struct declaration

Question

What is the difference between these 2 ways of declaring a struct?

First way:

struct x {};

Second way:

struct _x {} x;

Answer 1

The second way declares a variable named the type struct _x and a variable of this type named x. The first one only declares the type struct x.

The second way is essentially the same as

struct _x{};    // define a type
struct _x x;    // allocate a variable of type struct _x

Answer 2

The first defines only the type struct x. The second defines the type struct _x and defines a variable of that type named x.

Though it's probably not what you had in mind, names starting with an underscore like _x are reserved at file scope, so unless this is inside some other scope, the second has undefined behavior.