What is the usage of typedef

The usage of typedef is to create a new alias for an existing data type. Using typedefs can increase the readability and maintainability of your code, especially when dealing with complex data types. For simple data types, such as integers, floating point numbers, or characters, the benefits of using aliases are not obvious. However, for complex data types such as pointers, structures, arrays, and functions, the advantages of using aliases are obvious. A typedef cannot be used before a variable or function definition and is usually created at the top of a program file or after a structure definition.

# Operating system for this tutorial: Windows 10 system, Dell G3 computer.

typedef is a keyword in C language, which is used to create new aliases for existing data types. Using typedefs can increase the readability and maintainability of your code, especially when dealing with complex data types.

Basic usage

The general syntax of typedef is as follows:

typedef existing_type new_type;

For example, we can use typedef to create a new alias for the integer type:

typedef int my_int;

Now, my_int becomes an alias of int. We can use it like this:

my_int a = 10;

We can also create aliases for pointer types:

typedef int* my_int_ptr;  
my_int_ptr p = malloc(sizeof(int));

More complicated Usage

In addition to simple data types, we can also create aliases for complex data types. For example, we can create aliases for struct types:

typedef struct {  
    int x;  
    int y;  
} my_struct;

Now, we can use my_struct to declare variables:

my_struct s;  
s.x = 10;  
s.y = 20;

We can also create aliases for array types. For example, the following code creates an alias for an array containing 5 integers:

typedef int my_array[5];  
my_array arr;

It is important to note here that aliases for arrays are not pointers, although their syntax is very similar. In fact, the alias of an array is the same data type as the array itself. This means that we can assign an array to another array, but we cannot assign an alias of an array to another array. With pointers, we can initialize one pointer with the value of another pointer. For example:

my_array arr1 = {1, 2, 3, 4, 5};  
my_array arr2 = arr1;  // 错误！不能将数组别名赋值给另一个数组  
int *p1 = arr1;  // 正确！可以将数组的地址赋值给指针  
int *p2 = p1;  // 正确！可以将一个指针的值赋值给另一个指针

In addition, we can also create aliases for function types. For example:

typedef int (*my_func_ptr)(int);

Here, my_func_ptr is an alias for a function pointer that accepts an integer parameter and returns an integer. We can use it like this:

int square(int x) {  
    return x * x;  
}  
my_func_ptr fp = square;  // fp现在是一个指向square函数的指针  
int result = fp(5);  // 通过fp调用square函数，结果为25

In C, aliases can be created using class names as typedefs. For example:

class my_class {  
public:  
    int x;  
};  
typedef my_class my_class_alias;  // my_class_alias成为my_class的别名  
my_class_alias obj;  // 现在我们可以像这样使用my_class_alias来声明对象了  
obj.x = 10;  // 设置x的值为10

When using typedef, you need to pay attention to the following points:

typedef cannot be used before a variable or function definition. For example, you cannot create an alias for the return type of a function before the function is defined. Therefore, typedefs are usually created at the top of the program file or after the structure definition. In C, you can create a typedef inside a class definition.

Typedef is usually used for complex data types. For simple data types, such as integers, floating point numbers, or characters, the benefits of using aliases are not obvious. However, for complex data types such as pointers, structures, arrays, and functions, the advantages of using aliases are obvious. This makes the code easier to read and understand.

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