c语言 const常量_C编程中的常量（const）

我就是我 2023-03-05 09:55 13阅读 0赞

c语言 const常量

***const*** is a keyword in C language, it is also known as [type qualifier][] (which is to change the property of a variable). ***const*** is used to define a constant whose value may not be changed during the program execution. It prevents the accidental changes of the variable.

***const***是C语言中的关键字，也称为[类型限定符][type qualifier] (用于更改变量的属性)。 ***const***用于定义一个常量，其值在程序执行期间不得更改。 这样可以防止变量的意外更改。

Consider these two definitions,

考虑这两个定义，

int value1 = 10;
    const int value2 = 20;

Here, *value1* is an integer variable, the value of *value1* can be changed any time during the run time. But, the *value2* is an integer constant, and the value of *value2* cannot be changed during the run time.

这里， *value1*是一个整数变量，在运行期间可以随时更改*value1*的值。 但是， *value2*是一个整数常量，并且在运行期间无法更改*value2*的值。

Here, *value1* is an integer variable while *value2* is an integer constant.

此处， *value1*是整数变量，而*value2*是整数常量。

## 定义一个常数 **(**Defining a constant**)** ##

The ***const*** keyword is used to define a constant, include the keyword ***const*** before the data type or after the data type, both are valid.

***const***关键字用于定义常量，在数据类型之前或之后的关键字***const***都有效。

const data_type constant_name = value;
    or 
    data_type const constant_name = value;

**Does a constant occupy memory?**

**常量会占用内存吗？**

**Yes**, a **constant** always occupies memory at compile time. In the above statements, value2 will take *sizeof(int)* bytes (that maybe 2, 4, or 8 according to the system architecture) in the memory.

**是的** ，一个**常量**总是在编译时占用内存。 在上面的语句中，value2将在内存中占用*sizeof(int)*个字节(根据系统体系结构可能是2、4或8个字节)。

## C程序演示常量示例 **(**C program to demonstrate the example of constants**)** ##

#include <stdio.h>
    
    int main()
    {
        const int a = 10; //integer constant
        const float b = 12.3f; // float constant
        const char c = 'X'; // character constant
        const char str[] = "Hello, world!"; // string constant
    
        // printing the values
        printf("a = %d\n", a);
        printf("b = %f\n", b);
        printf("c = %c\n", c);
        printf("str = %s\n", str);
    
        return 0;
    }

**Output:**

**输出：**

a = 10
    b = 12.300000
    c = X
    str = Hello, world!

**What does happen, if we try to change the value of a constant?**

**如果我们尝试更改常量的值，会发生什么？**

If we try to change the value of a constant, the compiler produces an error that constant is read-only.

如果我们尝试更改常数的值，则编译器会产生一个错误，指出该常数是只读的。

**Let's consider this example,**

**让我们考虑这个例子，**

#include <stdio.h>
    
    int main()
    {
        const int a = 10; //integer constant
    
        // printing the value
        printf("a = %d\n", a);
    
        // changing the values
        a = 20;
        // again, printing the value
        printf("a = %d\n", a);
    
        return 0;
    }

**Output:**

**输出：**

main.c: In function ‘main’:
    main.c:11:7: error: assignment of read-only variable ‘a’
         a = 20;
           ^

See the output – *a* is an integer constant here, and when we try to change it, the error is there.

看到输出- *一个*是一个整型常量这里，当我们试图改变它，错误是存在的。

> 翻译自: [https://www.includehelp.com/c/const-in-c-programming.aspx][https_www.includehelp.com_c_const-in-c-programming.aspx]

c语言 const常量

[type qualifier]: https://www.includehelp.com/c/type-qualifiers-in-c-language.aspx
[https_www.includehelp.com_c_const-in-c-programming.aspx]: https://www.includehelp.com/c/const-in-c-programming.aspx