2 quick questions about passing by const pointer/const reference

Question

1) Can someone explain the following?

void OnCreate(HWND hWnd, const LPCREATESTRUCT lpCreateStruct)
{
   lpCreateStruct->x = 2; // this compiles
}

void OnCreate(HWND hWnd, const CREATESTRUCT * lpCreateStruct)
{
   lpCreateStruct->x = 2; // this does not compile
}

2) Is it faster to pass by pointer or by reference? Or the same?

Answer 1

Why does the following assign 2 to x when it is const?

It shouldn't, any standard-compliant compiler makes that an error because ms points to a const my_structure.

edit:

With your updated case, its a problem of how typedefs work:

typedef T* TPtr;

void f(const TPtr);
void g(T* const);
void h(const T*);

Both f() and g() are equivalent and take a const pointer to a T, while h() a pointer to a const T.

Answer 2

const LPCREATESTRUCT expands to CREATESTRUCT* const, i.e. a constant pointer to non-const structure, while const CREATESTRUCT* is a non-constant pointer to constant structure, so the compiler is right.

Answer 3

Assuming you have some control over the signature of the OnCreate method, I would suggest writing it as follows

void OnCreate(HWND hWnd, CREATESTRUCT& createStruct) 
{ 
   createStruct.x = 2; // this compiles 
}

or, if you don't want to allow modification, then

void OnCreate(HWND hWnd, const CREATESTRUCT& createStruct) 
{ 
   createStruct.x = 2; // this does not compile 
}

In addition to avoiding any confusion about exactly what is const, by using C++ references you've also said in code that the argument "can't" be NULL. (Yes, somebody could write OnCreate(hWnd, *(static_cast<CREATESTRUCT*>(NULL))) to create a null reference).