Self referential structs in c

Question

I'm trying to move some code that was compiling fine as C++ in VS2010 to c (gcc c99) and I'm getting compilation errors. It's a little different than the other self-referential struct questions, because I have 2 user defined types, each of which contains pointers to one another. It seems my forward declartions aren't enough.

struct potato; //forward declare both types
struct tomato;

struct potato
{
    potato* pPotato; //error: unknown type name ‘potato’
    tomato* pTomato;

};

struct tomato
{
    potato* pPotato;
    tomato* pTomato;
};

Why does this not work in gcc 99? Why is it ok as C++ code? How should I modify this to get the same behavior as c99?

Answer 1

Alternatively, typedef them both

typedef struct potato potato; //forward declare both types
typedef struct tomato tomato;

struct potato
{
    potato* pPotato;
    tomato* pTomato;

};

struct tomato
{
    potato* pPotato;
    tomato* pTomato;
};

Answer 2

Self-referential types are valid in C, but in C structs live in a different namespace to variables/constants and must be prefixed with struct when their name is used.

Also, avoid Hungarian Notation, in your case the p prefix.

Try this:

struct potato; //forward declare both types
struct tomato;

struct potato
{
    struct potato* potato;
    struct tomato* tomato;

};

struct tomato
{
    struct potato* potato;
    struct tomato* tomato;
};

The traditional way of avoiding having to constantly type struct foo was to use a typedef:

typedef struct potato potato;

The definition of struct potato can be used anonymously and inline:

typedef struct { ... } potato;

I have made a personal observation that use of typedef struct seems to be in decline and using the "longhand" form of always specifying struct when used is back in vogue.