c+const_如何在C ++中使用const？初学者指南

た入场券 2023-02-28 08:40 21阅读 0赞

c+const

Hey, folks! In this tutorial, we will be focusing on **const in C++**.

嘿伙计！ 在本教程中，我们将重点介绍**C ++中的const** 。

--------------------

## C ++中const的基础 **(****Basics of const in C++****)** ##

C++ has a huge set of keywords that help us give special functionality to the data variables on a whole.

C ++有大量的关键字，可以帮助我们整体上为数据变量提供特殊功能。

`const` in C++ makes a particular data entity, a constant one i.e. during the scope of the entire program, the value of the const variable remains the same.

C ++中的`const`使特定的数据实体成为一个常数，即在整个程序范围内，const变量的值保持不变。

In the actual scenarios, the const keyword is very essential to lock a particular value as static and constant throughout its use.

在实际方案中，const关键字对于在使用过程中将特定值锁定为静态和常量非常重要。

**Let us take an example.**

**让我们举个例子。**

For an automatic emailing system, wherein the owner of a company wants to send a mail at the last date of every month to his employees, the email address of the owner would stay the same. So we can declare a variable to save the owner’s email address and make it constant using the const keyword.

对于自动电子邮件系统，其中公司的所有者希望在每个月的最后一天向其员工发送邮件，所有者的电子邮件地址将保持不变。 因此，我们可以声明一个变量来保存所有者的电子邮件地址，并使用const关键字使其保持不变。

Thus, we would actually save time and reduce the complexity of the system.

因此，我们实际上将节省时间并降低系统的复杂性。

--------------------

## C ++中const的变体 **(****Variants of const in C++****)** ##

The const keyword in C++ can be used alongside different data entities of programming such as:

C ++中的const关键字可以与编程的不同数据实体一起使用，例如：

*  **Data variables**
    
    **资料变数**
 *  **Function arguments**
    
    **函数参数**
 *  **[Pointers][]**
    
    **[指针][Pointers]**
 *  **Objects of a class**
    
    **类的对象**
 *  **Class data members**
    
    **类数据成员**

--------------------

### 1.数据变量为const **(****1. Data variables as const****)** ###

When a data variable is declared as const, the value equated to the variable is treated as **constant** and is **immutable**.

当数据变量声明为const时，等于该变量的值被视为**常量**并且是**不可变的** 。

**Syntax:**

**句法：**

const data-type variable = value;

**Example:**

**例：**

#include<iostream>
    using namespace std;
    int main()
    {
        const int A=10,B=20;
        cout<<"Addition: "<<(A+B);
      
    }

**Output:**

**输出：**

Addition: 30

**Error and Exceptions:**

**错误和异常：**

If we try to change or manipulate the value of a const data variable, the compiler would throw an error saying that we cannot assign values to a read-only variable.

如果尝试更改或操纵const数据变量的值，则编译器将引发错误，表明我们无法将值分配给只读变量。

#include<iostream>
    using namespace std;
    int main()
    {
        const int A=10;
        A+=10;
        cout<<A;
      
    }

**Output:**

**输出：**

main.cpp: In function ‘int main()’:
    main.cpp:6:8: error: assignment of read-only variable ‘A’
         A+=10;

--------------------

### 2.函数参数在C ++中为const **(****2. Function arguments as const in C++****)** ###

We can even assign an argument to a function/method as constant, thus restricting all the changes to the function argument.

我们甚至可以将一个参数分配给函数/方法作为常量，从而将所有更改限制在该函数参数上。

**Syntax:**

**句法：**

data-type function(const data-type variable)
    {
      //body
    }

**Example:**

**例：**

#include<iostream>
    using namespace std;
    
    int add(const int x)
    {   
        int res=0;
        int error=20;
        res = x+error;
        return res;
    }
    int main()
    {
      int ans = add(50);
      cout<<"Addition:"<<ans;
    }

**Output:**

**输出：**

Addition:70

**Error and Exceptions:**

**错误和异常：**

As seen below, if we try to manipulate the value of a const function argument, an error would be raised by the compiler.

如下所示，如果我们尝试操纵const函数参数的值，则编译器将引发错误。

#include<iostream>
    using namespace std;
    
    int add(const int x)
    {   x=x+100;
        return x;
    }
    int main()
    {
      int ans = add(50);
      cout<<"Addition:"<<ans;
    }

In the above example, we are trying to change the value of the const function argument “x”, resulting in a compiler error.

