Integer Array to Vector

Question

I'm trying to add an int array into a vector. No matter what I do, the debugger indicates the new vector element is simply "0".

std::vector< int * > level_collection;
for( auto & i : levels )
{
     auto size = std::get< 1 >(i).size();
     int level_data[size];
     for( size_t x = 0; x < size; x ++ )
     {
          level_data[x] = std::get< 1 >(i)[x];
     }

     for( auto x : level_data)
     {
          std::cout << x << std::endl; // This works. All the values print correctly. So it did store the information as it should.
     }

     level_collection.push_back( level_data );
 }
 for( auto & i : level_collection)
 {
     std::cout << i[1] << std::endl; // This prints ALL 0s. Despite the new element not having that value.
 }

I've been looking hours for a solution. I can't seem to find any.

What I've tried:

Changing the location of level_data so it's not always reinitialized every time.
Removing level_data and instead adding a new element to level_collection and copying the values returned in std::get directly into the vector
Checked through the debugger and output that level_data does indeed return the correct array with the correct values. However, the moment it's added to level_collection, it turns to 0. (Doesn't matter if I try the other things I mentioned above)

I'm sure there's an easy solution here. I might just be overlooking something.

Edit: I unfortunately cannot use std::array in this assignment. We were told to work off of what's available. The function level_collection gets passed to is a "const int *"

Which int array are you to add to which vector here? I'm assuming level_data to level_collection? — user3684240
– user3684240, Commented Sep 23, 2019 at 19:42
Is size a constant expression? If not, int level_data[size]; is not standard C++. Also, level_data's "pointer" you put in the vector is invalid outside the scope in which level_data is declared. — crashmstr
– crashmstr, Commented Sep 23, 2019 at 19:43
Consider using std::array. Each array, that you push_backed gets destroyed at the end of each loop iteration. Hence, each pointer is a dangling pointer, dereferencing which, leads to undefined behavior. — Algirdas Preidžius
– Algirdas Preidžius, Commented Sep 23, 2019 at 19:44

user3684240 · Accepted Answer · 2019-09-23 19:49:15Z

When you push_back a pointer to a vector, you do not actually preserve the memory that this pointer points to. Therefore, this does not work here.

Instead, you should use a vector of an object which owns the int array, such as another std::vector. You only have to change two lines:

std::vector< std::vector<int> > level_collection; // CHANGED
for( auto & i : levels )
{
     auto size = std::get< 1 >(i).size();
     std::vector<int> level_data{size}; // CHANGED
     for( size_t x = 0; x < size; x ++ )
     {
          level_data[x] = std::get< 1 >(i)[x];
     }

     for( auto x : level_data)
     {
          std::cout << x << std::endl; // This works. All the values print correctly. So it did store the information as it should.
     }

     level_collection.push_back( level_data );
 }
 for( auto & i : level_collection)
 {
     std::cout << i[1] << std::endl; // This prints ALL 0s. Despite the new element not having that value.
 }

Note that that's assuming you actually want a two-dimensional vector. If you don't, you can just push_back your ints directly to level_collection which should then be defined as a std::vector<int>.

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