How to declare 2 structs contain each other in c

April 25, 2022

I am implementing a graph structure for a maze solving program. I declare 2 structs: vertex contains label to label that vertex, 4 directions of edge; edge contains weight and a vertex pointer to 4 other vertices.

extern struct graph
{
    int label;
    struct Edge up;
    struct Edge down;
    struct Edge left;
    struct Edge right;
}vertex;

extern struct Edge
{
    int weight;
    struct graph *neighbour;
}edge;

typedef struct graph vertex;
typedef struct Edge edge;

This way of declaration leads to some errors: unknown type name ‘vertex’; field ‘up’ has incomplete type,..

(I have tried to us extern but it doesn’t seem to be the problem).

So how do I declare it properly?

Any help would be appreciated.

>Solution :

Not sure why a graph is also a Vertex, but you can reverse the order you declare the 2 structs and use a forward declaration. You can also combine the struct declarations and the typedefs.

struct graph;  // Forward declaration

typedef struct 
{
    int weight;
    graph *neighbour;
} edge;

typedef struct graph 
{
    int label;
    edge up;
    edge down;
    edge left;
    edge right;
} graph;