objective-c - NSString 分配中的摘要无效

Question

我正在尝试实现一个简单的计算器，我必须在 NSString 类型的变量中更新当前操作(+、-、* 等)。当前操作作为参数接收到模型。

 -(double)doOperation:(NSString *)operation withOperand:(double)operand
 {
     if (!currentOperation)
     {
         anOperand = operand;
     }   

     currentOperation = operation; //Invalid Summary in currentOperation; but + in operation
 }

这有什么问题？如果 NSString 不允许直接赋值指针，替代方法是什么？

编辑

1. 更准确地说，在 iOs 中使用 = 号分配 NSString 是否合法？
2. 如果没有，该怎么走？

注意:currentOperation是Controller类中的私有(private)变量，operation是方法的参数。

完整代码如下

@interface CalculatorBrain : NSObject {
    double anOperand;
    NSString * currentOperaion;
}

- (double) doOperation: (NSString *) operation 
           withOperand: (double) operand;

@end



@implementation CalculatorBrain


- (double) doOperation: (NSString *) operation 
           withOperand: (double) operand
{
    if (!currentOperaion)
    {
        anOperand = operand;
    }
    else if([currentOperaion isEqual: @"+"])
    {
        anOperand += operand;
    }
    else if([currentOperaion isEqual: @"-"])
    {
        anOperand -= operand;
    }
    else if([currentOperaion isEqual: @"*"])
    {
        anOperand *= operand;
    }
    else if([currentOperaion isEqual: @"/"])
    {
        anOperand /= operand;
    }

    currentOperaion = operation; //This is where the statement doesn't work as expected

    if ([currentOperaion isEqual: @"="])
    {
        currentOperaion = nil;
    }

    return anOperand;
}

@end

Answer 1

To be more precise, is it legal in iOs to assign NSString with = sign?

是的。

What could be wrong in this?

您没有取得字符串的所有权，如果您想稍后使用该实例，这将导致问题，请参阅 memory management guide :

You can take ownership of an object using retain.
Remember that an object may have more than one owner. Taking ownership of an object is your way of saying that you need it to be kept alive.

您可以-retain 字符串，或-copy 以避免外部代码的突变，例如:

// in -doOperation::
[currentOperation release];
currentOperation = [operation copy]; // copy if operation might be mutable

// relinquish ownership:
- (void)dealloc {
    [currentOperation release];
    [super dealloc];
}

另请查看 declared properties .