Using Reference of Pointers

[V. de Bellabre]

Hello all,
I'm currently working on a 3D graphic engine but I'm stuck with a
small problem on references of pointers.
Some of my c++ class are complex and use many pointers so I decided to
force initialisation of objects through their static function Create,
which has a parameter IN-OUT that will contain the instance of the
created object.

Here is a sample that speaks better than words :

--------------------------------------------------------------------------
typedef class Texture* PTEXTURE;
// With this, PTEXTURE& is a reference of a pointer to Texture

class Texture:
{
protected:
Texture( void ){} // Constructor, do nothing, use Create

public:
static void Create( PTEXTURE& ); // PTEXTURE& is a reference of a
pointer to the texture to create

// Many other stuff that is not important here

};

// Implementation of the Create Function
void Texture::Create( PTEXTURE& pTexToCreate )
{
pTexToCreate = new Texture; // memory alloc

// Some init...

if( badInit )
{
delete pTexToCreate;
pTexToCreate = NULL;
}
}

// A small example of how I use this
void main( void )
{
PTEXTURE pMyTexture;
Texture::Create( pMyTexture );
if( pMyTexture == NULL )
// Error creating the texture
}

--------------------------------------------------------------------------

Hope this code is clear enough
Then there is my problem : pMyTexture will never be NULL!! even if
there IS a problem in the Create function and the line "pTexToCreate =
NULL" is executed by the program. Using the debugger shows that
pTexToCreate is NULL, however when going out of the function,
pMyTexture has a random value pointing to nothing with sense.
I really don't understand that, because using references should affect
the original value passed as a parameter (which is true because when
initialization is successfull, pMyTexture is pointing to a valid
Texture). And I don't want to use pointers of pointers to solve this
problem.

Can anyone help please ? I just would like pMyTexture to be NULL when
Create fails /

Vianney

 

[Victor Bazarov]

I'm currently working on a 3D graphic engine but I'm stuck with a
small problem on references of pointers.
Some of my c++ class are complex and use many pointers so I decided to
force initialisation of objects through their static function Create,
which has a parameter IN-OUT that will contain the instance of the
created object.

Here is a sample that speaks better than words :

--------------------------------------------------------------------------
typedef class Texture* PTEXTURE;
// With this, PTEXTURE& is a reference of a pointer to Texture

class Texture:

What's the colon doing there?

{
protected:
Texture( void ){} // Constructor, do nothing, use Create

public:
static void Create( PTEXTURE& ); // PTEXTURE& is a reference of a
pointer to the texture to create

// Many other stuff that is not important here

};

// Implementation of the Create Function
void Texture::Create( PTEXTURE& pTexToCreate )
{
pTexToCreate = new Texture; // memory alloc

// Some init...

if( badInit )
{
delete pTexToCreate;
pTexToCreate = NULL;
}
}

// A small example of how I use this
void main( void )

^^^^^^^^^^^^^^^^^^
Change this to

int main()

{
PTEXTURE pMyTexture;

^^^
Initialise the pointer here, to 0 if there is no better value.

Texture::Create( pMyTexture );
if( pMyTexture == NULL )
// Error creating the texture
}

--------------------------------------------------------------------------

Hope this code is clear enough
Then there is my problem : pMyTexture will never be NULL!! even if
there IS a problem in the Create function and the line "pTexToCreate =
NULL" is executed by the program. Using the debugger shows that
pTexToCreate is NULL

Where? Inside the function?

, however when going out of the function,
pMyTexture has a random value pointing to nothing with sense.

Seems that your stack gets blown to pieces.

I really don't understand that, because using references should affect
the original value passed as a parameter (which is true because when
initialization is successfull, pMyTexture is pointing to a valid
Texture). And I don't want to use pointers of pointers to solve this
problem.

Can anyone help please ? I just would like pMyTexture to be NULL when
Create fails /

Something else can be a problem. If I take the code you posted, get it
to compile (by fixing it up a bit), it behaves as expected. So, the
cause of your trouble is not in the code you showed.

Try to find uninitialised pointers or memory overruns.

V

 

[niklasb]

Some of my c++ class are complex and use many pointers so I decided to
force initialisation of objects through their static function Create,
which has a parameter IN-OUT that will contain the instance of the
created object.

An alternative would be to simply have your Create function return
a pointer. OUT parameters have their place, but all else being equal,
simply returning a value is usually more clear.

typedef class Texture* PTEXTURE;
// With this, PTEXTURE& is a reference of a pointer to Texture

Personally, I think hiding a pointer behind a typedef like this
obfuscates more than it clarifies. Yes, I know the Windows header
files do this all the time, but I suspect that's a holdover from
the bad old 16-bit days of __near and __far pointers.

class Texture:
{
protected:
Texture( void ){} // Constructor, do nothing, use Create

Is Texture meant to be derived from? If so, you should probably
add a virtual destructor. If not, the ctor should be private
instaed of protected.

public:
static void Create( PTEXTURE& ); // PTEXTURE& is a reference of a
pointer to the texture to create

Or:

static Texture* Create();

Problem solved, and no need for reference-to-pointer.

// Implementation of the Create Function
void Texture::Create( PTEXTURE& pTexToCreate )
{
pTexToCreate = new Texture; // memory alloc

// Some init...

if( badInit )
{
delete pTexToCreate;
pTexToCreate = NULL;
}
}

The code above should work as you intended. The only thing I
can think of is there's undefined behavior somewhere else in
code you snipped. You might try taking the stripped-down code
you posted and edit it to be actually compilable, then run it
to see if the problem still manifests.

As a matter of style, I'd prefer to see Create return a
pointer. There's no need for an OUT or IN/OUT parameter here.

Better yet, why not do the initialization in the ctor as C++
is meant to be used? If initialization failed, the ctor would
throw an exception. (My guess is you're using a factory method
instead because you want to return NULL instead of throwing an
exception on failure.)

// A small example of how I use this
void main( void )
{
PTEXTURE pMyTexture;
Texture::Create( pMyTexture );
if( pMyTexture == NULL )
// Error creating the texture
}

The return value of main must be int. The above is not legal
C++, although some compilers accept it.