Question

I have strange behavior with typedef struct constructions in classes and functions. The difference is that in first case all definitions are performed during class definition. Another case — during function definition. Look at my code.

class C
{
public:
    //struct xStruct;
    typedef struct xStruct* xPtr;
    typedef struct xStruct {xPtr F;} xStructR;
    typedef struct { xPtr First; } xPtr_Type;
    void F(void **Var)
    {
        xPtr Ptr = 0;
        ((xPtr_Type*)Var)->First = Ptr->F; //errors
    }
};

void Fu()
{
    typedef struct qxStruct* qxPtr;
    typedef struct qxStruct {qxPtr qF;} qxStructR;
    typedef struct { qxPtr qFirst; } qxPtr_Type;
    qxPtr qPtr = 0;
    void **qVar = 0;
    ((qxPtr_Type*)qVar)->qFirst = qPtr->qF;
}

During compilation with MS compiler (cl.exe) i have two errors:

error C2027: use of undefined type 'xStruct'
error C2227: left of '->F' must point to class/struct/union

If i uncomment struct xStruct declaration my code works. But i can’t understand why the similar code works fine in Fu function without struct xStruct declaration? It seems that compiler has more strict algorithms of code analysing during class definition…

Answer 1

I see one difference:

In class C compiler treats first occurence of struct xStruct as from global namespace. The second is from class C namespace – that’s why the error during assignment.

class C
{
public:
    //struct xStruct;
    typedef struct xStruct* xPtr; 
    //      ^^^^^^^^^^^^^^  forward declaration of global type ::xStruct
    typedef struct xStruct {xPtr F;} xStructR;
    //      ^^^^^^^^^^^^^^^^^^^^^^^^  definition of local type C::xStruct
    typedef struct { xPtr First; } xPtr_Type;
    //                    ^^^^^ pointer to ::xStruct not C::xStruct
    void F(void **Var)
    {
       xPtr Ptr = 0;
       //   ^^^ pointer to global incomplete type
       ((xPtr_Type*)Var)->First = Ptr->F; //errors
       //                         ^^^^^^ using of ::xStruct which is incomplete
    }

In global function example both definitions are from global namespace, so no problem there.

To prove it I extended your example – now it compiles without problems:

class C
{
public:
    //struct xStruct;
    typedef struct xStruct* xPtr;
    typedef struct xStruct {xPtr F;} xStructR;
    typedef struct { xPtr First; } xPtr_Type;
    void F(void **Var);
};

struct xStruct {
  ::xStruct* F;    
};
void C::F(void **Var)
{
    xPtr Ptr = 0;
    ((xPtr_Type*)Var)->First = Ptr->F; //no errors
}