What are the pros/cons of this class definition style?

I have seen following style class definition several times. What exactly are the pros/cons of this style?

typedef class _MyClass
{
public :
  _MyClass();
开发者_JAVA百科} MyClass;

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It's pretty rare in C++, but common in C (where struct Foo does not automatically alias to just Foo.) You might see it in a library that has different classes for different platforms (e.g. a "Canvas" class, which is very implementation-specific.) The library would use the typedef to let its users simplify their code:

#if WINDOWS
    typedef class WindowsCanvas {
        private: HWND handle;
    } Canvas;
#elif MAC
    typedef class MacCanvas {
        private: CGContextRef context;
    } Canvas;
#endif

In such an example, one would only need to use the Canvas type, never the platform-specific types.

---

In C++ there are no pros. This style came from C where you couldn't just use the struct's name as a type. E.g.

struct X
{
   int x;
};

X a; //compiler error
struct X b; //OK

in order to avoid using the elaborated type specifier, like struct X a, or enum E e; etc. in C it is a common practice to typedef the name. E.G.

typedef struct X_ { ... } X;

Now X a; is OK too.

Naturally in C++ there is no need to do this.

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One possible advantage is illustrated by a highly contrived example, but since the Standard speaks of it, it must be having an impliciation for sure.

$7.1.3/6- "Similarly, in a given scope, a class or enumeration shall not be declared with the same name as a typedef-name that is declared in that scope and refers to a type other than the class or enumeration itself. [

typedef struct S{} MYS;

int MYS;                     // Error due to $7.1.3/6

struct A{};

int A;                       // No error, subsequent use required fully elaborated name

int main(){}