Question

I have a struct like so

typedef struct person {
 int id;
 char name[20];
} Person;

Then, outside of the function, I have a pointer array of pointers to these structs, like so

Person **people;

Then in the function I am adding people to the array like so (in a loop)

Person person;

for (i = 0; i < 50; i++)
{
  person.id = i;
  person.name = nameArray[i];
  people[i] = &person;
}

person is being added to the people array but when (in VS2010) I go to the Watch screen and type people, 50
I just see the same person in every slot as if when adding the next person, it changes all previous as well. What am I doing wrong here?

Also, to retrieve a certain person’s name, is this the right syntax?

people[0] -> name; Or is it people[0][0].name?

Thanks!

Answer 1

What do you expect? You are making all the pointers point to the same Person. And when person goes out of scope, all the pointers in your array (which are all the same) will be invalid and point to a deallocated block of memory. You have to use malloc in each iteration of the loop to allocate dynamic storage and create a Person that won’t go away till you free it:

for (i = 0; i < 50; i++)
{
  Person *person = malloc(sizeof(Person));
  person->id = i;
  person->name = nameArray[i];
  people[i] = person;

  /* or:
  people[i] = malloc(sizeof(Person));
  people[i]->id = i;
  people[i]->name = nameArray[i];

  it does the same thing without the extra temporary variable
  */
}

// then when you are done using all the Person's you created...
for (i = 0; i < 50; ++i)
    free(people[i]);

Alternatively, you could have an array of Persons instead of Person*s and what you are doing would work:

Person people[50];

Person person;

for (i = 0; i < 50; i++)
{
  person.id = i;
  person.name = nameArray[i];
  people[i] = person; // make a copy
}

And with that way you don’t have to free anything.