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.
与恶龙缠斗过久,自身亦成为恶龙；凝视深渊过久,深渊将回以凝视…
