c - typedef struct vs struct definitions [重复](typedef struct vs struct definitions [duplicate])

Question

This question already has an answer here:

(这个问题已经在这里有了答案：)

• Why should we typedef a struct so often in C?

  (为什么要在C语言中这么频繁地对结构进行typedef？)

  15 answers

  (15个答案)

I'm a beginner in C programming, but I was wondering what's the difference between using typedef when defining a structure versus not using typedef .

(我是C编程的初学者，但我想知道在定义结构时使用typedef与不使用typedef什么区别。)

It seems to me like there's really no difference, they accomplish the same goal.

(在我看来，它们之间并没有什么区别，它们实现了相同的目标。)

struct myStruct{
    int one;
    int two;
};

vs.

(与)

typedef struct{
    int one;
    int two;
}myStruct;

  ask by user69514 translate from so

Answer 1

The common idiom is using both:

(常见用法同时使用：)

typedef struct X { 
    int x; 
} X;

They are different definitions.

(它们是不同的定义。)

To make the discussion clearer I will split the sentence:

(为了使讨论更加清晰，我将拆分句子：)

struct S { 
    int x; 
};

typedef struct S S;

In the first line you are defining the identifier S within the struct name space (not in the C++ sense).

(在第一行中，您将在结构名称空间中定义标识符S （不是C ++含义）。)

You can use it and define variables or function arguments of the newly defined type by defining the type of the argument as struct S :

(您可以使用它并通过将参数的类型定义为struct S来定义新定义类型的变量或函数参数：)

void f( struct S argument ); // struct is required here

The second line adds a type alias S in the global name space and thus allows you to just write:

(第二行在全局名称空间中添加类型别名S ，从而使您可以编写：)

void f( S argument ); // struct keyword no longer needed

Note that since both identifier name spaces are different, defining S both in the structs and global spaces is not an error, as it is not redefining the same identifier, but rather creating a different identifier in a different place.

(请注意，由于两个标识符名称空间都不相同，因此在结构和全局空间中定义S都不是错误，因为它不是在重新定义相同的标识符，而是在不同的地方创建不同的标识符。)

To make the difference clearer:

(为了使区别更加清楚：)

typedef struct S { 
    int x; 
} T;

void S() { } // correct

//void T() {} // error: symbol T already defined as an alias to 'struct S'

You can define a function with the same name of the struct as the identifiers are kept in different spaces, but you cannot define a function with the same name as a typedef as those identifiers collide.

(您可以定义一个具有相同结构名称的函数，因为标识符被保存在不同的空间中，但是您不能定义一个与typedef相同名称的函数，因为这些标识符会发生冲突。)

In C++, it is slightly different as the rules to locate a symbol have changed subtly.

(在C ++中，由于定位符号的规则已经微妙地改变了，因此它略有不同。)

C++ still keeps the two different identifier spaces, but unlike in C, when you only define the symbol within the class identifier space, you are not required to provide the struct/class keyword:

(C ++仍然保留两个不同的标识符空间，但是与C语言不同，当您仅在类标识符空间中定义符号时，不需要提供struct / class关键字：)

 // C++
struct S { 
    int x; 
}; // S defined as a class

void f( S a ); // correct: struct is optional

What changes are the search rules, not where the identifiers are defined.

(什么是搜索规则更改，而不是定义标识符的位置。)

The compiler will search the global identifier table and after S has not been found it will search for S within the class identifiers.

(编译器将搜索全局识别符表后S尚未发现它会搜索S的类标识符内。)

The code presented before behaves in the same way:

(前面提供的代码的行为方式相同：)

typedef struct S { 
    int x; 
} T;

void S() {} // correct [*]

//void T() {} // error: symbol T already defined as an alias to 'struct S'

After the definition of the S function in the second line, the struct S cannot be resolved automatically by the compiler, and to create an object or define an argument of that type you must fall back to including the struct keyword:

(在第二行中定义了S函数之后，编译器将无法自动解析struct S，要创建对象或定义该类型的参数，您必须使用struct关键字：)

// previous code here...
int main() {
    S(); 
    struct S s;
}