static variables in Objective-C - what do they do?

问题:

I've seen a few posts discussing what a static variable is and I think I get it - but I'd love to quickly write (or find) a program that utilizes both a regular and a static variable, side by side, and see how/when they operate differently. Some quick n dirty code, maybe two int vars and a couple of NSLog tracking statements just to see HOW they're different.

Anybody got any tips/ideas/code out there that would illustrate how a static var differs from a regular one?

回答1:

In both C and Objective-C, a static variable is a variable that is allocated for the entire lifetime of a program. This is in contrast to automatic variables, whose lifetime exists during a single function call; and dynamically-allocated variables like objects, which can be released from memory when no longer used. More simply put, a static variable's value is maintained throughout all function/method calls. When declared outside of a function, a static variable is visible to everything within the file in which it is declared; when declared inside a function or method, it is visible only within that function or method, but the value is retained between calls.

Say you have this:

int f(void)
{
    int i = 5;
    i += 10;
    return i;
}

Every call to f() will return the value 15.

Now say you have this:

int g(void)
{
    static int i = 5;
    i += 10;
    return i;
}

The first time g() is called, the value 15 will be returned. The second time, 25 will be returned, as i maintained its value of 15 and then incremented itself by 10. The third call, 35 will be returned. And so on.

In the context of Objective-C classes, static variables are often used to mimic class variables, as Objective-C does not have class variables (other languages, such as Java, do). For instance, say you want to lazily initialize an object, and only return that object. You might see this:

static MyObject *obj = nil;

@implementation MyObject

+ (id)sharedObject
{
    if (obj == nil) obj = [[MyObject alloc] init];
    return obj;
}

@end

obj will be initialized the first time classObject is called; subsequent invocations of classObject will return the same object. You could check this by logging the address of the object:

NSLog(@"obj is at %p", [MyObject sharedObject]);
NSLog(@"obj is at %p", [MyObject sharedObject]);    // Will print the same address both times

Furthermore, obj will be visible to all methods in MyObject.

This technique is used to implemented singleton classes in Objective-C as well.

回答2:

static works mostly like in C.

1. It can either initialize a variable only once.

2. Declaring a variable static in a file above @implementationblock will be available for the whole file only.

回答3:

"static" refers more to the attributes of the variable (who what where) rather than just the value. Unlike other languages where it refers exclusively to the value.

• Sticky

It's like pinning the var to a specific location, be it inside a function or in the implementation.

• Private

It has similar attributes to a "private" var in that it's not visible to sibling or parents, but children can access.

• Classy

It is a declaration with default value. Like in other languages where you define vars within a class and assign their "default" value:

private int myNumber = 3;

This gives us "class-like" variables within functions. Declare them once, then as the function manipulates the value, the value is retained. The next time the function is called, the value will be the same as it was after the previous "cycle", just like you would expect a class variable's value to remain after manipulation.