T *与char *指针算术 [英] T* versus char* pointer arithmetic

问题描述

假设我们有一个包含N个类型为T的元素的数组.

Assume we have an array that contains N elements of type T.

T a[N];

根据C ++ 14标准，在这种情况下，我们可以保证

According to the C++14 Standard, under which conditions do we have a guarantee that

 (char*)(void*)&a[0] + n*sizeof(T) == (char*)(void*)&a[n],  (0<=n<N) ?

尽管对于许多类型和实现都是如此，但该标准在脚注中并以一种含糊的方式提及了这一点:

While this is true for many types and implementations, the standard mentions it in a footnote, and in an ambiguous way:

第5.7.6节，脚注85)处理指针算术的另一种方法...

§5.7.6, footnote 85) Another way to approach pointer arithmetic ...

几乎没有迹象表明其他方式被认为等同于标准方式.对于实施者而言，它可能暗示了许多一致的实施中的一种.

There is little indication that this other way was thought of being equivalent to the standard's way. It might rather be a hint for implementers that suggests one of many conforming implementations.

人们低估了这个问题的难度.

People have underestimated the difficulty of this question.

这个问题不是关于您在教科书中可以读到的内容，而是关于您可以通过使用逻辑和理由从C ++ 14标准中得出什么.

This question is not about what you can read in textbooks, it is about what what you can deduce from the C++14 Standard through the use of logic and reason.

如果您使用连续"或连续"，还请说出连续的内容.

If you use 'contiguous' or 'contiguously', please also say what is being contiguous.

虽然T []和T *密切相关，但是它们是抽象的，并且T * x N上的加法可以由实现以任何一致的方式定义.

While T[] and T* are closely related, they are abstractions, and the addition on T* x N may be defined by the implementation in any consistent way.

使用指针加法重新排列了方程式.如果p指向char，则始终使用(§5.7(4))或一元加法来定义p + 1，因此我们不会遇到UB.原始文件包含指针减法，这可能早早导致了UB. (只比较char指针，不取消引用).

The equation was rearranged using pointer addition. If p points to a char, p+1 is always defined using (§5.7 (4)) or unary addition, so we don't run into UB. The original included a pointer subtraction, which might have caused UB early on. (The char pointers are only compared, not dereferenced).

推荐答案

在[dcl.array]中:

In [dcl.array]:

一个数组类型的对象包含一个连续分配的非空对象 T类型的N子对象的集合.

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

连续表示T类型的任何连续子对象之间的偏移为sizeof(T)，这表示第n个子对象的偏移为n*sizeof(T).

Contiguous implies that the offset between any consecutive subobjects of type T is sizeof(T), which implies that the offset of the nth subobject is n*sizeof(T).

n < N的上限来自[expr.add]:

The upper bound of n < N comes from [expr.add]:

当将具有整数类型的表达式添加到指针或从指针中减去时，结果将具有指针操作数的类型.如果表达式P指向具有n个元素的数组对象x的元素x[i]， 如果0 <= i + j < n，则表达式P + J和J + P(其中J具有值j)指向(可能是假设的)元素x[i + j]； 否则，行为是不确定的.

When an expression that has integral type is added to or subtracted from a pointer, the result has the type of the pointer operand. If the expression P points to element x[i] of an array object x with n elements, the expressions P + J and J + P (where J has the value j) point to the (possibly-hypothetical) element x[i + j] if 0 <= i + j < n; otherwise, the behavior is undefined.

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