C_const

https://www.sharetechnote.com/html/C_const.html

你是一个经验丰富的中英文翻译，对我说的话进行中英文互译。你只需要简洁的给我翻译。即使你说的是一个词，你也要翻译。我说的是"const"关键字与变量一起使用。它使变量成为常量值。这意味着用const关键字声明的变量在之后不能被更改。实际上，这个语句’它使变量成为常量值’让很多人感到困惑。当我们说’变量’时，意味着它可以随时更改，但是用const关键字声明的变量不能被更改。这对许多人来说可能很困惑。

我理解这可能让人感到困惑，但就把它当作是这样的。这就是它的实现方式。

Example 01 >

 

Let's take a look at a simple example showing the implication of 'const' keyword.

 

void main ()

{

    int a = 0;

    int b = 0;

 

    a = b;

    b = a;

}

 

 

void main ()

{

    int a = 0;

    const int b = 0; // this means this integer type variable called 'b' should not (will not) changed.

 

    a = b;

    b = a; // this line gives an error '[Error] assignment of read-only variable 'b'

              // this is because you tried to change a value for a 'const variable'.

 

}

 

Example 02 >```

好的，我会尽量简洁地为你翻译。

假设我们有一个值类型变量，那么'const'的行为是非常直接的。然而，如果'const'与指针类型一起使用（例如char *），情况就会变得复杂许多。让我们看看下面的例子。这是C库中著名的strcpy函数。如你所见，_Dest被定义为'char *'，而_Source被定义为'const char *'。这意味着在strcpy()函数内，_Dest的值会被改变，但_Source的值不会改变。考虑到strcpy()的功能，这是有意义的。

```c
char * __cdecl strcpy(char * __restrict__ _Dest,const char * __restrict__ _Source);

现在让我们看下面的例子，看看它是如何工作的。

#include<stdio.h>

#include <string.h>

 

int main()

{

    char dest1[255];

    const char dest2[255];

    //const char dest2[255] = "Any String Here";

    //const char *dest2 = "Any String Here";

 

    char src1[] = "source 2";

    const char *src2 = "source 1";

 

    strcpy(dest1,src1);

    printf("dest1 = %s\n",dest1);

    

    strcpy(dest1,src2);    

    printf("dest1 = %s\n",dest1);

 

    printf("dest2 = %s\n",dest2);

    

    // you would see Compiler Warning as shown below.

   

    strcpy(dest2,src1);  // [Warning] passing argument 1 of 'strcpy' discards 'const' qualifier from pointer target type  

    printf("dest2 = %s\n",dest2);

    

    strcpy(dest2,src2);  // [Warning] passing argument 1 of 'strcpy' discards 'const' qualifier from pointer target type  

    printf("dest2 = %s\n",dest2);

 

    return 0;

}

 

 

Example 03 >

 

Let's create a function in two different way as shown below and see what happens.

 

// cAry here is defined without 'const'.

void toUpper(char cAry[255],int n)

{

    

    int i = 0;

    for( i = 0; i <n; i++)

    {

        if(cAry[i]=='\0') break;

        cAry[i] = toupper(cAry[i]); // the contents of cAry[] is being changed here.

                                     // no problem in this case.

        }   

}

 

 

 

// cAry here is defined with 'const'.

void toUpper(const char cAry[255],int n)

{

    

    int i = 0;

    for( i = 0; i <n; i++)

    {

        if(cAry[i]=='\0') break;

        cAry[i] = toupper(cAry[i]); // the contents of cAry[] is being changed here.

                                     // you would see complier error as follows.

                                     // [Error] assignment of read-only location '*(cAry + (sizetype)...

        }   

}