Type Redefinition of Struct In C++?

Type Redefinition of Struct In C++?

This is not a type redefinition. This is called an elaborated type specifier. It can be used to disambiguate a class name from another name that hides it. It is not necessary to use it in this case. It also doesn't break anything to use it.

The code may be written for an API usable from C as well. In C, this is necessary.

In the code

struct LinkNode { int data; struct LinkNode* next; // Why there use `struct` type define LinkNode again? }; 

The line struct LinkNode* next; does not define a new type. It declares a member variable next of the (already existing) type LinkNode.

What threw you off here is probably the keyword struct, which, as you already figured out, is purely optional in this context. The reason why you are allowed to put struct here in the first place is a heritage from C. C distinguishes between tag names and type names. A declaration of the form struct Foo {}; in C only introduces a tag name Foo, but not a type name. This means that to instantiate an object of Foo in C you would need to write struct Foo f;, with the struct keyword there to turn the tag name into the corresponding type.

To get a proper type name in C, you'd have to use a typedef: typedef struct Foo FooType; which would then allow you to just write FooType f;. Since this distinction between tag names and type names proved to be not very useful in practice, C++ simply got rid of the distinction, allowing the simpler declaration notation you are familiar with.