Question

Static variables are variables allocated statically at compile time. My doubt is for what purpose some variables are declared statically? I didn’t have used any static variable in my code till now. From the apple code http://developer.apple.com/library/ios/#samplecode/CryptoExercise/Introduction/Intro.html
in securityWrapper.m, there is a line

static SecKeyWrapper * __sharedKeyWrapper = nil; 

what is the use of statically allocating __sharedKeyWrapper here?

I have seen the use of static key word in so many codes. So kindly explain the use and benefits when static keyword.

Thanks in advance.

Answer 1

That file global is a singleton, used for sharing an instance of a class.

The reason it’s static in Objective-C is internal linkage. In practice, this means the variable can not be seen outside the file it is declared in. If another file declares a variable with the same name, they’re two different variables.

Keep in mind that the way Objective-C instances work, the instance won’t actually be allocated automatically. Rather, you have a pointer to the instance. The code will still have to do something like:

if ( !_sharedKeyWrapper ) {
    _sharedKeyWrapper = [[SecKeyWraper alloc] initBlahBlah];
}

See links for more.