Objective c write file example

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Does this work? It seems fine at first glance. This takes as arguments the path of the symbolic link, the path to the file to which the link is to refer and an optional NSError object. The NSFileManager class includes some basic file reading and writing capabilities. Firstly, the contents of a file may be read and stored in an NSData object through the use of the contentsAtPath method:. Having stored the contents of a file in an NSData object, that data may subsequently be written out to a new file using the createFileAtPath method:.

This, however, gives us no control over how much data is to be read or written and does not allow us to append data to the end of an existing file. Clearly some more flexible mechanism is required.

The NSFileHandle class provides a range of methods designed to provide a more advanced mechanism for working with files. In addition to files, this class can also be used for working with devices and network sockets. In the following sections we will look at some of the more common uses for this class. An NSFileHandle object can be created when opening a file for reading, writing or updating reading and writing. Having opened a file, it must subsequently be closed when we have finished working with it using the closeFile method.

If an attempt to open a file fails, for example because an attempt is made to open a non-existent file for reading, these methods return nil.

For example, the following code excerpt opens a file for reading and writing and then closes it without actually doing anything to the file:. NSFileHandle objects maintain a pointer to the current position in a file. This is referred to as the offset. When a file is first opened the offset is set to 0 the beginning of the file. This means that any read or write operations we perform using the NSFileHandle methods will take place at offset 0 in the file.

Previous Page. Next Page. Live Demo. Useful Video Courses. Understanding S. In other words, before you call an instance method, you must first create an instance of the class. Instance methods are the most common type of method. It does not require an instance of an object to be the receiver of a message. The declaration of a method consists of the method type identifier, a return type, one or more signature keywords, and the parameter type and name information.

The colon characters declare the presence of a parameter. In the above example, the method takes two parameters. If a method has no parameters, you omit the colon after the first and only signature keyword. When you want to call a method, you do so by sending a message to the object that implements the method. A message is the method name along with the parameter information the method needs properly conforming to type.

All messages you send to an object are dispatched dynamically, thus facilitating the polymorphic behavior of Objective-C classes. Polymorphism refers to the ability of different types of objects to respond to the same message. Sometimes the method invoked is implemented by a superclass of the class of the object receiving the message. To dispatch a message, the runtime requires a message expression.

A message expression encloses with brackets [ and ] the message itself along with any required parameters and, just inside the leftmost bracket, the object receiving the message. For example, to send the insertObject:atIndex: message to an object held by the myArray variable, you would use the following syntax:. To avoid declaring numerous local variables to store temporary results, Objective-C lets you nest message expressions.

The return value from each nested expression is used as a parameter, or as the receiving object, of another message. For example, you could replace any of the variables used in the previous example with messages to retrieve the values. Thus, if you had another object called myAppObject that had methods for accessing the array object and the object to insert into the array, you could write the preceding example to look something like the following:.

Objective-C also provides a dot-notation syntax for invoking accessor methods. Accessor methods get and set the state of an object, and thus are key to encapsulation, which is an important feature of all objects. Objects hide, or encapsulate , their state and present an interface common to all instances for accessing that state.

Using dot-notation syntax, you could rewrite the previous example like this:. You cannot use a reference to a dynamically typed object object of type id in a dot-notation expression.

Although the preceding examples sent messages to an instance of a class, you can also send messages to the class itself. A class is an object of type Class created by the runtime. When messaging a class, the method you specify must be defined as a class method instead of an instance method.

You often use class methods either as factory methods to create new instances of the class or to access some piece of shared information associated with the class. The following example illustrates how you use a class method as a factory method for a class. In this case, the array method is a class method on the NSArray class—and inherited by NSMutableArray —that allocates and initializes a new instance of the class and returns it to your code.

A property in the general sense is some data encapsulated or stored by an object. It is either an attribute—such as a name or a color—or a relationship to one or more other objects. The class of an object defines an interface that enables users of its objects to get and set the values of encapsulated properties. The methods that perform these actions are called accessor methods. There are two types of accessor methods, and each method must conform to a naming convention.

Objective-C offers declared properties as a notational convenience for the declaration and implementation of accessor methods. Declared properties eliminate the need to implement a getter and setter method for each property exposed in the class. Instead, you specify the behavior you want using the property declaration. The compiler can then create—or synthesize —actual getter and setter methods based on that declaration.

Declared properties reduce the amount of boilerplate code you have to write and, as a result, make your code much cleaner and less error prone. You include property declarations with the method declarations in your class interface. You declare public properties in the class header files; you declare private properties in a class extension in the source file. Properties of controller objects such as delegates and view controllers should typically be private.