Messages without a matching method signature will be assume to return id

I'm get开发者_运维问答ting a warning from the compiler :

"warning: 'NSSortDescriptor' may not respond to '+sortDescriptorWithKey:ascending:' "

...on this line of code:

id sortDescriptor=[NSSortDescriptor sortDescriptorWithKey:@"value" ascending:YES];

the compiler is also saying the following:

"Messages without a matching method signature will be assume to return id..."

+sortDescriptorWithKey:ascending:is a standard method of the platorm. It's not my own class.

I think this message usually appears when you don't declare the method in the interface but again, this is not my method.

Any ideas...

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The sortDescriptorWithKey:ascending: method was introduced in iOS 4.0. So it sounds like you are compiling with a Base SDK set to something lower than 4.0.