C++ Date and Time
The C++ standard library does not provide a proper date type. C++ inherits the structs and functions for date and time manipulation from C. To access date and time related functions and structures, you would need to include <ctime> header file in your C++ program.
There are four time-related types: clock_t, time_t, size_t, and tm. The types clock_t, size_t and time_t are capable of representing the system time and date as some sort of integer.
The structure type tm holds the date and time in the form of a C structure having the following elements:
struct tm { int tm_sec; // seconds of minutes from 0 to 61 int tm_min; // minutes of hour from 0 to 59 int tm_hour; // hours of day from 0 to 24 int tm_mday; // day of month from 1 to 31 int tm_mon; // month of year from 0 to 11 int tm_year; // year since 1900 int tm_wday; // days since sunday int tm_yday; // days since January 1st int tm_isdst; // hours of daylight savings time }
Following are the important functions, which we use while working with date and time in C or C++. All these functions are part of standard C and C++ library and you can check their detail using reference to C++ standard library given below.
| SN | Function & Purpose |
|---|---|
| 1 | time_t time(time_t *time);This returns the current calendar time of the system in number of seconds elapsed since January 1, 1970. If the system has no time, .1 is returned. |
| 2 | char *ctime(const time_t *time);This returns a pointer to a string of the form day month year hours:minutes:seconds year\n\0. |
| 3 | struct tm *localtime(const time_t *time);This returns a pointer to the tm structure representing local time. |
| 4 | clock_t clock(void);This returns a value that approximates the amount of time the calling program has been running. A value of .1 is returned if the time is not available. |
| 5 | char * asctime ( const struct tm * time );This returns a pointer to a string that contains the information stored in the structure pointed to by time converted into the form: day month date hours:minutes:seconds year\n\0 |
| 6 | struct tm *gmtime(const time_t *time);This returns a pointer to the time in the form of a tm structure. The time is represented in Coordinated Universal Time (UTC), which is essentially Greenwich Mean Time (GMT). |
| 7 | time_t mktime(struct tm *time);This returns the calendar-time equivalent of the time found in the structure pointed to by time. |
| 8 | double difftime ( time_t time2, time_t time1 );This function calculates the difference in seconds between time1 and time2. |
| 9 | size_t strftime();This function can be used to format date and time a specific format. |
Current date and time:
Consider you want to retrieve the current system date and time, either as a local time or as a Coordinated Universal Time (UTC). Following is the example to achieve the same:
#include <iostream> #include <ctime> using namespace std; int main( ) { // current date/time based on current system time_t now = time(0); // convert now to string form char* dt = ctime(&now); cout << "The local date and time is: " << dt << endl; // convert now to tm struct for UTC tm *gmtm = gmtime(&now); dt = asctime(gmtm); cout << "The UTC date and time is:"<< dt << endl; }
When the above code is compiled and executed, it produces the following result:
The local date and time is: Sat Jan 8 20:07:41 2011 The UTC date and time is:Sun Jan 9 03:07:41 2011
Format time using struct tm:
The tm structure is very important while working with date and time in either C or C++. This structure holds the date and time in the form of a C structure as mentioned above. Most of the time related functions makes use of tm structure. Following is an example which makes use of various date and time related functions and tm structure:
While using structure in this chapter, I’m making an assumption that you have basic understanding on C structure and how to access structure members using arrow -> operator.
#include <iostream> #include <ctime> using namespace std; int main( ) { // current date/time based on current system time_t now = time(0); cout << "Number of sec since January 1,1970:" << now << endl; tm *ltm = localtime(&now); // print various components of tm structure. cout << "Year: "<< 1900 + ltm->tm_year << endl; cout << "Month: "<< 1 + ltm->tm_mon<< endl; cout << "Day: "<< ltm->tm_mday << endl; cout << "Time: "<< 1 + ltm->tm_hour << ":"; cout << 1 + ltm->tm_min << ":"; cout << 1 + ltm->tm_sec << endl; }
When the above code is compiled and executed, it produces the following result:
Number of sec since January 1, 1970:1294548238 Year: 2011 Month: 1 Day: 8 Time: 22: 44:59
C++ Basic Input/Output
The C++ standard libraries provide an extensive set of input/output capabilities which we will see in subsequent chapters. This chapter will discuss very basic and most common I/O operations required for C++ programming.
