Difference between 'struct' and 'typedef struct' in C++?

原址

摘抄：

In C++, there is only a subtle difference. It's a holdover from C, in which it makes a difference.

1. In C, there are two different namespaces of types: a namespace of 

struct

/

union

/

enum

 tag names
and a namespace of 

typedef

 names.
If you just said:

struct Foo { ... };
Foo x;

You would get a compiler error, because 

Foo

 is
only defined in the tag namespace. You'd have to declare it as:

struct Foo x;

2. Any time you want to refer to a 

Foo

,
you'd always have to call it a 

struct
Foo

. This gets annoying fast, so you can add a 

typedef

:

struct Foo { ... };
typedef struct Foo Foo;

Now both 

struct
Foo

 (in the tag namespace) and just plain 

Foo

 (in
the typedef namespace) both refer to the same thing, 

3. and you can freely declare objects of type 

Foo

 without
the struct keyword. The construct

typedef struct Foo { ... } Foo;

is just an abbreviation for the declaration and 

typedef

. 

4. Finally,

typedef struct { ... } Foo;

declares an anonymous structure and creates a 

typedef

 for
it. Thus, with this construct, it doesn't have a name in the tag namespace, only a name in the typedef namespace. This means it also can't be forward-declared. If you want to make a forward declaration, you have to give it a name in the tag namespace.(注：匿名的结构体定义也无法自定义各种构造函数)

5. In C++, all 

struct

/

union

/

enum

/class
declarations act like they are implicitly 

typedef

'ed,
as long as the name is not hidden by another declaration with the same name. See Michael
Burr's answer for the full details.