Question

This one is a little strange to me.

I have a pointer to a class member (mGeometry), which, in turn, holds a pointer to a QList< GLushort > data member (mFaces). I am trying to get the index of mFaces via the class Cube.

So, the more simplified version would look like this:

struct Geometry
{   
    Geometry( void );
    ~Geometry( void );
    void someFunc( void );
    QList< GLushort > *mFaces;
};

class Cube
{   
 public:
    Cube( void );
    ~Cube( void );
    void anotherFunc( void );
    Geometry *mGeometry;
};

Let’s say, in anotherFunc, we’re trying to do the following:

GLushort *indeces = new GLushort;

*indeces = ( *mGeometry ).mFaces[ 0 ];

Error(s)

error: cannot convert ‘QList<short unsigned int>’ to ‘GLushort {aka short unsigned int}’ in assignment

So, we try:

*indeces = mGeometry->( *mFaces )[ 0 ]; //which, is originally how I've accessed pointers-to-containers' indexes.

Error(s)

error: expected unqualified-id before ‘(’ token

error: ‘mFaces’ was not declared in this scope

And, of course, the obvious:

*indeces =  mGeometry->mFaces[ 0 ];

Error(s)

error: cannot convert ‘QList<short unsigned int>’ to ‘GLushort {aka short unsigned int}’ in assignment

Constructor for Geometry

Geometry::Geometry( void )
    : mFaces( new QList< GLushort > )
{
}

Is there anything wrong I’m doing here? If not, what is the correct method to obtain the index of the pointer to mFaces?

Answer 1

Because mFaces is a pointer, you have to dereference mGeometry with -> then dereference mFaces with * then use QList<>‘s operator[] to get the number:

*indeces = (*mGeometry->mFaces)[0]; // note that * has lower precedence than ->
                                    // so this is like (*(mGeometry->mFaces))[0]

It’s a little weird because [0] does the same thing as *. Indices on pointer types, such as x[i], follow the formula *(x + i), so you can also do this for the same effect (but don’t):

*indeces = mGeometry->mFaces[0][0]; // or *indeces = (*mGeometry).mFaces[0][0];

Which is the same as (*(mFaces + 0))[0], which is exactly the same as (*mFaces)[0].

That is also why you got that error cannot convert ‘QList<short unsigned int>’ to ‘GLushort’ when you tried to do

( *mGeometry ).mFaces[ 0 ];

Because ( *mGeometry ).mFaces[ 0 ]; (which, again, is equivalent to the above *mGeometry->mFaces) gets you a QList<GLushort>, and you have to use the operator[] of QList<> to get your data.

And now for something completely unrelated, you misspelled indices 🙂