为什么这个C代码编译？ C struct typdef

Question

I wrote the following program:

typedef struct blahblah {
    int x;
    int y;
} Coordinate;

int main () {
   Coordinate p1;
   p1.x = 1;
   p1.y = 2;

   //blah blah has not been declared as a struct, so why is it letting me do this?
   struct blahblah p2;
   p2.x = 5;
   p2.y = 6; 
}

Can anyone explain to me what's going on?

Answer 1

You said:

blah blah has not been declared as a struct,

Actually, it has:

typedef struct blahblah {
    int x;
    int y;
} Coordinate; 

This is both a typedef Coordinate , and a definition of struct blahblah . What the definition says is:

• define a data-type called struct blahblah
• It has two members, int x and int y .
• Also, make a type definition called Coordinate that is equivalent to struct blahblah

Answer 2

Your struct declaration is equivalent to

struct blahblah {
    int x;
    int y;
};
typedef struct blahblah Coordinate;

Since this creates two names for the struct type ( struct blahblah ) and Coordinate , both type names are permissible for declaring variables.

Answer 3

typedef defines a new user-defined data type but DOES NOT invalidate the old definition . For example typedef int INT will not invalidate int . Likewise your blahblah is still a valid defined structure! And Coordinate is just a new type!

Answer 4

You declared blahblah as a struct in your typedef. A typedef is simply an easy way of referencing struct blahblah. But struct blahblah exists and that's why you can give it a typedef.

Answer 5

typedef is used to create an alias of one type to another. You're actually declaring the 'struct blahblah' in the typedef itself. It's a bit confusing but as @Timothy and others note it's a valid definition.