引用/指针的无效到底是什么? [英] What exactly is invalidation of reference/pointer?

问题描述

在标准中找不到指针/引用无效的任何定义.我问是因为我刚刚发现C ++ 11禁止对字符串进行写时复制(COW).据我了解，如果应用了COW，则在以下命令之后， p 仍然是有效的指针，而 r 仍然是有效的引用:

  std :: string s("abc");std :: string s2(s);char * p =&(s2 [0]);字符和r = s2 [0];s2 [1] ="B"; 

只是他们不再指向/引用 s2 的第一个字符，而只是指向/引用 s 的第一个字符.

在C ++ 11标准中，据说非恒定的 std :: basic_string :: operator [] 可能不会使对字符串元素的指针/引用(以及迭代器)无效./p>

哪个规则说，如果实施了COW，则上面显示的示例实际上会使 p 和 r 无效?

解决方案

标准中没有对无效"的定义，因为该术语是从英语继承而来的.它的意思是英语:无效的引用/指针不再有效.无法使用.

但是，在某些地方明确指出了此约束.例如，将指针左值转换为右值时(发生在表达式求值过程中):

[conv.lval/2] 否则，如果glvalue引用的对象包含无效的指针值(3.7.4.2、3.7.4.3)，则该行为是实现定义的

我现在找不到引用的用语，但是无论如何，您不能使用对不再存在的引用进行引用是不言而喻的.

I cannot find any definition for invalidation of pointers/references in the Standard. I ask because I just found out that C++11 forbids copy-on-write (COW) for strings. As far as I understand, if COW was applied then p would be still a valid pointer and r a valid reference after the following commands:

std::string s("abc");
std::string s2(s);
char * p = &(s2[0]);
char & r = s2[0];
s2[1] = "B";

Just they would no longer point/refer to the first character of s2, but merely to the first character of s.

In the C++11 Standard, it is said that non-constant std::basic_string::operator[] may not invalidate pointers/references (and also iterators) to string elements.

Which rules say that the above shown example would actually invalidate p and r if COW was implemented?

解决方案

There is no definition for "invalidation" in the standard because the term is inherited from English. It means what it means in English: an invalidated reference/pointer is no longer valid. It cannot be used.

There are, however, places where this constraint is explicitly noted. For example, when converting a pointer lvalue to an rvalue (which happens during evaluation of an expression):

[conv.lval/2] Otherwise, if the object to which the glvalue refers contains an invalid pointer value (3.7.4.2, 3.7.4.3), the behavior is implementation-defined

I can't find the wording for references right now, but it's anyway self-explanatory that you cannot use a reference to something that no longer exists.

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