给定 int p1 和 const intp2，p1 == p2 是合法的吗？

Question

给定 int p1 和 const intp2，p1 == p2 是合法的吗？

34

给定以下函数：

void g(int **p1, const int**p2)
{
   if (p1 == p2) { }  
}

clang（回到版本3.0）会产生这个警告(现场查看):

warning: comparison of distinct pointer types ('int **' and 'const int **')
uses non-standard composite pointer type 'const int *const *' 
[-Wcompare-distinct-pointer-types]
  if (p1 == p2) { }
      ~~ ^ ~~

使用-pedantic-errors标志将其转换为错误。根据标准，gcc（回到4.3.6）和Visual Studio（2013年）都不会产生警告，比较如下：

使用-pedantic-errors标志会使其成为错误。根据标准，gcc（返回至4.3.6版本）和Visual Studio（2013年版）都不会产生警告，比较的内容是：

p1 == p2

格式正确？

更一般地，如果两个多级指针除了第一级以外的cv-qualifications不同，使用等号运算符或关系运算符进行比较是否格式正确？

- Shafik Yaghmour

非常有用的知识，已点赞。希望 MSVC 和 G++ 也能跟进实现 C++14。 - Cheers and hth. - Alf

1

如果gcc和VS允许此代码而没有警告或错误，则它们已经遵循了C++14。clang需要修复。@Cheersandhth.-Alf - M.M

@MattMcNabb：谢谢。我的脑海里有一些二进制切换。 - Cheers and hth. - Alf

@Cheersandhth.-Alf在我的答案中添加了一个来自五年前的clang提交，表明这是被clang、gcc和EDG视为扩展，我认为VS也是如此。请注意，在会议上，它被接受并指出这一变化只是将现有实践编码化。 - Shafik Yaghmour

1个回答

网页内容由stack overflow 提供, 点击上面的

可以查看英文原文，
原文链接

- Shafik Yaghmour · Accepted Answer

在C++14之前，这种情况是不规范的，更一般的带有一些例外情况也是不规范的。这在缺陷报告1512：指针比较与限定转换中有所涵盖，其中提到：

According to 5.9 [expr.rel] paragraph 2, describing pointer comparisons,

Pointer conversions (4.10 [conv.ptr]) and qualification conversions (4.4 [conv.qual]) are performed on pointer operands (or on a pointer operand and a null pointer constant, or on two null pointer constants, at least one of which is non-integral) to bring them to their composite pointer type.

This would appear to make the following example ill-formed,
bool foo(int** x, const int** y) {
   return x < y;  // valid ?
}
because int** cannot be converted to const int**, according to the rules of 4.4 [conv.qual] paragraph 4. This seems too strict for pointer comparison, and current implementations accept the example.

这份缺陷报告指出，尽管它格式不正确，但实现仍接受这样的比较。这个clang commit表明它被视为扩展，并且表明gcc和EDG也将其视为扩展，据推测Visual Studio也是如此。

这在标准中已得到解决，N3624: Core Issue 1512: Pointer comparison vs qualification conversions提到：

This paper presents the modifications to the Working Draft necessary to resolve core issues 583 and 1512. In particular, it makes

[...]

and
void g(int **p1, const int**p2)
{
   if (p1 == p2) { ... }
}
well-formed.

请注意，在会议中已经接受的情况下，已经注意到这只是将现有做法编码。

在标准的其他更改中，该段落被添加到第5节的结尾[expr]，其中包括新术语cv-combined type。

The cv-combined type of two types T1 and T2 is a type T3 similar to T1 whose cv-qualification signature (4.4) is:

for every j > 0, cv3,j is the union of cv1,j and cv2,j ;

if the resulting cv3,j is different from cv1,j or cv2,j , then const is added to every cv3,k for 0 < k < j.

[ Note: Given similar types T1 and T2, this construction ensures that both can be converted to T3. —end note ] The composite pointer type of two operands p1 and p2 having types T1 and T2, respectively, where at least one is a pointer or pointer to member type or std::nullptr_t, is:

if both p1 and p2 are null pointer constants, std::nullptr_t;

if either p1 or p2 is a null pointer constant, T2 or T1, respectively;

if T1 or T2 is “pointer to cv1 void” and the other type is “pointer to cv2 T”, “pointer to cv12 void”, where cv12 is the union of cv1 and cv2 ;

if T1 is “pointer to cv1 C1” and T2 is “pointer to cv2 C2”, where C1 is reference-related to C2 or C2 is reference-related to C1 (8.5.3), the cv-combined type of T1 and T2 or the cv-combined type of T2 and T1, respectively;

if T1 is “pointer to member of C1 of type cv1 U1” and T2 is “pointer to member of C2 of type cv2 U2” where C1 is reference-related to C2 or C2 is reference-related to C1 (8.5.3), the cv-combined type of T2 and T1 or the cv-combined type of T1 and T2, respectively;

if T1 and T2 are similar multi-level mixed pointer and pointer to member types (4.4), the cv-combined type of T1 and T2;

otherwise, a program that necessitates the determination of a composite pointer type is ill-formed.

[ Example:
    typedef void *p;
    typedef const int *q;
    typedef int **pi;
    typedef const int **pci;
The composite pointer type of p and q is “pointer to const void”; the composite pointer type of pi and pci is “pointer to const pointer to const int”. —end example ]

给定 int **p1 和 const int**p2，p1 == p2 是合法的吗？

给定 int p1 和 const intp2，p1 == p2 是合法的吗？