什么是“向前声明”和“typedef结构X'和'结构X'之间的区别？ [英] What is 'forward declaration' and the difference between 'typedef struct X' and 'struct X'?

问题描述

我在C编程初学者，我知道结构类型声明和typedef结构的声明之间的区别。我碰到知道一个答案说，如果我们定义诸如结构：

  typedef结构
{
一些成员;
} struct_name;
 

然后，它会像提供一个别名匿名结构（因为它是不是有一个标签名）。因此，它不能被用于正向声明。我不知道什么是向前声明的意思。

另外，我想知道，下面的code：

  typedef结构NAME
{
一些成员;
} struct_alias;
 

有名称之间有什么区别和 struct_alias ？或者，都等于为struct_alias是结构名的别名？

此外，我们可以声明类型的变量结构名称这样的：

  struct_alias变量1;
 

和/或类似：

 结构名变量2;
 

或类似：

  NAME variable3;
 

解决方案

结构时，你需要有循环结构声明前置声明可能是有用的。例如：

 结构A {
    结构的b * b_pointer;
    INT℃;
};结构B {
    结构体一个* a_pointer;
    无效* D;
};
 

在结构A 声明，它不知道的结构规格b ，但你可以转发引用它。

当您的typedef匿名结构，然后编译器将不允许你使用它之前的typedef的名字。

这是非法的：

 结构A {
    B * b_pointer;
    INT℃;
};typedef结构{
    结构体一个* a_pointer;
    无效* D;
} B：//结构b体从未声明或定义
 

这虽然是合法的：

 结构A {
    结构的b * b_pointer;
    INT℃;
};typedef结构B {
    结构体一个* a_pointer;
    无效* D;
} B：//结构b的定义，并有一个别名类型称为B
 

所以是这样的：

  typedef结构B B;
// B型referes一个尚未定义的类型结构b结构A {
    B * struct_b_pointer;
    INT℃;
};结构B {
    结构体一个* a_pointer;
    无效* D;
};
 

这：

 的typedef INT B：结构A {
    结构的b * struct_b_pointer;
    b b_integer_type;
    INT℃;
};结构B {
    结构体一个* a_pointer;
    无效* D;
};//结构b和b是两个不同类型的所有在一起。注：这是不是在C ++允许
 

I am a beginner in C programming and I know the difference between struct type declaration and typedef struct declaration. I came across to know an answer saying that if we define a struct like:

typedef struct
{ 
some members;
}struct_name;

Then it will be like providing an alias to an anonymous struct(as it is not having a tag name). So it cant be used for forward declaration. I don't know what the "forward declaration" means.

Also, I wanted to know that for the following code:

typedef struct NAME
{ 
some members;
}struct_alias;

Is there any difference between NAME and struct_alias? Or are both equal as struct_alias is an alias of struct NAME?

Furthermore, can we declare a variable of type struct NAME like these:

struct_alias variable1;

and/or like:

struct NAME variable2;

or like:

NAME variable3; 

解决方案

struct forward declarations can be useful when you need to have looping struct declarations. Example:

struct a {
    struct b * b_pointer;
    int c;
};

struct b {
    struct a * a_pointer;
    void * d;
};

When struct a is declared it doesn't know the specs of struct b yet, but you can forward reference it.

When you typedef an anonymous struct then the compiler won't allow you to use it's name before the typedef.

This is illegal:

struct a {
    b * b_pointer;
    int c;
};

typedef struct {
    struct a * a_pointer;
    void * d;
} b;

// struct b was never declared or defined

This though is legal:

struct a {
    struct b * b_pointer;
    int c;
};

typedef struct b {
    struct a * a_pointer;
    void * d;
} b;

// struct b is defined and has an alias type called b

So is this:

typedef struct b b;
// the type b referes to a yet undefined type struct b

struct a {
    b * struct_b_pointer;
    int c;
};

struct b {
    struct a * a_pointer;
    void * d;
};

And this:

typedef int b;

struct a {
    struct b * struct_b_pointer;
    b b_integer_type;
    int c;
};

struct b {
    struct a * a_pointer;
    void * d;
};

// struct b and b are two different types all together. Note: this is not allowed in C++

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