cannot access integers in a vector - c++

Question

The following program is running into segmentation fault and I cannot figure why.
vector<int> *A::B(vector<int> *prefix, vector<int> *projected_tids, int support) {
vector<int> *result = NULL;
for(it_vec it = projected_tids->begin(); it != projected_tids->end(); ++it) {
if(projected_tids == NULL) {
cout<<"NULL";
} else {
cout<<"abc"<<endl;
cout<<&projected_tids<<"address"<<endl;
}
cout<<"here"<<projected_tids->size()<<" "<<prefix->size()<<endl;
cout<<"iteration: "<<projected_tids->at(1)<<endl; //seg fault here
map<int, bool> *transaction = (*processed_input)[*it]; //seg fault here as well, because of *it
bool found = true;
//some more code
}
}
The prompt looks like this:
abc
0x7ffe3894a198address
here16 1
Segmentation fault (core dumped)
GDB is also confusing, might be because I am using C++ after a long time, but I cannot figure why? Below is GDB output:
(gdb) print projected_tids == (void *)0
$14 = false
(gdb) print *(projected_tids._M_impl._M_start)#1
Cannot access memory at address 0x0
(gdb) print *(projected_tids._M_impl._M_start)#3
Cannot access memory at address 0x0
(gdb) print projected_tids
$15 = (std::vector<int, std::allocator<int> > *) 0x60e000
The allocation of pointer to vector is done as in the calling class:
vector<int> *projected_tids = new vector<int>();
Please let me know if any more information is needed.

Answer 1

OK So a number of things going wrong. Concentrating on the question, it looks like:
projected_tids is not NULL (as you're done your check with cout)
Your code then dumps out the address of the projected_tids pointer, (which is on the stack), not the address of the vector. There's nothing wrong with that, but will be a bit misleading.
It reports that projected_tids contains 16 elements
It then seg faults (i.e. invalid use of pointer) when you look at the 2nd item (at(1) use a 0-index). This is the big hint that something is a bit wrong.
It looks like projected_tids is not a valid vector. Whatever is passing the data into your A::B method is passing in a bad pointer somehow. Perhaps uninitialised? You say it's allocated, but maybe it was deleted then?
A few other bits
As mentioned, you're using projected_tids before checking if it was NULL
What's the processed_input pointer all about? There's no reference to it anywhere
Why not use references? They're safer and just as performant as passing pointers.

Answer 2

You already call a member function on a pointer-to-object without checking for NULL first:
for(it_vec it = projected_tids->begin(); it != projected_tids->end(); ++it) {
^^^^^^^^^^^^^^^^^^^^^^^
unconditional call to member function
if(projected_tids == NULL) {
^^^^^^^^^^^^^^^^^^^^^^^^^^
Only here you test for NULL or not
So the logic in your program is flawed (big time) and anything might happen.
Also you cannot use ->at(1) without checking that the size is at least 2. In your example the size of the vector is 0.
As an aside: why don't you pass the vectors in by reference?

Answer 3

The logic does not seem correct.
if(projected_tids == NULL) {
cout<<"NULL";
} else {
you check for null.If it is null, you print null and then go ahead and dereference it.
cout<<"iteration: "<<projected_tids->at(1)<<endl;
If it is NULL, donot dereference it.