Allocating Array of Pointers on Heap - c++

Question

Does the following create x new objects, or simply allocate space for x objects?:
Vector3D* binArray = new Vector3D[size];
I need to build an array with space for x Vector3D objects on the heap. However, a Vector3D object can only be created when an "add" function is called - this will take the parameters, construct the object on the heap and add its address to the array of Vector3D pointers.

Answer 1

This does create an array of Vector3D objects on the heap.
Each vector is created by calling the Vector3D constructor.
Put a little debugging print statement in the default constructor for Vector3D, and watch the constructor get called the same number of times as you have vectors in your array.
Example:
#include <iostream>
using namespace std;
class C {
public:
C() { cout << "Hello default constructor here\n"; }
};
int main() {
C* cs = new C[5];
}
Output is:
Hello default constructor here
Hello default constructor here
Hello default constructor here
Hello default constructor here
Hello default constructor here
If your class does not have a default constructor, you cannot allocate the array in one shot (thank you for the comment #Everyone), so in this case consider using a std::vector or a std::array and adding your Vector3D objects dynamically --- or even "statically"! Example:
#include <iostream>
#include <vector>
using namespace std;
class Vector3D {
double i, j, k;
public:
Vector3D(double i, double j, double k): i(i), j(j), k(k) {}
};
int main() {
vector<Vector3D> v = {
Vector3D(3, 4, 5),
Vector3D(6, 8, 10),
Vector3D(7, 24, 25)
};
v.push_back(Vector3D(1, 2, 3));
cout << v.size() << '\n';
}
This outputs 4.
You can also make your vector contain pointers to Vector3D objects.

Answer 2

Just to add based on the asker's comments on RayToal's excellent answer. If you don't know the size of the binArray prior to runtime then you must use std::vector. If you want to allocate each item alone, I would recommend using std::vector<Vector3D*>.
This way you can resize the std::vector at runtime and when you do, it will hold a bunch of nullptrs that are not allocated. Then you can allocate each one of them separately.
std::vector<Vector3D*> binArray;
binArray.resize(x); // now you have binArray of size x and no allocated elements
binArray[0] = new Vector3D(...);
Please keep in mind that you need to delete them after you're not using them in order to not have a memory leak:
for(size_t i=0;i<binArray.size(); i++)
if(binArray[i]!=nullptr) delete binArray[i];