为什么我可以更改此const NSMutableSring？(Why can I change this const NSMutableSring?)

为什么我可以更改此const NSMutableSring？(Why can I change this const NSMutableSring?)

我有这个示例代码：

const NSMutableString *const foobar = [[NSMutableString alloc] initWithFormat:@"Hello"]; [foobar appendString:@" World"]; NSLog(@"String: %@", foobar);

它输出：

字符串：Hello World

我的变量不应该是const （以及指针）吗？ 因此我不能修改它。

C ++的行为与我期望的一样

int main() { const std::string *const s = new std::string("hello"); s->append(" world"); std::cout << s << std::endl; }

输出：

$ g++ test.c++ test.c++: In function ‘int main()’: test.c++:7:20: error: passing ‘const string {aka const std::basic_string<char>}’ as ‘this’ argument of ‘std::basic_string<_CharT, _Traits, _Alloc>& std::basic_string<_CharT, _Traits, _Alloc>::append(const _CharT*) [with _CharT = char; _Traits = std::char_traits<char>; _Alloc = std::allocator<char>]’ discards qualifiers [-fpermissive] s->append(" world");

我知道我不应该使用const和mutable，但这并没有改变事实，即mutable应该是const 。

I have this sample code:

const NSMutableString *const foobar = [[NSMutableString alloc] initWithFormat:@"Hello"]; [foobar appendString:@" World"]; NSLog(@"String: %@", foobar);

and it outputs:

String: Hello World

Shouldn't my variable be const (as well as the pointer)? Therefore I should not be able to modify it.

C++ behaves the way I expect

int main() { const std::string *const s = new std::string("hello"); s->append(" world"); std::cout << s << std::endl; }

output:

$ g++ test.c++ test.c++: In function ‘int main()’: test.c++:7:20: error: passing ‘const string {aka const std::basic_string<char>}’ as ‘this’ argument of ‘std::basic_string<_CharT, _Traits, _Alloc>& std::basic_string<_CharT, _Traits, _Alloc>::append(const _CharT*) [with _CharT = char; _Traits = std::char_traits<char>; _Alloc = std::allocator<char>]’ discards qualifiers [-fpermissive] s->append(" world");

I know I shouldn't use const with mutable, but that doesn't change the fact, that the mutable should be const.

最满意答案

您的观察是正确的，在T是Objective-C类的情况下， 指向 常量T的类型指针会自动更改为指向T的指针 。

这在技术上不是偏离C（++）中const的语义，因为C（++）中没有Objective-C类类型，因此它们不被C（++）标准所涵盖。

Objective-C本身没有任何形式的描述，足够标准。 Objective-C 2.0编程语言 ，2009，没有提到常量语义，也很久以来作为参考被遗弃并且没有被替换。

换句话说，我无法指引您使用任何明确的文档，该文档声明const默默地省略了Objective-C类类型，但这就是编译器的行为方式。

如果你想要“常量”对象，你必须遵循Objective-C可变/不可变类模式。

Your observation is correct, the type pointer to a constant T is silently changed to pointer to a T in the cases where T is an Objective-C class.

This isn't technically a deviation from the semantics of const in C(++) as there are no Objective-C class types in C(++) and they are therefore not covered by the C(++) Standards.

Objective-C itself is not defined by any formal description, well enough a Standard. The Objective-C 2.0 Programming Language, 2009, makes no mention of constant semantics, and is also long since abandoned as a reference and has not been replaced.

In other words I cannot direct you to any definitive document that states that const is silently elided for Objective-C class types, but that is how the compilers behave.

If you want "constant" objects you have to follow the Objective-C mutable/immutable class pattern.