在上面的示例中，我们试图更改const函数参数“ x”的值，从而导致编译器错误。

**Output:**

**输出：**

main.cpp: In function ‘int add(int)’:
    main.cpp:5:9: error: assignment of read-only parameter ‘x’
     {   x=x+100;

--------------------

### 3.指针为const **(****3. Pointer as const****)** ###

[Pointer variables][Pointers] can be declared as constant too using the below syntax:

[指针变量][Pointers]也可以使用以下语法声明为常量：

data-type const *variabl;

**Example**:

**范例** ：

#include<iostream>
    using namespace std;
    int main()
    {   int age = 21;
        int const *point = & age;
        cout<<"Pointer value: "<<*point;
      
    }

**Output:**

**输出：**

Pointer value: 21

**Error and Exceptions:**

**错误和异常：**

If we try to manipulate the value of a const pointer, that leads to a compilation error.

如果我们尝试操纵const指针的值，则会导致编译错误。

#include<iostream>
    using namespace std;
    int main()
    {   int age = 21;
        int const *point = & age;
        *point=*point+10;
        cout<<"Pointer value: "<<*point;
      
    }

**Output:**

**输出：**

main.cpp: In function ‘int main()’:
    main.cpp:6:19: error: assignment of read-only location ‘* point’
         *point=*point+10;

--------------------

### 4.类的数据变量为const **(****4. Data variables of a class as const****)** ###

The Class variables can also be treated as const to make the value of the data variable as constant and immutable.

也可以将Class变量视为const，以使数据变量的值恒定且不变。

**Syntax:**

**句法：**

const data-type variable = value;

**Example:**

**例：**

#include<iostream>
    using namespace std;
    class Demo
    {
        public:
        const int X = 10;
        
    };
    int main()
    {
       Demo d;
       cout<<d.X;
      
    }

**Output:**

**输出：**

**Error and Exceptions:**

**错误和异常：**

#include<iostream>
    using namespace std;
    class Demo
    {
        public:
       const int X = 10;
        
    };
    int main()
    {
       Demo d;
       d.X=100;
       cout<<d.X;
      
    }

**Output:**

**输出：**

main.cpp: In function ‘int main()’:
    main.cpp:12:8: error: assignment of read-only member ‘Demo::X’
        d.X=100;

--------------------

### 5.类的对象为const **(****5. Object of a class as const****)** ###

Even the objects of a class can work with the const keyword. As soon as we declare the object of a class as constant, the data variables of the same becomes **immutable** i.e. **the values of the data variables that belongs to the class pointed by the object cannot be changed**.

甚至类的对象也可以使用const关键字。 一旦我们将一个类的对象声明为常量，该类的数据变量就变得**不可变，**即**，属于该对象所指向的类的数据变量的值无法更改** 。

**Syntax:**

**句法：**

const Class object;

**Example:**

**例：**

#include<iostream>
    using namespace std;
    class Demo
    {
        public:
        int X = 10;
    
    };
    int main()
    {
       const Demo d;
       cout<<d.X;
      
    }

**Output:**

**输出：**

**Error and Exceptions**:

**错误和异常** ：

#include<iostream>
    using namespace std;
    class Demo
    {
        public:
        int X = 10;
    
    };
    int main()
    {
       const Demo d;
       d.X=100;
       cout<<d.X;
      
    }

**Output:**

**输出：**

main.cpp: In function ‘int main()’:
    main.cpp:12:8: error: assignment of member ‘Demo::X’ in read-only object
        d.X=100;

--------------------

## 结论 **(****Conclusion****)** ##

Thus, in this article, we have unleashed the working of const keyword alongside data variables, pointers, Class objects, etc.

因此，在本文中，我们释放了const关键字以及数据变量，指针，Class对象等的工作。

--------------------

## 参考资料 **(****References****)** ##

[C++ const keyword — Documentation][C_ const keyword _ Documentation]

[C ++ const关键字—文档][C_ const keyword _ Documentation]

> 翻译自: [https://www.journaldev.com/39812/const-in-c-plus-plus][https_www.journaldev.com_39812_const-in-c-plus-plus]

c+const

[Pointers]: https://www.journaldev.com/30481/pointers-in-c-and-c-plus-plus
[C_ const keyword _ Documentation]: https://docs.microsoft.com/en-us/cpp/cpp/const-cpp?view=vs-2019
[https_www.journaldev.com_39812_const-in-c-plus-plus]: https://www.journaldev.com/39812/const-in-c-plus-plus