1. Common references

1. Concept description

In the previous [C++] Detailed Explanation of C++ References ① ~ ⑦ blogs, all the explanations are ordinary references, that is, assigning ordinary variables to references. The process is as follows: first define the ordinary variable a, and then define the reference b of the existing variable a;

The reference b here is a normal reference;

// 定义变量 a
int a = 10;

// 定义变量 a 的引用 b
int& b = a;

2. Code example - common reference

Common reference code example:

// 包含 C++ 头文件
#include "iostream"

// 使用 std 标准命名空间
//		该命名空间中 , 定义了很多标准定义
using namespace std;

// 导入 C 头文件
#include <stdio.h>

int main()
{
    
    
	// I . 普通引用
	// 定义变量 a
	int a = 10;

	// 定义变量 a 的 普通引用 b
	int& b = a;

	// 打印 普通引用 b 的值
	printf("b = %d\n", b);

	// 修改普通引用 b 的值
	b = 20;

	// 打印 普通引用 b 的值
	printf("b = %d\n", b);


	// 控制台暂停 , 按任意键继续向后执行
	system("pause");
    return 0;
}

Results of the :

b = 10
b = 20
Press any key to continue . . .

insert image description here

2. Constant reference

1. The introduction of constant references

The opposite of a variable reference is a constant reference;

By ordinary reference, the value of the variable can be modified;
Constant references are not modifiable. Declare a constant reference of an ordinary variable, and then no longer use the variable, but use the constant reference instead, which is equivalent to converting the variable into a constant;

Constant references can convert variable references into constants; that is, variables cannot be modified through constant references;

If you modify the value referenced by a constant, an error will be reported at compile time:

error C3892: “b”: 不能给常量赋值

2. Constant reference concept and syntax

In the C++ language, "constant reference", the English name is Const Reference, is a type of reference, and the other kind of reference is "common reference";

Use "constant reference" to pass the value of a variable to the constant reference formal parameter of a function, which can ensure that the variable value will not be modified inside the function;

"Constant reference" is often used in the following scenarios:

as a function parameter
as function return value
define object members

Constant reference syntax:

const T& variable_name = value;

T is the type name of the constant reference , such as: int , string ;
variable_name is the name of the constant reference , calling the constant reference, the value of the value variable cannot be modified;
value is the referenced variable, calling this variable can modify the value of the variable;

2. Code example - constant reference cannot be modified

In the following function, the ordinary variable a is declared first,

Then define the constant reference b of the variable a,

Constant reference b is essentially a constant, and the value of variable a can be obtained through constant reference b, but the value of variable a cannot be modified;

If you forcibly modify the value of variable a through constant reference b, an error will be reported:

error C3892: “b”: 不能给常量赋值

Error code example:

// 包含 C++ 头文件
#include "iostream"

// 使用 std 标准命名空间
//		该命名空间中 , 定义了很多标准定义
using namespace std;

// 导入 C 头文件
#include <stdio.h>

int main()
{
    
    
	// II . 常量引用
	// 定义变量 a
	int a = 10;

	// 定义变量 a 的 常量引用 b
	const int& b = a;

	// 打印 常量引用 b 的值
	printf("b = %d\n", b);

	// 修改 常量引用 b 的值
	// 报错 : error C3892: “b”: 不能给常量赋值
	b = 20;


	// 控制台暂停 , 按任意键继续向后执行
	system("pause");
    return 0;
}