- C++ Basics
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- C++ Object Oriented
- C++ Advanced
- C++ Useful Resources
- Selected Reading
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 −
Sr.No | Header File & Function and Description |
---|---|
1 | <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. |
2 | <iomanip> This file declares services useful for performing formatted I/O with so-called parameterized stream manipulators, such as setw and setprecision. |
3 | <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.
When the above code is compiled and executed, it produces the following result −
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.
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 following result −
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 −
This will be equivalent to the following two statements −
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.
When the above code is compiled and executed, it produces the following result −
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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.
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When the above code is compiled and executed, it produces the following result −
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 the difference becomes obvious. So it is good practice to display error messages using cerr stream and while displaying other log messages then clog should be used.
-->A GUI application must respond to events from the user and from the operating system.
- Events from the user include all the ways that someone can interact with your program: mouse clicks, key strokes, touch-screen gestures, and so on.
- Events from the operating system include anything 'outside' of the program that can affect how the program behaves. For example, the user might plug in a new hardware device, or Windows might enter a lower-power state (sleep or hibernate).
These events can occur at any time while the program is running, in almost any order. How do you structure a program whose flow of execution cannot be predicted in advance?
To solve this problem, Windows uses a message-passing model. The operating system communicates with your application window by passing messages to it. A message is simply a numeric code that designates a particular event. For example, if the user presses the left mouse button, the window receives a message that has the following message code.
Some messages have data associated with them. For example, the WM_LBUTTONDOWN message includes the x-coordinate and y-coordinate of the mouse cursor.
To pass a message to a window, the operating system calls the window procedure registered for that window. (And now you know what the window procedure is for.)
The Message Loop
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An application will receive thousands of messages while it runs. (Consider that every keystroke and mouse-button click generates a message.) Additionally, an application can have several windows, each with its own window procedure. How does the program receive all these messages and deliver them to the correct window procedure? The application needs a loop to retrieve the messages and dispatch them to the correct windows.
For each thread that creates a window, the operating system creates a queue for window messages. This queue holds messages for all the windows that are created on that thread. The queue itself is hidden from your program. You cannot manipulate the queue directly. However, you can pull a message from the queue by calling the GetMessage function.
This function removes the first message from the head of the queue. If the queue is empty, the function blocks until another message is queued. The fact that GetMessage blocks will not make your program unresponsive. If there are no messages, there is nothing for the program to do. If you have to perform background processing, you can create additional threads that continue to run while GetMessage waits for another message. (See Avoiding Bottlenecks in Your Window Procedure.)
The first parameter of GetMessage is the address of a MSG structure. If the function succeeds, it fills in the MSG structure with information about the message. This includes the target window and the message code. The other three parameters let you filter which messages you get from the queue. In almost all cases, you will set these parameters to zero.
Although the MSG structure contains information about the message, you will almost never examine this structure directly. Instead, you will pass it directly to two other functions.
The TranslateMessage function is related to keyboard input. It translates keystrokes (key down, key up) into characters. You do not really have to know how this function works; just remember to call it before DispatchMessage. The link to the MSDN documentation will give you more information, if you are curious.
The DispatchMessage function tells the operating system to call the window procedure of the window that is the target of the message. In other words, the operating system looks up the window handle in its table of windows, finds the function pointer associated with the window, and invokes the function.
For example, suppose that the user presses the left mouse button. This causes a chain of events:
- The operating system puts a WM_LBUTTONDOWN message on the message queue.
- Your program calls the GetMessage function.
- GetMessage pulls the WM_LBUTTONDOWN message from the queue and fills in the MSG structure.
- Your program calls the TranslateMessage and DispatchMessage functions.
- Inside DispatchMessage, the operating system calls your window procedure.
- Your window procedure can either respond to the message or ignore it.
When the window procedure returns, it returns back to DispatchMessage. This returns to the message loop for the next message. As long as your program is running, messages will continue to arrive on the queue. Therefore, you must have a loop that continually pulls messages from the queue and dispatches them. You can think of the loop as doing the following:
As written, of course, this loop would never end. That is where the return value for the GetMessage function comes in. Normally, GetMessage returns a nonzero value. When you want to exit the application and break out of the message loop, call the PostQuitMessage function.
The PostQuitMessage function puts a WM_QUIT message on the message queue. WM_QUIT is a special message: It causes GetMessage to return zero, signaling the end of the message loop. Here is the revised message loop.
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As long as GetMessage returns a nonzero value, the expression in the while loop evaluates to true. After you call PostQuitMessage, the expression becomes false and the program breaks out of the loop. (One interesting result of this behavior is that your window procedure never receives a WM_QUIT message. Therefore, you do not have to have a case statement for this message in your window procedure.)
The next obvious question is when to call PostQuitMessage. We'll return to this question in the topic Closing the Window, but first we have to write our window procedure.
Posted Messages versus Sent Messages
The previous section talked about messages going onto a queue. Sometimes, the operating system will call a window procedure directly, bypassing the queue.
The terminology for this distinction can be confusing:
- Posting a message means the message goes on the message queue, and is dispatched through the message loop (GetMessage and DispatchMessage).
- Sending a message means the message skips the queue, and the operating system calls the window procedure directly.
For now, the difference is not very important. The window procedure handles all messages. However, some messages bypass the queue and go directly to your window procedure. However, it can make a difference if your application communicates between windows. You can find a more thorough discussion of this issue in the topic About Messages and Message Queues.