C语言 类型定义/结构声明
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Typedef/struct declarations
提问by user2780061
What is the difference between these two declarations, if someone could explain in detail:
这两个声明有什么区别，如果有人能详细解释一下：
typedef struct atom {
int element;
struct atom *next;
};
and
和
typedef struct {
int element;
struct atom *next;
} atom;
回答by Gangadhar
This is Normal structure declaration
这是正常的 structure declaration
struct atom {
int element;
struct atom *next;
}; //just declaration
creation of object
的制作 object
struct atom object;
struct atom {
int element;
struct atom *next;
}object; //creation of object along with structure declaration
And
和
This is Type definition of struct atomtype
这是类型定义 struct atom类型
typedef struct atom {
int element;
struct atom *next;
}atom_t; //creating new type
Here atom_tis alias for struct atom
这里atom_t是别名struct atom
creation of object
对象的创建
atom_t object;
struct atom object; //both the ways are allowed and same
回答by Barmar
The purpose of typedefis to give a name to a type specification. The syntax is:
的目的typedef是为类型规范命名。语法是：
typedef <specification> <name>;
After you've done that, you can use <name>much like any of the built-in types of the language to declare variables.
完成此操作后，您可以<name>像使用该语言的任何内置类型一样来声明变量。
In your first example, you the <specification>is everything starting with struct atom, but there's no <name>after it. So you haven't given a new name to the type specification.
在您的第一个示例中，您的<specification>一切都以 开头struct atom，但<name>后面没有。所以你没有给类型规范一个新的名字。
Using a name in a structdeclaration is not the same as defining a new type. If you want to use that name, you always have to precede it with the structkeyword. So if you declare:
在struct声明中使用名称与定义新类型不同。如果要使用该名称，则始终必须在其前面加上struct关键字。所以如果你声明：
struct atom {
...
};
You can declare new variables with:
您可以使用以下命令声明新变量：
struct atom my_atom;
but you can't declare simply
但你不能简单地声明
atom my_atom;
For the latter, you have to use typedef.
对于后者，您必须使用typedef.
Note that this is one of the notable differences between C and C++. In C++, declaring a structor classtype doesallow you to use it in variable declarations, you don't need a typedef. typedefis still useful in C++ for other complex type constructs, such as function pointers.
请注意，这是 C 和 C++ 之间的显着差异之一。在 C++ 中，声明 astruct或class类型确实允许您在变量声明中使用它，您不需要typedef. typedef在 C++ 中对于其他复杂类型构造仍然很有用，例如函数指针。
You should probably look over some of the questions in the Relatedsidebar, they explain some other nuances of this subject.
您可能应该查看相关侧栏中的一些问题，它们解释了该主题的其他一些细微差别。
回答by Deep
The general syntax of typedef keyword will be:
typedef existing_data_type new_data_type;
typedef 关键字的一般语法为：
typedef existing_data_type new_data_type;
typedef struct Record {
char ename[30];
int ssn;
int deptno;
} employee;
