Static variable "Declaring a variable static limits its scope to just the class—and to just the part of the class that’s implemented in the file" (Apple doc).
And I think a variable was defined in class extension has a limit scope in just only the class define it.
That is similar !
What is different between static variable and variable in class extension ?
The static variable is tied to the file it's defined in. It's not accessible from outside of that file, and there is only one place for storage created for it in your entire program.
The distinction about storage also applies to non-static global variables -- there will only be one in your program.
A variable in a class extension is likewise limited in visibility to the file in which it's declared, but it's an instance variable. There's a new piece of storage attached to each instance of the class you create.
If you create a static variable and change its value from several instances of the class, every instance will see the same value. That is not the case with an ivar -- each object can change and retain its own value for that variable.
(This is why static variables are sometimes used in ObjC to simulate class variables, which are present in other languages.)
Related
After using Java for a long time I almost forgot how it is to be allowed to define global variables. I read that it is recommended to use top-level functions in Kotlin instead of wrapping them inside objects like in Scala. How about top-level variables (which I guess we call global variables)? What is the recommendation about using them? What are the pitfalls? How are they deallocated?
Global variables are supposed to be accessible from anywhere at any point in your program, so it would most likely be a bad idea to deallocate global variables, even if you could because it could lead to instability or crashes in your application.
Now Kotlin is a JVM language, it compiles to Java code and therefore is garbage-collected. As for how the garbage collector works, when a variable is no longer referenced by the program, it gets flagged for deletion and will be removed by the garbage collector later (not necessarily instantly).
Global variable's scope is the entire program so they are never garbage-collected. What could you do tho is to set its reference to null (if it is declared nullable) to allow at least the referenced memory to be collected.
The top-level variables that you are referring about aren't exactly free since Kotlin will compile them into a java class named with the file name anyway(that's how you can access kotlin file-specific functions inside java code), so they are just like static fields in Java. For a better explanation you could check this thread: Kotlin: Difference between constant in companion object and top level.
Now we know that top-level variables are compiled into static variables inside a Java class. Those don't get flagged for collection unless the program somehow drops the class loader associated to that class itself but that's something I can't speak of more since I never used that.
Now in the end, should you use global variables? They are usually considered a bad practice and this is in big part due to the fact that they can be referenced by any part of the program and it can be hard to remember when and why did they change, especially in large applications. If you keep track of their usages and use them in moderation then they are alright. Personally, if I have to use them, I define them inside a class as a companion object the same way you would define static variables in java, like this:
class GlobalData {
companion object {
var varEx= 0
const val constString = "CONSTANT GLOBAL"
}
}
I do not have a background on C/C++. I just started learning Objective-C after a past using other languages.
In which situations should I use static declaration of a variable over regular ivars or properties? What do I gain doing this?
thanks
Global variables and functions
By default, all symbols (global variables and functions) are exported (made visible to code in other source files). If a global variable is declared static, it is not exported. That means it is only accessible to code in the current source file.
This is useful when you have a global variable that you want to restrict access to, and don't want to worry about a name collision. For example, if you wanted to maintain a counter to track how many instances of a class have been created, you could create a static int gInstanceCount. Since it is static, you would know that (1) no other code can modify the variable and (2) if some other file uses a global with the same name, there won't be any collisions.
Static declarations in header files
Note that when you put something in a header file, it is as if you copied-and-pasted that code into every other file that includes it. That means if you declare something as static in a header file, every file that includes it gets its own copy of that thing.
That means if you declare static int foo in Foo.h and then write execute foo = 4 in Bar.m, when you try to access that value in Other.m you won't necessarily get 4 back.
Local static variables
You can also define a local variable (inside a function or method body) as static. Normally, local variables are allocated on the "stack" which means they are created when your function is executed and deallocated when the function exits. If two threads enter the same function at the same time (or one thread makes a recursive call back into a function) each thread gets a fresh chunk of memory to work with, and anything it does won't affect any other thread.
However, a local static variable is stored on the "heap" instead. All executions of the function share that same memory location. Also, when the function ends, the value stays where it is. That's why a local static variable is often used in sharedInstance methods on Objective-C singleton objects.
In most ways, a local static variable basically acts like a global variable that can only be seen inside the function which declared it.
the static keyword is used to provide scope to global variables. Normally, global variables defined outside of a function have a public scope, and are visible to all .m or .c files in a project. making a variable static allows you to have a "global" variable which is scoped (visible) to only the .m or .c file containing the variable definition. This allows you to have a variable which can be shared by multiple functions in a single source file, while avoiding potential naming conflicts. Also of note, the extern keyword, which allows you to indicate that a specific global variable is initialized in a different source file but used in this source file.
The keyword static is overused in C. It means several different things. In some contexts, it just means that field is only understood in the remainder of that file (compilation unit). In other words, it can't be linked to from some other file.
In Java and C++, static class members are defined at the class level, not the individual object level, so the one value is shared by all objects of that class (or one of its subclasses). Unfortunately, IMHO, Objective C does not support this. Instead you use statics at the file level.
If you don't know if you need a static variable, you probably don't need one.
One reason you might use a static variable is to provide a class variable (that is, one variable for an entire class, rather than an instance variable, which each instance of a class would have it's own copy of).
