Question

I have this struct in a file “vector.c”:

struct Vector
{
int * m_items;
int m_size;
int m_extSize;
int m_numItems;
};

On “main.c” I’m trying to check if the value m_items of a certain vector is NULL :

if (! vec->m_items)
        printf("not fail\n");

I do it after initializing “vec” with values – the vector has 1 value (I checked it).
However, gcc is writing an error for the above line :

Error: dereferencing pointer to incomplete type

Why is it?

Answer 1

You have to move the entire definition of Vector to a header file and include it in both vector.c and main.c.

Adding a typedef struct Vector Vector; is not enough. That merely tells the compiler that there is a type Vector and is defined elsewhere, so, it is incomplete. It lets you declare pointers to it because it doesn’t need to know which members it has in order to allocate a pointer. All pointers are the same size.