C ++ learning began to die Series: 6 basic grammar

Date & Time

There are four time-related types: clock_t, time_t, size_t  and  tm . Type clock_t, size_t time_t and the system time and date can be represented as a certain integer. Structure type  tm  the date and time stored in the form C structure, tm structure defined as follows:

struct tm {
  int tm_sec;   // 秒，正常范围从 0 到 59，但允许至 61
  int tm_min;   // 分，范围从 0 到 59
  int tm_hour;  // 小时，范围从 0 到 23
  int tm_mday;  // 一月中的第几天，范围从 1 到 31
  int tm_mon;   // 月，范围从 0 到 11
  int tm_year;  // 自 1900 年起的年数
  int tm_wday;  // 一周中的第几天，范围从 0 到 6，从星期日算起
  int tm_yday;  // 一年中的第几天，范围从 0 到 365，从 1 月 1 日算起
  int tm_isdst; // 夏令时
}

Use date and time functions and structures need to reference <ctime> header file in a C ++ program. C ++ standard library operations functions Date and time:

function	description
time_t time(time_t *t)	Returns the timestamp, if the parameter is not null * t, the return value is also stored in the memory space a pointer pointing to t, if there is no current system time, -1
char *ctime(const time_t *timer)	Returns a string representing local time, local time is based on the parameters of  the Timer . Returns the string format is as follows:  Www Mmm dd HH: mm: SS yyyy  where, Www  represents a day of the week, Mmm  is a month of letters, dd  represents the day of the month, HH: mm: SS  indicates the time, yyyy  indicate the year.
struct tm *localtime(const time_t *timer)	A timer value used to fill  tm  structure. timer  value is decomposed into  tm  structure, and is represented by local time zone.
clock_t clock(void)	Since initiating the program returns, the time used by the processor clock. If it fails, it returns a value of -1.
char *asctime(const struct tm *timeptr)	C returns a string containing a readable format date and time information  Www Mmm dd HH: mm: SS yyyy , where, Www  represents a day of the week, Mmm  is a month of letters, dd  represents the day of the month , HH: mm: SS  indicates the time, yyyy  represents the year.
struct tm *gmtime(const time_t *timer)	This function returns a pointer pointing to time, time is tm structure, with Coordinated Universal Time (UTC), also known as Greenwich Mean Time (GMT) representation.
time_t mktime(struct tm *timeptr)	Time_t returns a value that corresponds to the calendar time to the transmission parameters. If an error occurs, a value of -1 is returned.
double difftime(time_t time1, time_t time2)	Returns the seconds of time to phase difference between two double precision floating-point value representation (time1 - time2)
size_t strftime(char *str, size_t maxsize, const char *format, const struct tm *timeptr)	The  format  formatting rules defined in the formatting structure  timeptr  represents time, and stores it in the  str  in. If C is smaller than the size string produced characters (characters including null terminator), it returns to the copy number of characters in str (not including null terminator character), otherwise, returns zero.

The current date & time and time format tm

#include <iostream>
#include <ctime>
 
using namespace std;
 
int main( )
{
   // 基于当前系统的当前日期/时间
   time_t now = time(0);
   
   // 把 now 转换为字符串形式
   char* dt = ctime(&now);
 
   cout << "本地日期和时间：" << dt << endl;
 
   // 把 now 转换为 tm 结构
   tm *gmtm = gmtime(&now);
   dt = asctime(gmtm);
   cout << "UTC 日期和时间："<< dt << endl;
   
   // 输出 tm 结构的各个组成部分
   cout << "年: "<< 1900 + gmtm->tm_year << endl;
   cout << "月: "<< 1 + gmtm->tm_mon<< endl;
   cout << "日: "<<  gmtm->tm_mday << endl;
   cout << "时间: "<< gmtm->tm_hour << ":";
   cout << gmtm->tm_min << ":";
   cout << gmtm->tm_sec << endl;
}

tm  time structure with dates and time-related operations in C / C ++, a particularly important. Arrow -> operator to access the structure members.

input Output

The C ++ I / O occurs in the stream, the stream is a sequence of bytes. If the flow stream of bytes from the memory device (e.g., keyboard, disk drive, network connection, etc.), which is called the input operation . If the byte stream flowing from the memory device (e.g., display screen, printer, disk drive, network connection, etc.), which is called the output operation .