C++ I/O occurs in streams, which are sequences of bytes. If bytes flow from a device like a keyboard, a disk drive, or a network connection etc. to main memory, this is called input operation and if bytes flow from main memory to a device like a display screen, a printer, a disk drive, or a network connection, etc, this is called output operation.
I/O Library Header Files:
There are following header files important to C++ programs:
| Header File | Function and Description |
|---|---|
| <iostream> | This file defines the cin, cout, cerr and clog objects, which correspond to the standard input stream, the standard output stream, the un-buffered standard error stream and the buffered standard error stream, respectively. |
| <iomanip> | This file declares services useful for performing formatted I/O with so-called parameterized stream manipulators, such as setw andsetprecision. |
| <fstream> | This file declares services for user-controlled file processing. We will discuss about it in detail in File and Stream related chapter. |
The standard output stream (cout):
The predefined object cout is an instance of ostream class. The cout object is said to be “connected to” the standard output device, which usually is the display screen. The cout is used in conjunction with the stream insertion operator, which is written as << which are two less than signs as shown in the following example.
#include <iostream> using namespace std; int main( ) { char str[] = "Hello C++"; cout << "Value of str is : " << str << endl; }
When the above code is compiled and executed, it produces the following result:
Value of str is : Hello C++
The C++ compiler also determines the data type of variable to be output and selects the appropriate stream insertion operator to display the value. The << operator is overloaded to output data items of built-in types integer, float, double, strings and pointer values.
The insertion operator << may be used more than once in a single statement as shown above and endl is used to add a new-line at the end of the line.
The standard input stream (cin):
The predefined object cin is an instance of istream class. The cin object is said to be attached to the standard input device, which usually is the keyboard. The cin is used in conjunction with the stream extraction operator, which is written as >> which are two greater than signs as shown in the following example.
#include <iostream> using namespace std; int main( ) { char name[50]; cout << "Please enter your name: "; cin >> name; cout << "Your name is: " << name << endl; }
When the above code is compiled and executed, it will prompt you to enter a name. You enter a value and then hit enter to see the result something as follows:
Please enter your name: cplusplus Your name is: cplusplus
The C++ compiler also determines the data type of the entered value and selects the appropriate stream extraction operator to extract the value and store it in the given variables.
The stream extraction operator >> may be used more than once in a single statement. To request more than one datum you can use the following:
cin >> name >> age;
This will be equivalent to the following two statements:
cin >> name; cin >> age;
The standard error stream (cerr):
The predefined object cerr is an instance of ostream class. The cerr object is said to be attached to the standard error device, which is also a display screen but the object cerr is un-buffered and each stream insertion to cerr causes its output to appear immediately.
The cerr is also used in conjunction with the stream insertion operator as shown in the following example.
#include <iostream> using namespace std; int main( ) { char str[] = "Unable to read...."; cerr << "Error message : " << str << endl; }
When the above code is compiled and executed, it produces the following result:
Error message : Unable to read....
The standard log stream (clog):
The predefined object clog is an instance of ostream class. The clog object is said to be attached to the standard error device, which is also a display screen but the object clog is buffered. This means that each insertion to clog could cause its output to be held in a buffer until the buffer is filled or until the buffer is flushed.
The clog is also used in conjunction with the stream insertion operator as shown in the following example.
#include <iostream> using namespace std; int main( ) { char str[] = "Unable to read...."; clog << "Error message : " << str << endl; }
When the above code is compiled and executed, it produces the following result:
Error message : Unable to read....
You would not be able to see any difference in cout, cerr and clog with these small examples, but while writing and executing big programs then difference becomes obvious. So this is good practice to display error messages using cerr stream and while displaying other log messages then clog should be used.