Ç - 这是什么2常量是什么意思？ [英] c - what does this 2 const mean?

问题描述

code：

const char * const key;

有上述指针2常量，我看到了这样的事情还是第一次。

There are 2 const in above pointer, I saw things like this the first time.

我认识的第一个常量使得由指针指向永恒不变的价值，
但没有第二个常量使指针本身不可变的？

I know the first const makes the value pointed by the pointer immutable, but did the second const make the pointer itself immutable?

任何人都可以帮助解释一下吗？

Anyone can help to explain this?

@Update：

和我写的证明，答案是正确的程序。

And I wrote a program that proved the answer is correct.

#include <stdio.h>

void testNoConstPoiner() {
    int i = 10;

    int *pi = &i;
    (*pi)++;
    printf("%d\n", i);
}

void testPreConstPoinerChangePointedValue() {
    int i = 10;

    const int *pi = &i;

    // this line will compile error
    // (*pi)++;
    printf("%d\n", *pi);
}


void testPreConstPoinerChangePointer() {
    int i = 10;
    int j = 20;

    const int *pi = &i;
    pi = &j;
    printf("%d\n", *pi);
}

void testAfterConstPoinerChangePointedValue() {
    int i = 10;

    int * const pi = &i;
    (*pi)++;
    printf("%d\n", *pi);
}

void testAfterConstPoinerChangePointer() {
    int i = 10;
    int j = 20;

    int * const pi = &i;
    // this line will compile error
    // pi = &j
    printf("%d\n", *pi);
}

void testDoublePoiner() {
    int i = 10;
    int j = 20;

    const int * const pi = &i;
    // both of following 2 lines will compile error
    // (*pi)++;
    // pi = &j
    printf("%d\n", *pi);
}

int main(int argc, char * argv[]) {
    testNoConstPoiner();

    testPreConstPoinerChangePointedValue();
    testPreConstPoinerChangePointer();

    testAfterConstPoinerChangePointedValue();
    testAfterConstPoinerChangePointer();

    testDoublePoiner();
}

在功能3取消注释行，将得到编译错误与技巧。

Uncomment lines in 3 of the functions, will get compile error with tips.

推荐答案

首先常量是为了告诉你不能改变 *键，键[I] 等

First const tells you can not change *key, key[i] etc

以下行的无效

*key = 'a';
*(key + 2) = 'b';
key[i] = 'c';

二常量告诉你不能改变键

以下行的无效

key = newkey;
++key;

另外，请查阅如何读

添加更多的细节。

1. 为const char *键：你可以改变的关键，但不能更改键指向的字符


2. char * const的关键：你不能改变的关键，但可以通过按键
指出的字符

3. 为const char * const的关键：您不能更改密钥以及指针字符

1. const char *key: you can change key but can not change the chars pointed by key.
2. char *const key: You can not change key but can the chars pointed by key
3. const char *const key: You can not change the key as well as the pointer chars.

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