objective c - "incompatible type in initializtion" -

my caller-code looks this:

nsdictionary *mydictionary = [nsdictionary dictionarywithobjectsandkeys:       @"a", a_key,       @"b", b_key,        @"x", x_key,       nil]; id exp = [[self brain] expression];  double result = [[self brain] evaluateexpression:exp     usingvariablevalues:mydictionary]; -> error line 

• expression defined as: @property (readonly) id expression; (i not think that's relevant)

the signature of function "evaluateexpression:exp..." is:

" + (double)evaluateexpression:(id)anexpression usingvariablevalues:(nsdictionary *)variables;" 

the error receiving: "incompatible type in initializtion"

i can't figure 1 out - it's initializing double variable method returns double. doing wrong?

try changing plus @ beginning of method declaration minus. plus indicates class method, , you're (as far can tell) calling against instance.

i think above mistake causing compiler not know method returning double, hence error.