how to create a instance of objective-c class by name?

I want get somethine like:

#define weaken(object) ...

----

ClassABCD * abcd = [ClassABCD new];
weaken(abcd);
weakAbcd.tag = 0;

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i have some code below:

#define weaken(x) __weak typeof(x) weak##x = x

but it only can use like "weakabcd" . not "weakAbcd"...