C++ const 指针赋值给一个指针

const pointer assign to a pointer

提问by Tony The Lion

Why can I not do this:

为什么我不能这样做：

char* p = new char[10];

void SetString(char * const str)
{
    p = str;
}


SetString("Hello");

I have a const pointer to a char, why can I not assign the const pointer to another pointer?

我有一个指向 char 的 const 指针，为什么我不能将 const 指针分配给另一个指针？

It just seems illogical, as by assigning it to another pointer, you are not essentially violating the const-ness of the char pointer. Or are you?

这似乎不合逻辑，因为通过将它分配给另一个指针，您实际上并没有违反 char 指针的常量性。还是你？

EDIT: When I compile this it says "error C2440: '=' : cannot convert from 'char *const *__w64 ' to 'char *'"

编辑：当我编译它时，它说“错误 C2440：'='：无法从'char *const *__w64' 转换为'char *'”

(I'm attempting to understand a concept from a book I'm reading. Just cannot get the code to compile.

（我试图从我正在阅读的书中理解一个概念。只是无法编译代码。

CODE:

代码：

int _tmain(int argc, _TCHAR* argv[])
{

    MyString *strg = new MyString(10);
    strg->SetString("Hello, ");

    MyString *secondstr = new MyString(7);
    secondstr->SetString("Tony");

    strg->concat(*secondstr, *strg);

}

CPP FILE:

CPP文件：

#include "MyStringClass.h"
#include <string.h>
#include "stdafx.h"

#include "MyStringClass.h"

void MyString::concat(MyString& a, MyString& b)
{
    len = a.len + b.len;
    s = new char[len + 1];
    strcpy(s, a.s);
    strcat(s, b.s);
    delete [] s; 

}

void MyString::SetString(char * const str)
{
    s = str;
}

MyString::MyString(int n)
{
    s = new char[n+1];
    s[n+1] = '
#include <string.h>
#include <stdio.h>

class MyString
{
private:
    char* s;
    int len;
public:
    MyString(int n = 80);

    void SetString (char * const str);

    void concat (MyString& a, MyString& b);
};
';
    len = n;
}

HEADER FILE:

头文件：

int * const const_pointer = &some_int_var; // will be always pointing to this var
const_pointer = &some_other_var; // illegal - cannot change the pointer
*const_pointer = 2; // legal, the pointer is a pointer to non-const

回答by adf88

There is difference between constant pointer and pointer to constant. Constant pointer is a pointer (a number - memory address) that cannot be changed - it always point to the same object given via initialization:

常量指针和指向常量的指针是有区别的。常量指针是一个不能改变的指针（一个数字 - 内存地址） - 它总是指向通过初始化给出的同一个对象：

const int * pointer_to_const = &some_int_var; // doesn't have to be always pointing to this var
pointer = &some_other_var; // legal, it's not a constant pointer and we can change it
*pointer = 2; // illegal, pointed value cannot be changed

Pointer to constant is a pointer whose pointed value cannot be changed:

指向常量的指针是一个指针，其指向的值不能改变：

int * pointer;
int * const const_pointer = &var;
const int * pointer_to_const;

/* a */
pointer = const_pointer; // OK, no cast (same type)

/* b */
pointer_to_const = pointer; // OK, casting 'int*' to 'const int*'

/* c */
pointer = pointer_to_const; // Illegal, casting 'const int*' to 'int*'

You can always assign constant to variable i.e. const pointer to non-const pointer (a). You can cast pointer to non-const to a pointer to const (b). But you cannot cast pointer to const to a pointer to non-const (c):

您始终可以将常量分配给变量，即指向非常量指针 (a) 的常量指针。您可以将指向非常量的指针转换为指向 const (b) 的指针。但是您不能将指向 const 的指针转换为指向非常量 (c) 的指针：

"Hello"

[EDIT] Below, this is not standard c++. However, this is common.[/EDIT]
String literal

[编辑] 下面，这不是标准的 C++。但是，这是常见的。[/EDIT]
字符串文字

char *pointer = "Hello"; // Illegal, cannot cast 'const char*' to 'char*'
char * const const_pointer = "Hello"; // Illegal, cannot cast 'const char*' to 'char*'
const char * pointer_to_const = "Hello"; // OK, we can assign a constant to a variable of the same type (and the type is 'const char*')
"Hello" = pointer_to_const; // Illegal cannot re-assign a constant

is converted to constant pointer to const (const char * const):

转换为指向 const ( const char * const) 的常量指针：

##代码##

In above examples the second is your case. You tried to initialize pointer-to-non-constwith a pointer-to-constwhen passing string literal as argument of your function. No matter if these pointers are constants or not, it's matter what do they point to.

在上面的例子中，第二个是你的情况。在将字符串文字作为函数的参数传递时，您尝试使用指向常量的指针初始化指向非常量的指针。不管这些指针是否是常量，它们指向什么很重要。

Summary:
1) If you cast a pointer of some type to a pointer of another type, you cannot cast pointer-to-const to pointer-to-non-const.
2) If you have constant pointer, the same rules applies as to other constants - you can assign a constant to a variable but you cannot assign a variable to a constant (except initializing it).

总结：
1）如果将某种类型的指针转​​换为另一种类型的指针，则不能将指向常量的指针转换为指向非常量的指针。
2) 如果你有常量指针，同样的规则适用于其他常量——你可以将常量分配给变量，但不能将变量分配给常量（初始化除外）。

// EDIT
As GMan pointed out, the C++98 standard (§4.2/2) allows to implicitly cast string literals (which are constant char arrays) to a non-const char pointer. This is because of backward compatibility (in C language there are no constants).

// 编辑
正如 GMan 所指出的，C++98 标准（第 4.2/2 节）允许将字符串文字（它们是常量字符数组）隐式转换为非常量字符指针。这是因为向后兼容（在 C 语言中没有常量）。

Of course such a conversion can lead to mistakes and compilers will violate the rule and show an error. However, GCC in compatibility mode shows only a warning.

当然，这样的转换会导致错误，编译器会违反规则并显示错误。但是，兼容模式下的 GCC 仅显示警告。