In the header file:

typedef struct _point * Point;

After the compiler sees this it knows:

• there is s struct called _point
• there is a pointer type Point that can refer to a _point

The compiler does not know:

• what the _point struct looks like
• what members it contains
• how big it is

And not only does the compiler not know it - we as programmers don't know it either. This means we can't write code that depends on those properties of _point, which means that our code may be more portable.

對於 C opaque pointer 的解譯