const char*和char const* - 它们是一样的吗？

Question

根据我的理解,const修饰符应该从右到左阅读.从那以后,我明白了:

const char*

是一个指针,其char元素不能被修改,但指针本身可以,和

char const*

是一个指向mutable字符的常量指针.

但是我得到以下代码的以下错误:

const char* x = new char[20];
x = new char[30];   //this works, as expected
x[0] = 'a';         //gives an error as expected

char const* y = new char[20];
y = new char[20];   //this works, although the pointer should be const (right?)
y[0] = 'a';         //this doesn't although I expect it to work

那么是哪一个呢？我的理解还是我的编译器(VS 2005)错了？

Answer 1

实际上,根据标准,const直接将元素修改为左侧.const在声明开头使用只是一个方便的心理捷径.所以以下两个陈述是等价的:

char const * pointerToConstantContent1;
const char * pointerToConstantContent2;

为了确保指针本身不被修改,const应该放在星号后面:

char * const constantPointerToMutableContent;

要保护指针和指向的内容,请使用两个consts.

char const * const constantPointerToConstantContent;

我个人采用总是将const放在我打算不修改的部分之后,即使指针是我希望保持不变的部分,我也保持一致性.

Answer 2

它的工作原因是两者都相同.可能你对此感到困惑,

const char*  // both are same
char const*

和

char* const  // unmutable pointer to "char"

和

const char* const  // unmutable pointer to "const char"

[要记住这一点,这是一个简单的规则,'*'首先影响其整个LHS ]

Answer 3

那是因为规则是:

规则:const左边绑定,除非左边没有任何东西,然后绑定吧:)

所以,看看这些:

(const --->> char)*
(char <<--- const)*

两者一样!哦,--->>并且<<---都没有的运营商,他们只是展示一下const结合到.

Answer 4

(来自2个简单变量初始化问题)

一个非常好的经验法则const:

从右到左阅读声明.

(参见Vandevoorde/Josutiss"C++模板:完整指南")

例如:

int const x; // x is a constant int
const int x; // x is an int which is const

// easy. the rule becomes really useful in the following:
int const * const p; // p is const-pointer to const-int
int const &p;        // p is a reference to const-int
int * const * p;     // p is a pointer to const-pointer to int.

自从我遵循这个经验法则之后,我再也没有误解过这样的声明.

(:sisab retcarahc-rep a no ton,sisab nekot-rep a tfel-ot-thgir naem I hguohT:tidE

Answer 5

以下是我总是试图解释的方式:

char *p

     |_____ start from the asterisk. The above declaration is read as: "content of `p` is a `char`".

char * const p

     |_____ again start from the asterisk. "content of constant (since we have the `const` 
            modifier in the front) `p` is a `char`".

char const *p

           |_____ again start from the asterisk. "content of `p` is a constant `char`".

希望能帮助到你!