help needed with void pointers.
in [Learn C or C++]
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From: pereges on 1 Jul 2008 05:51 Hi, I'm trying to create a generic link list. Here's the data structure for node: Code: typedef struct node_s { void *data; struct node_s *next; }node; Now let's say I want the data pointer to point to a vector structre: Code: typedef struct { double coord[3]; }vector; I can assign data (of some random node in list) address of some vector: Code: p->data = &v p is pointer to node in link list and v is the vector. But when I try an operation like this for eg: Code: p->data->coord[0] < p->data->coord[1] It gives me an error. Is there any way I can solve this ?
From: pereges on 1 Jul 2008 06:06 Btw this worked for me but I'm not sure if it is a legal or correct way because it contradicts what i have have heard : vector *a = (vector *) p->ptr; vector *b = (vector *) p->ptr; a->coord[0] < b->coord[0]
From: Francis Glassborow on 1 Jul 2008 07:51 pereges wrote: > Hi, I'm trying to create a generic link list. Here's the data > structure for node: > Code: > > typedef struct node_s > { > void *data; > struct node_s *next; > > }node; > > Now let's say I want the data pointer to point to a vector structre: > > Code: > > typedef struct > { > double coord[3]; > > }vector; > > I can assign data (of some random node in list) address of some > vector: > > Code: > > p->data = &v > > p is pointer to node in link list and v is the vector. > > But when I try an operation like this for eg: > > Code: > > p->data->coord[0] < p->data->coord[1] > > It gives me an error. Is there any way I can solve this ? I have heard > you can't typecast a void pointer to some other data type Then you heard wrong. Indeed in C, (but not in C++) void* implicitly convert to other data pointer types where the context provides the necessary information (it doesn't in the code you provide) However if you try to cast a void* to anything other than the original type and then dereference the result you have undefined behaviour. IOWs there must be an actual object of the correct type if you attempt to dereference a pointer. The error in your code is that a void* cannot be dereferenced. If you want to access the data in your 'generic' linked list you need to write something such as: (vector)(p->data) -> coord[0] However this is not the normal way to write a generic list in C. Instead we would normally use a typedef. For example: typedef int valuetype; typedef struct node { valuetype * data; struct node * next; } node; Yes, this causes some problems if you want more than one type of linked list in a single translation unit. But otherwise you can typedef the valuetype before #include the header file that contains the relevant declarations for your linked list.
From: Bart van Ingen Schenau on 1 Jul 2008 12:56 pereges wrote: > Hi, I'm trying to create a generic link list. Here's the data > structure for node: > Code: > > typedef struct node_s > { > void *data; > struct node_s *next; > }node; > > Now let's say I want the data pointer to point to a vector structre: > > Code: > > typedef struct > { > double coord[3]; > }vector; > > > I can assign data (of some random node in list) address of some > vector: > > Code: > > p->data = &v > > p is pointer to node in link list and v is the vector. > > But when I try an operation like this for eg: > > Code: > > p->data->coord[0] < p->data->coord[1] > > It gives me an error. Is there any way I can solve this ? You get an error, because p->data is a 'pointer to void' and the type void does not have a member called coord. If you are using C++, the best solution is to make node a template: template <class T> struct node { T data; node* next; }; In C, your only choice is to use a typecast: ((vector*)p->data)->coord[0] < ((vector*)p->data)->coord[1] Bart v Ingen Schenau -- a.c.l.l.c-c++ FAQ: http://www.comeaucomputing.com/learn/faq c.l.c FAQ: http://c-faq.com/ c.l.c++ FAQ: http://www.parashift.com/c++-faq-lite/
From: Ben Bacarisse on 1 Jul 2008 19:34
Francis Glassborow <francis.glassborow(a)btinternet.com> writes: In the context of: > pereges wrote: .... >> typedef struct node_s >> { >> void *data; >> struct node_s *next; >> }node; .... >> typedef struct >> { >> double coord[3]; >> }vector; .... >> p->data->coord[0] < p->data->coord[1] >> >> It gives me an error. Is there any way I can solve this ? .... > The error in your code is that a void* cannot be dereferenced. > > If you want to access the data in your 'generic' linked list you need > to write something such as: > > (vector)(p->data) -> coord[0] I imagine you intended to write: ((vector *)p->data) -> coord[0] -- Ben. |