Is it possible to have a linked list of different data types?

Question

This is just another interview question.

Can we have a linked list of different data types, i.e. each element in a linked list can have different structure or union

Answer 1

Yes, I do this by defining the list's element's value as a void pointer void*. In order to know the type stored in each element of the list I also have a .type field in there, so I know how to dereference what the pointer is pointing to for each element.

struct node {
    struct node* next;
    int type;
    void* value;
};

Here's a full example of this:

//                                                                                                                                                                                          
// An exercise to play with a struct that stores anything using a void* field.                                                                                                              
//                                                                                                                                                                                          

#include 

#define TRUE 1

int TYPE_INT = 0;
int TYPE_STRING = 1;
int TYPE_BOOLEAN = 2;
int TYPE_PERSON = 3;

struct node {
  struct node* next;
  int type;
  void* value;
};

struct person {
  char* name;
  int age;
};

int main(int args, char **argv) {

  struct person aPerson;
  aPerson.name = "Angel";
  aPerson.age = 35;

  // Define a linked list of objects.                                                                                                                                                       
  // We use that .type field to know what we're dealing                                                                                                                                     
  // with on every iteration. On .value we store our values.                                                                                                                                
  struct node nodes[] = {
    { .next = &nodes[1], .type = TYPE_INT    , .value=1                   },
    { .next = &nodes[2], .type = TYPE_STRING , .value="anyfing, anyfing!" },
    { .next = &nodes[3], .type = TYPE_PERSON , .value=&aPerson            },
    { .next = NULL     , .type = TYPE_BOOLEAN, .value=TRUE                }
  };

  // We iterate through the list                                                                                                                                                            
  for ( struct node *currentNode = &nodes[0]; currentNode;  currentNode = currentNode->next) {
    int currentType = (*currentNode).type;
    if (currentType == TYPE_INT) {
      printf("%s: %d\n", "- INTEGER", (*currentNode).value); // just playing with syntax, same as currentNode->value                                                                        
    } else if (currentType == TYPE_STRING) {
      printf("%s: %s\n", "- STRING", currentNode->value);
    } else if (currentType == TYPE_BOOLEAN) {
      printf("%s: %d\n", "- BOOLEAN (true:1, false:0)", currentNode->value);
    } else if (currentType == TYPE_PERSON) {
        // since we're using void*, we end up with a pointer to struct person, which we *dereference                                                                                        
        // into a struct in the stack.                                                                                                                                                      
        struct person currentPerson = *(struct person*) currentNode->value;
        printf("%s: %s (%d)\n","- TYPE_PERSON", currentPerson.name, currentPerson.age);
      }
  }

    return 0;
}

Expected output:

- INTEGER: 1
- STRING: anyfing, anyfing!
- TYPE_PERSON: Angel (35)
- BOOLEAN (true:1, false:0): 1