c中的普通指针和const指针的区别

Actually i don't know what is the difference between a normal pointer and a const pointer if i use the below code it will works fine . but when i change int *ptr=# to int *const ptr = &var1; then it will not work. can anybody explain what is the difference between a normal pointer and a const pointer ?

实际上我不知道普通指针和const指针的区别是什么如果我使用下面的代码它会很好地工作。但是当我改变int *ptr=&num时;至*const ptr = &var1;那就没用了。有人能解释一下普通指针和常量指针之间的区别吗?

 int main(void)
    {
        int num = 20;
     int  *ptr = &num ; // if i change to   `int *const ptr = &var1;` then it shows some error

     *ptr = 20 ;              // Valid 
     ptr ++ ;                 // valid

        return 0;
    }

5 个解决方案

#1

int* const ptr = &num ;

Will create a constant pointer to an int. The data it points to can be changed, but the pointer it self cannot.

将创建一个指向int的常量指针。它指向的数据可以更改，但是它自己不能。

You cannot change the pointer:

不能更改指针:

ptr++ ;

But you can change the data:

但你可以改变数据:

*ptr = 1234 ;

#2

We can do following operations on constant pointers

我们可以对常量指针进行后续操作

• Assigning value at address
• 价值分配地址
• Printing value or address
• 打印值或地址
• Assigning Address at the time of declaration.
• 在声明时分配地址。

We Can't to following operation on constant pointers

我们不能对常量指针进行后续操作

• Adding Integers to Constant pointers.
• 向常量指针添加整数。
• Subtracting integers to constant pointers.
• 将整数减为常数指针。
• Any Operation that can change address of pointer.
• 任何可以改变指针地址的操作。

So, Here in your question..

那么，在你的问题中。

If you declare

如果你声明

int* const ptr = &num ; // this is ok

next line

下一行

*ptr = 20 ;  // Assigning value at address this is ok

Now,

现在,

ptr ++ ;  // you can not change the value // Error!

Hope it helps!

希望它可以帮助!

#3

This:

这样的:

int* const ptr = &num ;

will create a constant pointer to an integer. You can use it to modify the integer's value but you can't change where the pointer points, thus ptr++ ; is invalid.

将创建一个指向整数的常量指针。你可以用它来修改整数的值但是你不能改变指针的位置，因此ptr++;是无效的。

The const keyword is usually applied to its left symbol, e.g.

const关键字通常用于它的左符号，例如。

int * const ptr; // A constant pointer (*)
int const * ptr; // A pointer to a constant integer
int const * const ptr; // A constant pointer to a constant integer
const int *ptr; // Shorthand for pointer to a constant integer (equivalent to int const * ptr;)

const pointers are useful when you want to pass a fixed memory location around and you want to make sure that nobody will modify the pointer's pointed address.

当您希望传递一个固定的内存位置，并且您希望确保没有人会修改指针的指定地址时，const指针是非常有用的。

#4

in c, const is a type qualifier. use of const in some variable definition means, the variable will not get modified (will be treated as read-only)during the entire lifetime of the program.

在c语言中，const是一个类型限定符。在某些变量定义中使用const意味着，在程序的整个生命周期中，变量不会被修改(将被视为只读)。

Usually, when defining a variable / data type with const, the pratice is to initialize it with required value, as normally, the value it holds cannot be modified at a later part.

通常，在使用const定义变量/数据类型时，实践是使用必需的值初始化它，通常，它所包含的值不能在后面的部分修改。

For example:

例如:

const int a = 10;

const int a = 10;

means, the integer a will hold the value 10 and it cannot be changed. at a later part,

表示，整数a将保存值10，不能更改。在稍后的部分,

a = 20;

will produce error.

会产生错误。

So, in your case

所以,在你的情况中

int *const ptr = &var;

here, ptr will always hold the address of var and it cannot be changed, i.e., we cannot write

在这里，ptr将始终保存var的地址，并且不能更改，即。我们不能写

ptr = &num2; // where num2 is another int, declared like int num2;

it will show compile-time error like :

它将显示编译时错误，如:

error:assignment of read-only variable "*ptr".

错误:为只读变量赋值“*ptr”。

You can find a nice and handy description here.

你可以在这里找到一个很好的描述。

#5

int* const pointer = &x ;

it create a constant pointer to an int. The data it points to can be changed, but the pointer it self cannot be changed

它创建一个指向int的常量指针，它指向的数据可以被改变，但是指针本身不能更改。

You cannot change the pointer:

不能更改指针:

pointer++ ;

here you can change the data:

在这里，您可以更改数据:

*pointer=1 ;