这个C ++代码是可移植的吗？ （假设多维数组具有连续的内存布局） [英] Is this C++ code portable? (assuming multidimensional arrays have continuous memory layout)

问题描述

首先，对于我的英语，如果我犯任何语法错误等等...

我的问题是，当我们有一个二维数组，那么如果我是对的，从计算机和C / C ++的角度来看，它只是一个长一维数组，索引只是帮助编译器映射到具体的地址。

此代码片段在Visual C ++中有效，但我想知道，此代码是可移植和符合 （C ++ 98），不会对其他架构和/或操作系统造成意外：

  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  cout< ptr [i]< endl 
} 
  

解决方案

b
$ b

多维数组的元素以行为主的顺序顺序存储，因此手动索引是可移植的：

C ++ 98，8.3.4 / 1：

数组类型的对象包含一个连续分配的非空的
- 类型为T的对象。

显然，对于多维数组，这是递归应用。

但是， reinterpret_cast 的使用不可移植。标准说（C ++ 98，5.2.10 / 1）

[...]否则结果是右值[...]，
数组到指针，[...]对
表达式执行标准转换。

换句话说，传递 arr 会立即触发数组衰减到指向其第一个元素的指针。然后（C ++ 98，5.2.10 / 3）来自catch-all




本节的其余部分列出了大量的异常，指定了铸造，总是很好定义。

最终结论

p>理论上讲，这不是便携式的。实际上，只要架构是相同的（例如x86），我一定会期望转换可靠地工作。

幸运的是，你不必假设因为其他人 已经提到过，例如 int * ptr = arr [0] 做同样的事情，并保证便携。

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.

这篇关于这个C ++代码是可移植的吗？ （假设多维数组具有连续的内存布局）的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！