Strange output when attempting to print from an array of pointers c++

Question

I'm attempting to create an array of pointers.

struct vertex
{
std::string id;
std::string name;
int networkID;
std::vector<adjVertex> friends;
bool visited;
};

struct hobbylist
{
std::string hobby;
std::vector<vertex*> list;
};

hobbylist * hobbies[HASHMAP_SIZE];

adding the user to the hobbies array:

int Graph::addUserToHobby(std::string hobby1, std::string id){
// initial key is based on the first 2 characters of the hobby name
int key = (hobby1[0] + hobby1[1]) % HASHMAP_SIZE;
cout << " initial hashmap key " << key << endl;
hobbylist *h = new hobbylist;
h->hobby = hobby1;
hobbies[key] = h;
}

my goal is to create an array of pointers with the hobbylist type, when attempting to print the contents of that array I end up with a very strange random symbol output:

GLIBC_2.2.5GLIBCXX_3.4.13GLIBCXX_3.4.14CXXABI_1.3GLIBCXX_3.4� P&y

I attempt to print it as so:

void Graph::displayHobbies(){
cout << "========================================\n";
cout << "DISPLAYING HOBBY INTERESTS =============" << endl;
for(auto const& value: hobbies)
{
    cout << value->hobby << ":" << endl;
}
}

I was wondering if I am printing incorrectly or if I am adding the hobby to the hobbies array incorrectly.

Changed Code:

hobbylist *h = new hobbylist;
h->hobby = hobby1;
if(hobbies[key] ==NULL){
h->list.push_back(user);
hobbies[key] = h;
}
else if (hobbies[key]!=NULL){
    h= hobbies[key];
    h->list.push_back(user);
}

Changed code is above and I am getting a seg fault at the last line in the else statement when running the function the first time and I am confused why the function would go to the else statement when the array should be empty and therefore hobbies[key] should be null the first time the function is run?

Initialize the pointers of array to nullptr (hobbylist* hobbies[HASHMAP_SIZE] {}), and check that before printing (if (value) {std::cout << value->hobby << ":" << std::endl;}). — Jarod42
– Jarod42, Commented Dec 1, 2016 at 23:50
Or even better, use array of std::unique_ptr<hobbylist > to avoid manual memory management. — Jarod42
– Jarod42, Commented Dec 1, 2016 at 23:53
Maybe std::map<std::string, hobbylist> (or std::unordered_map) would make more sense, since the hobbies are being keyed by name, which is what a map is good for. — Remy Lebeau
– Remy Lebeau, Commented Dec 2, 2016 at 0:04
Thanks for all the suggestions but unfortunately my class is very strict and I am unable to modify the header file in any way without losing lots of points. :( — andrew fay
– andrew fay, Commented Dec 2, 2016 at 3:48

Sam Varshavchik · Accepted Answer · 2016-12-01 23:57:21Z

3

You have at least two bugs.

hobbylist *h = new hobbylist;
h->hobby = hobby1;
hobbies[key] = h;

key is your hash key. If hobbies[key] already has a pointer, this is going to leak memory.

for(auto const& value: hobbies)
{
    cout << value->hobby << ":" << endl;
}

This assumes that every slot in the hobbies hash array contains a pointer. This is unlikely. If a particular value in hobbies has never been initialized (none of the previously inserted hobbies mapped to that hash key), the pointer will be NULL, and value->hobby will attempt to dereference a NULL pointer, resulting in undefined behavior. That's your likely crash.

answered Dec 1, 2016 at 23:57

Sam Varshavchik

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2 Comments

Sam Varshavchik Over a year ago

That seems to fall under the "memory leak" category, as described.

andrew fay Over a year ago

so i've changed it a little bit to:

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