const 指针到指针在 C 和 C++ 中是什么意思？
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What does a const pointer-to-pointer mean in C and in C++?
提问by Niklas
I know the rule-of-thumb to read declarations right-to-left and I was fairly sure I knew what was going on until a colleague told me that:
我知道从右到左阅读声明的经验法则，我相当确定我知道发生了什么，直到一位同事告诉我：
const MyStructure** ppMyStruct;
means "ppMyStruct is a pointer to a const pointer to a (mutable) MyStructure" (in C++).
表示“ppMyStruct 是指向（可变）MyStructure 的常量指针的指针”（在 C++ 中）。
I would have thought it meant "ppMyStruct is a pointer to a pointer to a const MyStructure". I looked for an answer in the C++ spec, but apparently I'm not very good at that...
我原以为这意味着“ppMyStruct 是一个指向 const MyStructure 的指针”。我在 C++ 规范中寻找答案，但显然我不太擅长......
What does in mean in C++, and does it mean the same thing in C?
in 在 C++ 中是什么意思，它在 C 中是什么意思？
回答by James Hopkin
Your colleague is wrong. That is a (non-const) pointer to a (non-const) pointer to a const MyStructure. In both C and C++.
你的同事错了。那是一个（非常量）指针，指向一个（非常量）指向一个 const MyStructure 的指针。在 C 和 C++ 中。
回答by flolo
In such cases the tool cdecl (or c++decl) can be helpfull:
在这种情况下，工具 cdecl（或 c++decl）可能会有所帮助：
[flolo@titan ~]$ cdecl explain "const struct s** ppMyStruct"
declare ppMyStruct as pointer to pointer to const struct s
回答by efotinis
You were right in your interpretation. Here's another way to look at it:
你的解释是对的。这是查看它的另一种方式：
const MyStructure * *ppMyStruct; // ptr --> ptr --> const MyStructure
MyStructure *const *ppMyStruct; // ptr --> const ptr --> MyStructure
MyStructure * *const ppMyStruct; // const ptr --> ptr --> MyStructure
These are all the alternatives of a pointer-to-pointer with one const qualifier. The right-to-left rule can be used to decipher the declarations (at least in C++; I'm no C expert).
这些都是带有一个 const 限定符的指针到指针的所有替代方案。从右到左的规则可用于破译声明（至少在 C++ 中；我不是 C 专家）。
回答by csl
Your colleague is wrong, and it's the same for C and C++. Try the following:
你的同事错了，C和C++都一样。请尝试以下操作：
typedef struct foo_t {
int i;
} foo_t;
int main()
{
foo_t f = {123};
const foo_t *p = &f;
const foo_t **pp = &p;
printf("f.i = %d\n", (*pp)->i);
(*pp)->i = 888; // error
p->i = 999; // error
}
Visual C++ 2008 gives the following errors for the last two lines:
Visual C++ 2008 给出最后两行的以下错误：
error C2166: l-value specifies const object
error C2166: l-value specifies const object
GCC 4 says:
GCC 4 说：
error: assignment of read-only location '**pp'
error: assignment of read-only location '*p'
G++ 4 says:
G++ 4 说：
error: assignment of data-member 'foo_t::i' in read-only structure
error: assignment of data-member 'foo_t::i' in read-only structure
回答by xtofl
Youare right.
你是对的。
Another answeralready pointed to the "Clockwise Spiral Rule". I liked that one very much - a little elaborate, though.
回答by MSalters
As a corollary to the other comments, don't put 'const' first. It really belongs after the type. That would have clarified the meaning immediately, just read it RTL as usual:
作为其他评论的必然结果，不要将“const”放在首位。它确实属于类型之后。那会立即澄清含义，只需像往常一样阅读RTL：
MyStructure const** ppMyStruct;
回答by MSalters
void Foo( int * ptr,
int const * ptrToConst,
int * const constPtr,
int const * const constPtrToConst )
{
*ptr = 0; // OK: modifies the pointee
ptr = 0; // OK: modifies the pointer
*ptrToConst = 0; // Error! Cannot modify the pointee
ptrToConst = 0; // OK: modifies the pointer
*constPtr = 0; // OK: modifies the pointee
constPtr = 0; // Error! Cannot modify the pointer
*constPtrToConst = 0; // Error! Cannot modify the pointee
constPtrToConst = 0; // Error! Cannot modify the pointer
}
