2个相似typedef定义的差异

Question

You can define a Point struct in this way:

typedef struct
{
    int x, y;
} Point;

and also in this way:

typedef struct Point
{
    int x, y;
};

What is the difference?

Answer 1

Consider the C code given below,

typedef struct
{
    int x, y;
} Point;

int main()
{
  Point a;
  a.x=111;
  a.y=222;
  printf("\n%d %d\n",a.x,a.y);
}

The above code will execute without any errors or warnings, whereas the following C code will give you an error ( error: 'Point' undeclared ) and a warning ( warning: useless storage class specifier in empty declaration ).

typedef struct Point
{
    int x, y;
};

int main()
{
    Point a;
    a.x=111;
    a.y=222;
    printf("\n%d %d\n",a.x,a.y);
}

To correct the error you have declare the structure variable a as follows,

 int main()
 {
    struct Point a;
    a.x=111;
    a.y=222;
    printf("\n%d %d\n",a.x,a.y);
 }

Answer 2

The second example, the typedef statement have no effect. The compiler will probably ignore it or give you a warning.

What differs from this code:

typedef struct Point
{
    int x, y;
} Point;

This allow you to use Point as a type, or as a struct. I consider a bad practice to use the struct name as a type, or even as a variable, but you are allowed to do this.