移动到第十个字节处
    location = infile.tellg();//-1
    infile.seekg(location, ios::cur);//从当前，移动到第十个字节处
    location = infile.tellg();//-1
    infile.seekg(5L, ios::end);//从尾数，移动到第十个字节处
    location = infile.tellg();//-1

    infile.close();
    outfile.close();


    return 0;
}

#include<iostream>
#include<fstream>
using namespace std;

class Student
{
public:
    char name[500], score[500];
};

int main()
{
    Student s;
    ofstream outfile("E:\stu.dat", ios::out | ios::binary);

    while (cin >> s.name >> s.score)
    {
        if (strcmp(s.name, "exit") == 0)
        {
            break;
        }
        outfile.write((char*)&s, sizeof(s));
    }
    outfile.close();

    ifstream infile("E:\stu.dat", ios::in | ios::binary);
    if (!infile)
    {
        cerr << "error" << endl;
    }

    //infile.get(c);读取一个字符
    //outfile.put(c);写入一个字符
    while (infile.read((char*)&s, sizeof(s)))
    {
        int ReadBytes = infile.gcount();
        cout << ReadBytes << " " << s.name << " " << s.score << endl;
    }
    infile.close();
    return 0;
}

五.若干特性

#include<iostream>
using namespace std;

int main()
{
    bool flag = false;
    cout << flag << endl; // 0
    cout << boolalpha << flag << endl; // false，boolalpha强制让bool类型输出true和false
    return 0;
}

#include<iostream>
#include<string>
using namespace std;

int main()
{
    //枚举
    enum Num{MinSize = 0, Mid, MaxSize = 1000, Last};
    Num x, y;
    int i = MinSize, j = MaxSize;
    cout << i << " " << j << endl; // 0 1000
    cout << Mid << " " << Last << endl;//1 1001

    //结构体默认public
    struct Point1
    {
    };
    Point1 p1, p2;

    typedef struct Point2
    {
    }*P;
    P p3, p4;

    //const
    int b = 100, c = 1000;

    const int *a = &b;
    *a = 4;//error
    a = &c;
    b = 4;

    int *const d = &b;
    *d = 4;
    d = &c;//error

    const int *p1;int *p2;
    p1 = new int(3);
    p2 = new int(2);
    p1 = p2;//ok
    p2 = p1;//error
    p2 = (int *)p1;
    *p2 = 5;
    cout << *p1 << endl;//5

    return 0;
}

#include <iostream>
#include <string>
using namespace std;

int main(int argc, char *argv[])
{
    //<<和>> 比 关系，比较，逻辑，赋值,三目操作符等级高
    int n = 1, m = 0;
    cout << n < m << endl;//error
    cout << (n < m) << endl;
    return 0;
}

#include <iostream>
#include <string>
#include <fstream>
using namespace std;

int v = 10;
int main(int argc, char *argv[])
{
    int v = 5;
    cout << v << endl; // 5 局部覆盖全局
    cout << ::v << endl; // 10 全局
    return 0;
}

六.输入输出

cerr：打印出错信息，不使用缓冲区，直接从屏幕输出

clog：打印出错信息，使用缓冲区，缓冲区满或者刷新时才输出到屏幕

ofstream out("text.txt", "w", stdout);

if (y == 0)
{
    cerr << "error" << endl;
}
else
{
    cout << x / y; 输出到text.txt文件，不是打印
}

七.typeid

#include <iostream>
#include <typeinfo>
using namespace std;

int main()
{
    float x;
    cout << (typeid(x) == typeid(float)) << endl; // true
    cout << typeid(x).name() << endl; //float
    return 0;
}