C ++ typedef中的struct关键字

Question

I was looking at Glew's header and I ran across something that raised a question for me:

// C
typedef struct __GLsync *GLsync;

I understand that Glew is a C library, therefore different from C++ and I know that this is a valid code in C++:

// C++
struct S { ... }; // S defined in the class name space
void f( S a ); // struct is optional

My question is, is it safe to remove the struct keyword from the typedef in C++ and include the header in a C++ only project?

typedef /*struct*/ __GLsync *GLsync;

If not, what would be the difference when I do a typedef with struct keyword and without it?

Answer 1

A typedef of the following form:

typedef struct S *SPtr;

does not need prior knowledge of what S is in the same translation unit. It assumes a forward declaration of some structure S and aliases SPtr as a pointer type to said-same. However, this:

typedef S *SPtr; // error unless S is declared earlier 

requires the declaration of the type S (which may be a structure) is already known prior to the declaration of the pointer alias type.

While this may seem trivial, it is a cornerstone of common implementations of the pimpl idiom . Note the typedef isn't really involved here, the announcement of struct MyClassImpl* as a pointer type to an unknown "thing" to-be-determined later (but prior to actual dereference) is what is important:

MyClass.h

#ifndef MY_CLASS
#define MY_CLASS

class MyClass
{
   ... members ...

private:
    struct MyClassImpl * m_pimpl; // note no MyClassImpl yet known
};

#endif

MyClass.cpp

#include "MyClass.h"  

// implementation formal declaration and implementation.
struct MyClassImpl
{
    ... members...
};

// MyClass implementation using the now-known MyClassImpl

Had that pointer been declared as:

MyClassImpl * m_pimpl;

the type MyClassImpl would require prior declaration or a compile-time error would ensue.

I hope that helps identify a key difference between the two methods of declaration. (and yes, I know, I should have used a smart pointer =P)

Best of luck.

Answer 2

The real answer here is maybe . It depends on what other code is found before it.

That being said, you can absolutely change this:

typedef struct __GLsync *GLsync;

To this:

struct __GLsync;
typedef __GLsync *GLsync;

Answer 3

If you want to remove struct keyword from typedef then you have to do things like this:

typedef struct S { ... }__GLsync; // S defined in the class name space

then you can use:

typedef  __GLsync  *GLsync;

Appended Question:

If struct is already definded by already some else then how to do work around then

 typedef struct __GLsync  *abc;
 typdef  *abc       GLsync

You can do things like this