C指针与Objective-C的指针 [英] C pointers vs. Objective-C pointers

问题描述

我是从一个Objective-C背景的，我试图扩大我在C.一件事知识有我困惑，但是，这在C指针和对象 - 之间的区别。正如你可以在下面的例子中看到，事情似乎表现不同的两种语言之间的一点，我想知道如果你能帮助解释为什么？

I'm coming from an Objective-C background and am trying to expand my knowledge in C. One thing has me confused, however, and that's the difference between pointers in C and Obj-C. As you can see in the examples below, things seem to behave a bit differently between both languages, and I was wondering if you could help explain why?

C code工作罚款：

void myFunction()
{
    int x, *pointerX; 
    pointerX = &x;
    *pointerX = 5;

    // Prints: "x is 5"
    printf("x is: %i", x);
}

的OBJ-C code失败：

- (void)myMethod
{
    NSString *string = @"Caramel coffee", *stringPointer;
    stringPointer = &string;                    // Warning:   Assignemnt from incompatible pointer type
    *stringPointer = @"Chocolate milkshake";    // Exception: Incompatible types in assignment

    NSLog(@"string is: %@", string);
}

问：为什么我不能指定stringPointer字符串的内存地址（ stringPointer =放大器;字符串; ），为什么我我能执行 * pointerX = 5; ℃下在，但我不能执行 * stringPointer = @巧克力奶昔; 下的Objective-C？

Question: Why can't I assign stringPointer to the memory address of string (stringPointer = &string;), and why am I able to perform *pointerX = 5; under C, but I can't perform *stringPointer = @"Chocolate milkshake"; under Objective-C?

我认识到，使用对象和C对象 - 交易没有，但我似乎无法找出背后的幕后细节，为什么它不OBJ - C的工作。任何帮助是极大的AP preciated。谢谢！ ：）

I realize that Obj-C deals with objects and C doesn't, but I can't seem to figure out the behind-the-scenes details as to why it doesn't work in Obj-C. Any help is greatly appreciated. Thanks! :)

推荐答案

在你的第一个例子中， X 和 pointerX 有不同的类型（（INT）和（INT *）分别）。在第二个例子中，字符串和 stringPointer 具备的相同的类型（的NSString * ）。尝试，而不是：

In your first example, x and pointerX have different types ((int) and (int *) respectively). In your second example, string and stringPointer have the same type (NSString *). Try instead:

NSString *string = @"Caramel coffee", **stringPointer;

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