对象指针的麻烦访问数组 [英] Trouble accessing array of object pointers

问题描述

我正在写一个有点code到一个总线模型。在code有一个错误，我已经简化在下面的代码片段问题：

I'm writing a bit of code to model a bus. The code has an error and I have simplified the problem in the following snippet:

struct luggageTag{
    int seat;
    bool luggage;
};

int main(){

    luggageTag *tagBox[36];
    tagBox[2]->luggage = true; // EXC_BAD_ACCESS on this line  
}

为什么该行

tagBox[2]->luggage = true;

结果不好的访问？

results in bad access?

推荐答案

一个指针是一个变量就像任何其他的，只是一个指针， T * PTR ，它的值预计是在存储器中的T实例的地址

A pointer is a variable much like any other, except that for a pointer, T* ptr, its value is expected to be the address of an instance of T in memory.

您已经创建未初始化的变量数组 - 你没有任何东西在他们指出

You've created an array of uninitialized variables - you haven't pointed them at anything.

想想一个指针作为后它音符与它事物的位置。你所做被撕裂36的空白贴字条从堆栈的顶部。

Think of a pointer as a post-it-note with the location of a thing on it. What you've done is torn 36 blank post-it notes off the top of the stack.

您需要创建一些luggageTag的指向，但随后也将负责释放这些对象。

You'll need to create some luggageTag's to point to, but then you'll also be responsible for freeing those objects.

struct luggageTag{
    int seat;
    bool luggage;
};

int main(){
    luggageTag *tagBox[36];
    for (size_t i = 0; i < 36; ++i) {
        tagBox[i] = new luggageTag;
    }
    tagBox[2]->luggage = true;
    // memory leak unless you do:
    // for (size_t i = 0; i < 36 ; ++i)
    //   delete tagBox[i];
}

另外，您可以创建一个指向36行李标签的数组：

Alternatively you could create a pointer to an array of 36 luggage tags:

struct luggageTag{
    int seat;
    bool luggage;
};

int main(){
    luggageTag *tagBox = new luggageTag[36];
    tagBox[2]->luggage = true;
    // ...
    delete [] tagBox;  // free the memory
}

如果这不是一所学校工作的一部分，你可能想看看使用的std ::阵列或的std ::矢量。

If this is not part of a school exercise, you might want to look at using a std::array or std::vector.

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