No visible @interface for 'class-name' declares the selector 'method-name'

Question

I'm writing a method as below in my View Controller:

- (IBAction)expressionEvaluation:(UIButton *)sender {
    NSDictionary *testValues = [NSDictionary dictionaryWithObjectsAndKeys:@"x", 2, @"y", 3, @"z", 4, nil];
    //the below line gives the error
    double result = [self.brain evaluateExpression:self.brain.expression usingVariableValues:testValues];
    NSString *resultString = [NSString stringWithFormat:@"%g", result];
    self.display.text = resultString;
}

And in my 'brain' class I have declared the not-yet-finished method:

#import <Foundation/Foundation.h>

@interface CalculatorBrain : NSObject
- (void) pushOperand:(double)operand;
- (void) setVariableAsOperand:(NSString *)variableName;
- (void) performWaitingOperation;
- (double) performOperation:(NSString *)operation;

@property (readonly) id expression;
+ (double)evaluateExpression: (id)anExpression
         usingVariableValues: (NSDictionary *)variables; //declared here
@end

...in the .h, and in the .m:

+ (double) evaluateExpression:(id)anExpression usingVariableValues:(NSDictionary *)variables {
    double result = 0;
    int count = [anExpression count];
    for (int i = 0; i < count; i++) {

    }

    return result;
}

Why am I getting this "No visible @interface for 'CalculatorBrain' declares the selector 'evaluateExpression:usingVariableValues'"error? I'm new to objective-c, but I imagine this means it's not seeing my declaration. I'm not sure if it's a syntax/formatting issue, though, because I'm new to the language.

Answer 1

notice that you declare evaluateExpression:usingVariableValues: as class method

+ (double)evaluateExpression: (id)anExpression  // + means class method
     usingVariableValues: (NSDictionary *)variables; //declared here

and use it like instance method

// assuming self.brain is an instance of CalculatorBrain
[self.brain evaluateExpression:self.brain.expression usingVariableValues:testValues];

so either change the method to

- (double)evaluateExpression: (id)anExpression  // - means instance method
     usingVariableValues: (NSDictionary *)variables; //declared here

or call it like this

[CalculatorBrain evaluateExpression:self.brain.expression usingVariableValues:testValues];

Answer 2

the "+" sign means class method. You can access it through your class name not an instance of it.

like

[MyClass methodName];

a "-" sign means instance method. You can access it through an instance of your class (after having allocated-inited it.

like

MyClass *myInstance = [[MyClass alloc] init];

[myInstance methodName];