NSMutable array - “Not an objective-C object” error

问题

I am not totally sure why it is that it is returning the error "not an objective-C object" at the commented line. Any help would be appreciated.

Additionally, I am very new to objective-C, and I do realize there is a high possibility this is a very silly mistake. However any advice will help.

#import "CalculatorBrain.h"

@interface CalculatorBrain()
@property (nonatomic, strong) NSMutableArray *operandStack;
@end

@implementation CalculatorBrain

@synthesize operandStack = _operandStack;

- (NSMutableArray *)operandStack
{
    if(!_operandStack){
    _operandStack = [[NSMutableArray alloc] init];
    }// end if
    return _operandStack;
}//end operandStack

- (void)pushOperand:(double)operand
{
    NSNumber *operandObject = [NSNumber numberWithDouble:operand];
    [self.operandStack addObject:operandObject];
}//end pushOperand

- (double)popOperand
{
    NSNumber *operandObject = [self.operandStack lastObject];//error "Not an objective-c object"
    if(operandObject) [self.operandStack removeLastObject];
    return [operandObject doubleValue];
}//end popOperand

- (double)performOperation:(NSString *)operation
{
    double result = 0;

    if([operation isEqualToString:@"+"]){
    result = [self popOperand] + [self popOperand];
} else if([operation isEqualToString:@"-"]){
    double subtrahend = [self popOperand];
    result = [self popOperand] - subtrahend;
} else if([operation isEqualToString:@"*"]){
    result = [self popOperand] * [self popOperand];
} else if([operation isEqualToString:@"/"]){
    double divisor = [self popOperand];
    if(divisor) result = [self popOperand] / divisor;
}//end if
[self pushOperand:result];
return result;

}//performOperation

@end

回答1:

I think this might be caused by the the incorrect operation order in the function popOperand. See my comments inline, the last line return [operandObject doubleValue] accessed a already released object. After running this code several time, it might cause memory problem and in turn, you might see the error at your comment line.

- (double)popOperand
{
    NSNumber *operandObject = [self.operandStack lastObject];
    if(operandObject) [self.operandStack removeLastObject];
    // When you remove the lastObject(operandObject) from the Array, the operandObject retainCount is zero.
    // Here, the operandObject is deallocated and should not be used.
    // But you call it after it was released.
    return [operandObject doubleValue];
}//end popOperand

This might be OK, when you only call popOperand once or several times. But this will result memory error. I guess the error doesn't occur every time when the popOperand is called. Here is my solution.

- (double)popOperand
{
    double result = 0.0;
    NSNumber *operandObject = [self.operandStack lastObject];
    if(operandObject) 
    {
        result = [operandObject doubleValue];
        [self.operandStack removeLastObject];
    }
    return result;
}

来源：https://stackoverflow.com/questions/11788000/nsmutable-array-not-an-objective-c-object-error

标签

objective-c

ios

nsmutablearray