"array type has incomplete element type" - c

Question

I am new to C programming, and have the following code. I am facing following error.
typedef struct Vertex Vertex;
typedef struct Edge Edge;
struct Vertex {
bool known;
char id[25];
Edge edges[20];
int distance;
};
struct Edge {
Vertex target;
int weight;
};
typedef struct {
Vertex Nodes[20];
int pass;
int infinity;
} Graph;
The error it gives is:
array type has incomplete element type
Can someone please help me understand what is the problem?

Answer 1

typedef struct Vertex Vertex;
typedef struct Edge Vertex;
this is probably generating some name conflict just change the name of one of them.

Answer 2

The only way this will be possible, is using a mixture of pointers and addressing how you implement your Vertex and Edge structures:
/*your typedefs didn't make sense to me as it was conflicting. So, I edited it accordingly*/
//typedef struct Vertex Vertex;
//typedef struct Edge Vertex;
struct Vertex;
struct Edge;
typedef struct Vertex {
bool known;
char id[25];
struct Edge *edges;//This HAS to be a pointer.
int distance;
} Vertex;
typedef struct Edge {
Vertex target;
int weight;
} Edge;
typedef struct {
Vertex Nodes[20];
int pass;
int infinity;
} Graph;
Why does this work? Because of something called forward declaration:
...a forward declaration is a declaration of an identifier (denoting
an entity such as a type, a variable, or a function) for which the
programmer has not yet given a complete definition. It is required for
a compiler to know the type of an identifier (size for memory
allocation, type for type checking, such as signature of functions),
but not a particular value it holds (in case of variables) or
definition (in the case of functions)...

Answer 3

In this declaration
struct Vertex {
bool known;
char id[25];
Edge edges[20];
int distance;
};
the type Edge is not yet declared. The compiler here only knows that it will correspond to a struct Edge, but that struct itself is not known.

Answer 4

Can someone please help me understand what is the problem?
Arrays have the following properties:
All its elements have the same size.
The elements are stored contigously.
This allows to calculate the memory address of each element (e.g. id[i]) from the size and memory address of the first element and the index i.
To do this, the compiler needs to know how large the array elements are. When you declare the Vertex::edges[20] member, the compiler does not yet know, how large objects of type Edge are. Hence the compiler error.
One way to avoid this is, to define the Edge struct before the Vertex struct. In your case, this won't help, because the Edge::target is of type Vertex and you will get a similar error. Memory addresses of struct members are calculated using the memory address of the object and adding the sizes of the members (and possibly some padding) that precede the requested member.
In such a case with circular dependencies, pointers as members can be used, because a pointer to a struct has the same size, no matter which members the struct has, that the pointer points to.

Answer 5

Think about it: the compiler has to know the size of the Vertex and Edge structures. If you make Edge contain a Vertex and Vertex contain an Edge, it can't really sort out the size. The solution is to provide just a pointer to the structure (pointer size should be known to the compiler). I would use a jrd1 version with a small change:
struct Edge {
struct Vertex* target;
int weight;
} Edge;
typedef struct Vertex {
bool known;
char id[25];
struct Edge edges[20];
int distance;
} Vertex;
typedef struct {
Vertex Nodes[20];
int pass;
int infinity;
} Graph;
That should work just fine.
Furthermore, if each edge is supposed to point the Vertex where it is contained, you don't really need to store that pointer, you could use something like the container_of macro in the linux kernel if you want.