Objective-c – How to access variables from another class

classcocoaimportobjective c

There is probably a very simple solution for this but I can't get it working.

I have got multiple classes in my Cocoa file. In one of the classes class1 I create a variable that I need to use in another class class2 as well. Is there a simple way to import this variable in class2?

Best Answer

You can either make the variable public, or make it into a property. For example, to make it public:

@interface Class1
{
@public
    int var;
}
// methods...
@end

// Inside a Class2 method:
Class1 *obj = ...;
obj->var = 3;

To make it a property:

@interface Class1
{
    int var;  // @protected by default
}
@property (readwrite, nonatomic) int var;
// methods...
@end

@implementation Class1
@synthesize var;
...
@end

// Inside a Class2 method:
Class1 *obj = ...;
obj.var = 3;  // implicitly calls [obj setVar:3]
int x = obj.var;  // implicitly calls x = [obj var];

Related Solutions

Python – Are static class variables possible in Python

Variables declared inside the class definition, but not inside a method are class or static variables:

>>> class MyClass:
...     i = 3
...
>>> MyClass.i
3

As @millerdev points out, this creates a class-level i variable, but this is distinct from any instance-level i variable, so you could have

>>> m = MyClass()
>>> m.i = 4
>>> MyClass.i, m.i
>>> (3, 4)

This is different from C++ and Java, but not so different from C#, where a static member can't be accessed using a reference to an instance.

See what the Python tutorial has to say on the subject of classes and class objects.

@Steve Johnson has already answered regarding static methods, also documented under "Built-in Functions" in the Python Library Reference.

class C:
    @staticmethod
    def f(arg1, arg2, ...): ...

@beidy recommends classmethods over staticmethod, as the method then receives the class type as the first argument, but I'm still a little fuzzy on the advantages of this approach over staticmethod. If you are too, then it probably doesn't matter.

Javascript – What techniques can be used to define a class in JavaScript, and what are their trade-offs

Here's the way to do it without using any external libraries:

// Define a class like this
function Person(name, gender){

   // Add object properties like this
   this.name = name;
   this.gender = gender;
}

// Add methods like this.  All Person objects will be able to invoke this
Person.prototype.speak = function(){
    alert("Howdy, my name is" + this.name);
};

// Instantiate new objects with 'new'
var person = new Person("Bob", "M");

// Invoke methods like this
person.speak(); // alerts "Howdy, my name is Bob"

Now the real answer is a whole lot more complex than that. For instance, there is no such thing as classes in JavaScript. JavaScript uses a prototype-based inheritance scheme.

In addition, there are numerous popular JavaScript libraries that have their own style of approximating class-like functionality in JavaScript. You'll want to check out at least Prototype and jQuery.

Deciding which of these is the "best" is a great way to start a holy war on Stack Overflow. If you're embarking on a larger JavaScript-heavy project, it's definitely worth learning a popular library and doing it their way. I'm a Prototype guy, but Stack Overflow seems to lean towards jQuery.

As far as there being only "one way to do it", without any dependencies on external libraries, the way I wrote is pretty much it.

Best Answer

Related Solutions

Python – Are static class variables possible in Python

Javascript – What techniques can be used to define a class in JavaScript, and what are their trade-offs

Related Topic