This question (and its accepted answer) explain how one would use static variables to simulate a class variable in Objective-C.
First: "object" in this context refer to "c objects", in very easy words, something, that lives at runtime. It has nothing to do with objects in the sense of OOP or Objective-C. You can think of an object as a var.
static has nothing to do with scope. static on an extern identifier (= declared outside a block) has internal linkage. The results of linkage are defined in ISO/IEC 9899:TC3, section 6.2.2. Scope is defined in section 6.2.1.
Internal linkage means, in short words, that two identifier in different translation units (".m files") do not denote the same object, but different objects. Every translation unit has its "own" object.
If an identifier is not extern (= inside a block) declared with static storage class it has a lifetime from begin of the program execution until the program stops running.
In objective-c, I can declare an int or bool etc in the .m file, outside any function. That allows me to use such variable everywhere in the class.
I can also declare such variables in the .h file, inside the interface block, achieving the same result.
Well, my question is: what is the difference? Is there? Or is it all a matter of organization?
In second case, it is a global variable that has external linkage. Meaning, it can be accessed other translation units / source files using extern keyword. But in first case, it is a part of interface, so it can be only used by it's member functions and any other interfaces derived from this interface depending on access specifier.
In first case they become gloabal variables in .m file and are are shared between all the instances of the interface. In second case the will be separate for multiple instances. The common way is to declare interface variables with in the interface
I have concept of static variables but what are the benefits of static methods in a class. I have worked on some projects but I did not make a method static. Whenever I need to call a method of a class, I create an object of that class and call the desired method.
Q: Static variable in a method holds it's value even when method is executed but accessible only in its containing method but what is the best definition of static method?
Q: Is calling the static method without creating object of that class is the only benefit of static method?
Q: What is the accessible range for static method?
Thanks
Your description of a static variable is more fitting to that found in C. The concept of a static variable in Object Oriented terms is conceptually different. I'm drawing from Java experience here. Static methods and fields are useful when they conceptually don't belong to an instance of something.
Consider a Math class that contains some common values like Pi or e, and some useful functions like sin and cos. It really does not make sense to create separate instances to use this kind of functionality, thus they are better as statics:
// This makes little sense
Math m = new Math();
float answer = m.sin(45);
// This would make more sense
float answer = Math.sin(45);
In OO languages (again, from a Java perspective) functions, or better known as methods, cannot have static local variables. Only classes can have static members, which as I've said, resemble little compared to the idea of static in C.
Static methods don't pass a "this" pointer to an object, so they can't reference non-static variables or methods, but may consequently be more efficient at runtime (fewer parameters and no overhead to create and destroy an object).
They can be used to group cohesive methods into a single class, or to act upon objects of their class, such as in the factory pattern.
Syntax (php) for static methods:
<?php
class Number {
public static function multiply($a, $b) {
return $a * $b;
}
}
?>
Client code:
echo Number::multiply(1, 2);
Which makes more sense than:
$number = new Number();
echo $number->multiply(1, 2);
As the multiply() method does not use any class variables and as such does not require an instance of Number.
Essentially, static methods let you write procedural code in an object oriented language. It lets you call methods without having to create an object first.
The only time you want to use a static method in a class is when a given method does not require an instance of a class to be created. This could be when trying to return a shared data source (eg a Singleton) or performing an operation that doesn't modify the internal state of the object (String.format for example).
This wikipedia entry explains static methods pretty well: http://en.wikipedia.org/wiki/Method_(computer_science)#Static_methods
Static variables and static methods are bound to the class, and not an instance of the class.
Static methods should not contain a "state". Anything related to a state, should be bound to an instantiated object, and not the class.
One common usage of static methods is in the named constructor idiom. See: http://www.parashift.com/c++-faq-lite/ctors.html#faq-10.8.
Static Methods in PHP:
Can be called without creating a class object.
Can only call on static methods and function.
Static variable is used when you want to share some info between different objects of the class.As variable is shared each object can update it and the updated value be available for all other objects as well.
As static variable can be shared,these are often called as class variable.
static elements are accessible from any context (i.e. anywhere in your script), so you can access these methods without needing to pass an instance of the class from object to object.
Static elements are available in every instance of a class, so you can set values that you want to be available to all members of a type.
for further reading a link!
How can we distinguish to create a class which is static?
A static class forces all of its methods to be static and prohibits an instance constructor therefor can't be instantiated. If your question extends to WHEN to use static and WHEN instance, please do a search on StackOverflow (or check out the Related box on this page)
At least in C#,
static classes and class members are used to create data and functions that can be accessed without creating an instance of the class.
If you want the class to be static in nature i.e. have only 1 copy within the program (VM) then there are two obvious mechanisms:
1. Make all members and methods of the class static (Java/C#).
2. Use Singleton design pattern.
For this case (static in nature), we don't have a language construct and hence one of the above technique is used.
As to your question for this case, such classes should be created if you want your functionality to be accessible globally, unchanged and instantly accessible e.g. utility methods, global constants etc.
Secondly, the keyword 'static' is used with classes to increase their visibility in the package. This keyword can only be applied on inner classes and allows the access to inner classes without the context of their parent class.
Such kind of static classes should be used only for those inner classes that serve their purpose within the parent class as well as are useful outside the class or the package e.g. Key of a POJO.