问题
What is the difference between these 2 ways of declaring a struct?
First way:
struct x {};
Second way:
struct _x {} x;
回答1:
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.
回答2:
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
来源：https://stackoverflow.com/questions/13651227/struct-declaration