摘自Dan Saks的一篇旧文章(http://www.ddj.com/cpp/184403396?pgno=3):)

The C language rules for naming structs are a little eccentric, but they're pretty harmless. However, when extended to classes in C++, those same rules open little cracks for bugs to crawl through. In C, the name s appearing in struct s { ... }; is a tag. A tag name is not a type name. Given the definition above, declarations such as s x; /* error in C */ s *p; /* error in C */ are errors in C. You must write them as struct s x; /* OK */ struct s *p; /* OK */ The names of unions and enumerations are also tags rather than types. In C, tags are distinct from all other names (for functions, types, variables, and enumeration constants). C compilers maintain tags in a symbol table that's conceptually if not physically separate from the table that holds all other names. Thus, it is possible for a C program to have both a tag and an another name with the same spelling in the same scope. For example, struct s s; is a valid declaration which declares variable s of type struct s. It may not be good practice, but C compilers must accept it. I have never seen a rationale for why C was designed this way. I have always thought it was a mistake, but there it is. Many programmers (including yours truly) prefer to think of struct names as type names, so they define an alias for the tag using a typedef. For example, defining struct s { ... }; typedef struct s S; lets you use S in place of struct s, as in S x; S *p; A program cannot use S as the name of both a type and a variable (or function or enumeration constant): S S; // error This is good. The tag name in a struct, union, or enum definition is optional. Many programmers fold the struct definition into the typedef and dispense with the tag altogether, as in: typedef struct { ... } S;

这篇链接的文章还讨论了不需要类型定义的c++行为是如何导致微妙的名称隐藏问题的。为了防止这些问题，在c++中对类和结构进行类型定义也是一个好主意，尽管乍一看似乎没有必要。在c++中，有了类型定义，隐藏的名称就变成了编译器告诉你的错误，而不是潜在问题的隐藏来源。

另一个总是对枚举和结构进行类型定义的原因是:

enum EnumDef
{
  FIRST_ITEM,
  SECOND_ITEM
};

struct StructDef
{
  enum EnuumDef MyEnum;
  unsigned int MyVar;
} MyStruct;

注意到结构(EnumDef)中的EnumDef中的错别字吗?这个编译没有错误(或警告)，并且(取决于C标准的字面解释)是正确的。问题是我刚刚在我的结构中创建了一个新的(空的)枚举定义。我没有(如预期)使用前面的定义EnumDef。

对于typdef，类似类型的打字错误将导致使用未知类型的编译器错误:

typedef 
{
  FIRST_ITEM,
  SECOND_ITEM
} EnumDef;

typedef struct
{
  EnuumDef MyEnum; /* compiler error (unknown type) */
  unsigned int MyVar;
} StructDef;
StrructDef MyStruct; /* compiler error (unknown type) */

我主张ALWAYS类型定义结构和枚举。

不仅是为了节省一些输入(没有双关语;))，而且因为它更安全。

使用typedef可以避免每次声明该类型的变量时都要写struct:

struct elem
{
 int i;
 char k;
};
elem user; // compile error!
struct elem user; // this is correct

在C99中，typedef是必需的。它已经过时了，但是很多工具(比如HackRank)使用c99作为它的纯C实现。这里需要typedef。

我不是说他们应该改变(也许有两个C选项)，如果要求改变，我们这些在网站上学习面试的人将是SOL。

让我们从最基本的开始，慢慢来。

下面是一个结构定义的例子:

struct point
  {
    int x, y;
  };

这里的名称点是可选的。

结构可以在定义期间声明，也可以在定义之后声明。

在定义期间声明

struct point
  {
    int x, y;
  } first_point, second_point;

在定义之后声明

struct point
  {
    int x, y;
  };
struct point first_point, second_point;

现在，仔细注意上面的最后一种情况;如果您决定稍后在代码中创建该类型，则需要编写struct point来声明该类型的结构。

输入类型。如果您打算在稍后使用相同的蓝图在程序中创建新的Structure (Structure是自定义数据类型)，那么在定义过程中使用typedef可能是一个好主意，因为您可以节省一些输入。

typedef struct point
  {
    int x, y;
  } Points;

Points first_point, second_point;

在命名自定义类型时要注意一点

没有什么可以阻止您在自定义类型名称的末尾使用_t后缀，但POSIX标准保留使用后缀_t来表示标准库类型名称。

Typedef不会提供一组相互依赖的数据结构。你不能用typdef做到这一点:

struct bar;
struct foo;

struct foo {
    struct bar *b;
};

struct bar {
    struct foo *f;
};

当然你还可以加上:

typedef struct foo foo_t;
typedef struct bar bar_t;

这到底有什么意义?