c++ pointer being freed was not allocated error - c++

Question

I am practcing c++'s new/delete, hashfunction and linked.
I made a practice by myself.
I have a struct which is
typedef struct student
{
int id;
string fName;
string lName;
student * nextStudent;
}Student;
Then in main function, I define an array of student
Student * table = new Student [10];
I have my own hash function which takes the id, and change to 0-9.
I want to add a student I do following
void addStudent(int studentId, string firstName, string lastName, Student *table)
{
// using hash function, convert the id into hashed id
int hashedID = hashFunction( studentId );
Student * pointer = &table[hashedID];
while(pointer->nextStudent !=NULL){
pointer = pointer->nextStudent;
}
// once we reach to the student who has NULL in nextStudent
// add student
Student *tmp = new Student;
tmp->id = studentId;
tmp->fName = firstName;
tmp->lName = lastName;
tmp->nextStudent = NULL;
// link
pointer->nextStudent = tmp;
}
I tested it, it seems fine.
The problem is deletion.
Since student variables are stored in dynamic memeory,
I need to use delete.
The following is my code.
void deleteAll(Student *table, int len)
{
for (int i = 0; i < len; i++)
{
Student* tmp = &table[i];
// delete student info except the last one
while ( tmp -> nextStudent !=NULL){
Student* tmp2;
tmp2 = tmp;
tmp = tmp->nextStudent;
delete tmp2;
}
}
}
I visited every student varialbes ane do the deletion.
I cannot find any probelm in my deletion funtion...
This is what I got after run..
malloc: *** error for object 0x7f85f1404b18: pointer being freed was not allocated
I have no clue what I have done wrong..
Can you help me?
EDIT...
As you guys metion
I added "delete [] table" in the main funtion..
Also, I remove "delete tmp" in deleteAll function; i think "delete [] table" will handle that part.
Still does not work..
By the way I forgot to added initTable function in the initial post.
initTable initialize the table...
void initTable (Student *table, int len)
{
for (int i = 0; i < len; ++i)
{
table[i].nextStudent = NULL;
}
}
Thank you.

Answer 1

The nextStudent field is never initialized, so all 10 elements created here have it pointing to unknown values.
Student * table = new Student [10];
This causes addStudent to loop until some pointer->nextStudent hits a NULL value by chance, then overwrite memory it doesn't own (unless it hits the lucky NULL on the first iteration).
while(pointer->nextStudent !=NULL) { ... }
The 'student` struct (btw, why the typedef?) should have a constructor to at least do this.
student::student() : nextStudent(NULL) { }
[ EDIT ] The other issue that #JonathanPotter duly pointed in a comment is that the head of each of the 10 student lists is a member of the table array. It is not dynamically allocated, and should not be individually deleted.
The qucik/easy fix would be to add a student destructor to recursively delete child nodes:
student::~student() { if(nextStudent) delete nextStudent; }
Then deleteAll would reduce to:
void deleteAll(student *table, int len)
{
for (int i = 0; i < len; i++)
{
student *tmp = &table[i];
if(tmp->nextStudent) delete tmp->nextStudent;
}
// this leaves the dynamically allocated table[] in place
// to delete it as well, just `delete [] table;`
}
However, such recursion may become impracticable once the lists grow large, and should better be rewritten as an iteration (without the recursive destructor).
student::~student() { }
// ...
void deleteAll(student *table, int len)
{
for(int i = 0; i < len; i++)
{
student *tmp = &table[i];
// delete student info except the *first* one
for(student *tmp2; tmp2 = tmp->nextStudent; )
{
tmp->nextStudent = tmp2->nextStudent;
delete tmp2;
}
}
// this leaves the dynamically allocated table[] in place
// to delete it as well, just `delete [] table;`
}

Answer 2

Then in main function, I define an array of student
Student * table = new Student [10];
First of all you are creating array of Student not Student*. And late you are trying to delete not allocated values. This is the reason of your program behavior.
To create pointer of array of pointers Student* you need the following:
Student** table = new Student*[10];
Than change your functions arguments from Student* table to Student** table and continue research.
Also do not forget to delete table using delete[] table;
Good Luck.