prinvt content of vectors - c++

Question

Resolved, thanks all. The vector needs to be initialized.
I am new in c++. I appreciate it if you cuold help me print the content of the following vector v. Here is the code:
#include <stdio.h>
#include <vector>
#include <iostream>
int main(int argc, char **argv)
{
std::cout << "hello world\n";
std::vector<int>* v; (<-- std::vector<int>* v = new std::vector<int>();]
v->push_back(17);
v->push_back(12);
v->push_back(23);
v->push_back(42);
for(std::vector<int>::iterator it = v->begin(); it!= v->end(); ++it) {
std::cout << *it << "\n";
}
std::cout <<"done\n";
return 0;
}

Answer 1

You missed to allocate memory for your vector pointer. I'd rather recommend just using a variable allocated on the stack:
#include <stdio.h>
#include <vector>
#include <iostream>
int main(int argc, char **argv)
{
std::cout << "hello world\n";
std::vector<int> v; // <<<<< Note that * was omitted here
v.push_back(17);
v.push_back(12);
v.push_back(23);
v.push_back(42);
for(std::vector<int>::iterator it = v.begin(); it!= v.end(); ++it) {
std::cout << *it << "\n";
}
std::cout <<"done\n";
return 0;
}
See the Live Demo.
As for your edit:
v will be used as follows:
void AddItem(uint64_t fkey, vector<uint64_t>* v) {
v->push_back(fkey);
}
Rather choose this signature:
void AddItem(uint64_t fkey, vector<uint64_t>& v) {
// ^ << Choose a reference
v.push_back(fkey);
// ^ << dereference accordingly
}

Answer 2

No need to make v a pointer here, use something like this instead:
#include <algorithm>
#include <iostream>
#include <iterator>
#include <vector>
int main() {
std::vector<int> v;
// push_back data...
std::copy(v.begin(), v.end(), std::ostream_iterator<int>(std::cout,","));
}
This will print out the contents of v from beginning to end separated by , chars.
If you really want to make v a pointer to a std::vector though, then try the following:
int main() {
std::vector<int>* v = new std::vector<int>(); // allocate memory on heap
v->push_back(17);
// etc...
std::copy(v->begin(), v->end(), std::ostream_iterator<int>(std::cout,","));
delete v; // make sure to delete from heap to prevent memory leak
}

Answer 3

I'm assuming this is not real code because there is no good reason to use a pointer to a vector in this case and the pointer doesn't point at anything.
But if your question is just how to print the contents of a vector given a pointer then you can simply dereference the pointer and use a range-based for loop.
#include <vector>
#include <iostream>
void print(std::vector<int>* v) {
for(int value : *v)
std::cout << value << "\n";
}
int main() {
std::cout << "hello world\n";
std::vector<int> v = {17, 12, 23, 42};
print(&v);
std::cout <<"done\n";
}
Live demo.