Static variable in C/C++

Thanks to the iPhone development platform, the C++ once again gets the notice of mobile developers. This post talks few issues about static keyword in c/c++.

1. Using static for variable in file scope
If the same named variable is defined in multiple files, compiler will report an error due to the duplicated definition. Adding static for the variable definition will make the variable's scope to the current file, so that they will not conflict with each other. Note, the static variable defined in each file has its own instance. Changing the static variable's value in one file will not affect another file's variable value.
If you want to share the same instance, using extern keywork.

2. Using static for variable in class scope
This is quite easy to understanding, since it is similar to .net or java. The variable belongs to the class instead of a particular class instance.

3. Using static for variable in method scope
The variable will be initialized once, and each time when the mothod gets called, the same instance will be used. It is more useful in c, as in c++, we can just define a class member to do the same thing.

In all cases, if adding const in the definition, then the variable can not be updated once it is initialized.

Jonathan

Create view without InterfaceBuilder for iPhone

Although InterfaceBuilder is an important tool for iPhone development, the tool is not as intuitive to use as Visual Studio UI designer.
To help understand what interfacebuilder does, we can create the interface without using InterfaceBuilder, and then comparing the corresponding steps done by InterfaceBuilder.

Steps to add a ViewController without using InterfaceBuild:
1. Create a Window based iPhone application
2. Add a new UIViewController subclass in the project. Do not select "With XIB for user interface" checkbox
3. In the UIViewContollerSubclass' viewDidLoad method, create some UI elements and then add them into UIViewController.view collection by [self.view addSubview:uiElement] method.
4. In WindowAppDelegate.m didFinishLaunchingWithOptions method, create a instance of this new UIViewControllerSubClass instance, add it into appliation's main window with [window addSubview:uiViewControllerInstance.view].

Comparing with InterfaceBuilder to do the same thing, the following steps are required
1. If a UI sub class is created in the app for a ui element, like UIViewControllerSubClass, then the identity class of the UI element added in InterfaceBuilder needs to set the corresponding sub class with Identity Inspector.
2. For UIViewController object, use Attribute Inspector to set the NIB name to load the xib file, this associates the xib file with the UI control added in InterfaceBuilder.
3. Any IBOutlet property , or IBAction defined in .h file need to be associated with the corresponding ui element added in InterfaceBuilder.
4. File owner is the class that will load the xib file at runtime, for MainWindow.xib, it must be UIApplication classor its subclass. For UIViewController.xib, it must be a subclass of UIViewController class.

Jonathan

How to hide and show soft keyboard on iPhone simulator

When testing with iPhone simulator, occasionaly, you will need to show or hide the soft keyboard. To do so, go to IOS simulator's Hardware menu, and select "Simulator Hardware Keyboard" submenu, it will toggle the soft keyboard's state.


Jonathan

Protocol in Objective C

Protocol in Objective C is quite similar to interface in java or .net, so there is not much to say about it.

To define a protocol in a class header as
@protocol MyProtocol
-(void) myfunction;
@end

The header file of the class that implements a protocol looks like the following, notice the syntax is quite similar to category but uses angle bracket instead of parenthesis.

@interface MyUserClass : NSObject .

Jonathan

Category in Objective C

Category in Objective C is a new concept to C++ developer. The catetory must associate with any existing class, even if the class implementation is not accessible to you. Note that a cateory can not declare additional instance variables for the class. It can only include instance methods.
While category is similar to subclass, the difference is with category, the new methods directly added into the original class without creating a new derived class.

The syntax is

Header file (name convention is "classname+categoryname.h"):
#interface "ClassName.h"

@interface ClassName (CategoryName )
- (int) categoryFunction;
@end


Implementation file (name convention is "classname+categoryname.m"):
#import "classnmae+categoryname.m"

@implementation ClassName (CategoryName )
- (int) categoryFunction {
return 0;
}
@end

Note another Objective C feature: Extension is like cateory but without an name, the extension methods must be implemented in the main @implementation block of the corresponding class. The extension example is shown below:
Header file ( "classname.h")
@interface ClassName ()
- (int) extensionFunction;
@end

implementation file ("classname.m")
@implementation classname

... class methods implementation

- (int) extensionFunction {
return 1;
}

@end

Jonathan

Object inheritance with Javascript

Prototype can be used to define data member and function member to a object. It also is used to realize inheritance for javascript. In order to inherit an object from another object, we set the new the new function's prototype to the existing function as show below:
function base()
{ this.inbase = 2;}

base.prototype.inbaseProto = 3;
base.prototype.getint = function()
{
return 3;
}

derived.prototype = new base;
function derived()
{ this.inderived = 4;}

derived.prototype.inderivedProto = 5;

x = new derived;
s = x.getint();

alert(base.inbase + ', ' + x.inbaseProto + ', ' + x.inderived + ', ' + x.inderivedProto + ', ' + s );
The result is "2, 3, 4, 5, 3";

Jonathan

Use prototype to make javascript object oriented

Unlike other object oriented language, javascript does not support the common way of defining data member or function member. However, this can be supported in javascript by using prototype.
For javascript, when a function is defined, it automatcally gets a data member of prototype, which also has a constructor method by default- i.e the function itself. The constructor is used to create the instance of the object. If any data members or function members are defined on this prototype object, then all instances created by the function will automatically have those data members and function members defined, similar to other object oriented lanaguages, the code is shown below,
function f(){
}
f.prototype.datamember=3.14159;

// create the object method
function f2(o){
alert(o);
}
f.prototype.funcmember=f2;

Note for prebuild types, only object, image, string, data and array can have their prototype changed to add new data members and function members, which means you can extend new function into those prebuild types. With the following code, then all instance of object will have the newprop as 123.
Object.prototype.newprop = "123";

Note if a same named property is defined in both object instance itself and object propotype, then the object instance property will take priority.
Jonathan