这是C ++ code便携式？ （假设多维数组有连续的内存布局） [英] Is this C++ code portable? (assuming multidimensional arrays have continuous memory layout)

问题描述

首先，对不起，我的英语，如果我做任何语法错误，等等...

我的问题是，当我们有一个二维数组，那么如果我是正确的，从计算机和C / C ++的角度来看，这只是一个很长的一维数组，该指数刚刚帮助编译器映射到具体地址。

这code段工程在Visual C ++，但是我想知道，这code是否便携和符合 应用于的标准（C ++ 98），不会造成对其他架构和/或操作系统的惊喜：

  INT ARR [] [3] = {1，5，3，7，5，2，7，8，9};
const int的ARR_NUM = sizeof的（ARR）/的sizeof（int）;在为int * PTR = reinter pret_cast＆LT; INT *＆GT;（ARR）; // NOT：INT（*）[] [3]！
的for（int i = 0; I＆LT; ARR_NUM ++我）{
    COUT＆LT;＆LT; ptr的[1]  - ;＆下; ENDL;
}
 

解决方案

Standardese

多维数组的元素存储在连续行优先顺序，因此人工索引是可移植的：

C ++ 98 8.3.4 / 1：

  

数组类型的对象包含一个连续分配的非空
  设置T型的N个子对象。

显然，对于多维数组适用递归。

然而，这种使用 reinter pret_cast 是不可移植的。标准说（C ++ 98，5.2.10 / 1）

  

...]否则，结果是一个rvalue和[...]，
  阵列到指针，[...]标准转换所执行
  前pression诉

在换句话说，传递改编立即触发所述阵列的衰变的指针其第一个元素。然后，（C ++ 98，5.2.10 / 3）来了包罗万象的

  

按 reinter pret_cast 执行的映射是实现定义的。

一节的其余部分列出了一些例外的，指定始终明确定义类型转换。看到作为他们没有适用于这里，得出的结论是，从技术上讲，实现定义默认。

最后结论

从理论上讲，这是不可移植。实际上，只要架构是相同的（如86）我当然希望中投能可靠地工作。

幸运的是，你不必承担这样的因为作为任何别人 <一个href=\"http://stackoverflow.com/questions/10618473/is-this-c-$c$c-portable-assuming-multidimensional-arrays-have-continuous-mem/10618866#comment13760969_10618473\">have上述，像为int * PTR = ARR [0] 做同样的事情，并保证便携性。

First of all, sorry for my english if i make any grammar mistakes, etc ...

My question is, that when we have a two dimensional array, then if i am right, from the point of the computer and C/C++, it's just a long one dimensional array, the indices are just helping the compiler to map to the concrete address.

This code fragment works in Visual C++, however I would like to know, whether this code is portable and conforms to the standard (C++98), not causing surprises on other architectures and/or operating systems:

int arr[][3] = { 1, 5, 3, 7, 5, 2, 7, 8, 9 };
const int ARR_NUM = sizeof(arr) / sizeof(int);

int* ptr = reinterpret_cast<int*>(arr);    // NOT: int(*)[][3] !!!
for (int i = 0; i < ARR_NUM; ++i) {
    cout << ptr[i] << endl;
}

解决方案

Standardese

The elements of the multidimensional array are stored sequentially in row-major order, so the manual indexing is portable:

C++98, 8.3.4/1:

An object of array type contains a contiguously allocated non-empty set of N sub-objects of type T.

Obviously for a multidimensional array this applies recursively.

However, this use of reinterpret_cast is not portable. The standard says (C++98, 5.2.10/1) that

[...] otherwise, the result is an rvalue and the [...], array-to-pointer, [...] standard conversions are performed on the expression v.

In other words, passing arr immediately triggers a decay of the array to a pointer to its first element. Then (C++98, 5.2.10/3) comes the catch-all

The mapping performed by reinterpret_cast is implementation-defined.

The rest of the section lists a number of exceptions to this, specifying casts that are always well-defined. Seeing as none of them applies here, the conclusion is that technically it's implementation-defined by default.

Final conclusion

Theoretically speaking, this is not portable. Practically, as long as the architectures are the same (e.g. x86) I would certainly expect the cast to work reliably.

Fortunately you don't have to assume anything like that because as others have mentioned, something like int* ptr = arr[0] does the same thing and is guaranteed portable.

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