I / O library header files

head File	description
<iostream>	This document defines the  CIN ( the standard input stream ), cout ( the standard output stream ), cerr,  (unbuffered standard error stream) and  the clog  (standard error buffer stream) object
<iomanip>	The document by so-called parametric flow manipulator (such  setw  and  setPrecision ), to declare useful for implementation of standardized I / O services
<fstream>	The file for the user control file processing service statement

Standard input stream cin

Predefined object  cin  is  iostream  an instance of a class. cin objects affiliated to the standard input device, typically a keyboard. cin  is the stream extraction operator   >>  used in combination, the stream extraction operator >>  can be used repeatedly in a statement

C ++ compiler according to the data type of the input values ​​to select the appropriate stream extraction operator to extract value, and stores it in a given variable.

Standard output stream cout

Predefined object  cout  is  iostream  an instance of a class. cout object "connected" to the standard output device, usually a display screen. cout  is a stream insertion operator << using binding, flow insertion operator <<  can be used repeatedly in a statement.

C ++ compiler according to the data type of the variable to be output, select the appropriate stream insertion operator to display the value. << operator is overloaded output built-in type (integer, float, double, string and pointer) data items.

Output stream function (template):

#include <iostream>
#include <iomanip>
using namespace std;
int main()
{
    cout<<setiosflags(ios::left|ios::showpoint);  // 设左对齐，以一般实数方式显示
    cout.precision(5);       // 设置除小数点外有五位有效数字 
    cout<<123.456789<<endl;
    cout.width(10);          // 设置显示域宽10 
    cout.fill('*');          // 在显示区域空白处用*填充
    cout<<resetiosflags(ios::left);  // 清除状态左对齐
    cout<<setiosflags(ios::right);   // 设置右对齐
    cout<<123.456789<<endl;
    cout<<setiosflags(ios::left|ios::fixed);    // 设左对齐，以固定小数位显示
    cout.precision(3);    // 设置实数显示三位小数
    cout<<999.123456<<endl; 
    cout<<resetiosflags(ios::left|ios::fixed);  //清除状态左对齐和定点格式
    cout<<setiosflags(ios::left|ios::scientific);    //设置左对齐，以科学技术法显示 
    cout.precision(3);   //设置保留三位小数
    cout<<123.45678<<endl;
    return 0; 
}

Which cout.setf with setiosflags the same, cout.precision like setprecision, cout.unsetf like resetiosflags.

setiosflags(ios::fixed) 固定的浮点显示 
setiosflags(ios::scientific) 指数表示 
setiosflags(ios::left) 左对齐 
setiosflags(ios::right) 右对齐 
setiosflags(ios::skipws 忽略前导空白 
setiosflags(ios::uppercase) 16进制数大写输出 
setiosflags(ios::lowercase) 16进制小写输出 
setiosflags(ios::showpoint) 强制显示小数点 
setiosflags(ios::showpos) 强制显示符号 

cout.setf common signs:

Mark	Features
boolalpha	You can use the word "true" and "false" input / output Boolean value.
oct	Display value in octal format.
dec	Displayed value in decimal format.
hex	Display values ​​in hexadecimal format.
left	Output justified on the left.
right	Adjust the alignment for the right output.
scientific	Display floating point numbers using scientific notation.
fixed	用正常的记数方法显示浮点数(与科学计数法相对应).
showbase	输出时显示所有数值的基数.
showpoint	显示小数点和额外的零，即使不需要.
showpos	在非负数值前面显示”＋（正号）”.
skipws	当从一个流进行读取时，跳过空白字符(spaces, tabs, newlines).
unitbuf	在每次插入以后，清空缓冲区.
internal	将填充字符回到符号和数值之间.
uppercase	以大写的形式显示科学记数法中的”e”和十六进制格式的”x”.

非缓冲标准错误流 cerr

预定义的对象 cerr 是 iostream 类的一个实例。cerr 对象附属到标准错误设备，通常也是显示屏，但是 cerr 对象是非缓冲的，且每个流插入到 cerr 都会立即输出。cerr 也是与流插入运算符 << 结合使用的

缓冲标准错误流 clog

预定义的对象 clog 是 iostream 类的一个实例。clog 对象附属到标准错误设备，通常也是显示屏，但是 clog 对象是缓冲的。这意味着每个流插入到 clog 都会先存储在缓冲在，直到缓冲填满或者缓冲区刷新时才会输出。clog 也是与流插入运算符 << 结合使用的

所以，使用 cerr 流来显示错误消息，而其他的日志消息则使用 clog 流来输出。