gcc - error: dereferencing pointer to incomplete type

Question

I have a rather convoluted set of nested structs/unions as shown:

typedef enum {
    expr_BooleanExpr,
    expr_ArithmeticExpr
} expr_type;

typedef union {
    struct BooleanExpr *_bool;
    struct ArithmeticExpr *_arith;
} u_expr;

typedef struct {
    expr_type type;
    u_expr *expr;
} Expression;

typedef struct {
    Expression *lhs;
    char *op;
    Expression *rhs;
} BooleanExpr;

typedef struct {
    Expression *lhs;
    char *op;
    Expression *rhs;
} ArithmeticExpr;

gcc is happy for me to create an Expression struct containing a BoolExpression value in its union field as shown:

Expression *BooleanExpr_init(Expression *lhs, char *op, Expression *rhs) {

    BooleanExpr *_bool = safe_alloc(sizeof(BooleanExpr));
    _bool->lhs = lhs;
    _bool->op = op;
    _bool->rhs = rhs;

    Expression *the_exp = safe_alloc(sizeof(Expression));
    the_exp->type = expr_BooleanExpr;
    the_exp->expr->_bool = _bool;

    return the_exp;
}

although it gives a warning: assignment from incompatible pointer type [enabled by default] for the line: the_exp->expr->_bool = _bool;

However, when accessing the inner expressions such as lhs and rhs, with an expression like

an_expr->expr->_bool->rhs

where an_expr is a previously created Expression struct, I get the error specified in the title of this post.

Much of what I've read says that this results from the use of the -> operator where the . operator is required. However this is not appropriate since everything is a pointer, so the implicit dereference of the -> operator is required.

Any ideas?

Solution

You are mixing typedef identifiers and struct scope identifiers. This can't work. Do something like

typedef struct  BooleanExpr BooleanExpr;

before all your struct declarations and have these only as

struct BooleanExpr { ...

without the typedef.

In your code you never defined struct BooleanExp but only an anonymous struct that you alias to the identifier BooleanExp.