C++ Dynamic Arrays

1. What is dynamic array

   A dynamic array is quite similar to a regular array, but its size is modifiable during program runtime. It automatically grows when we try to make an insertion and there is no more space left for the new item. In C++ there is std::vector class that is dynamic array.

2. How to use std::vector class

   To create dynamic array we use std::vector class from #include <vector> and then in <> you specify data type that vector will contain.

   #include <vector>
    
   struct Entity{
       float m_x, m_y;
       Entity(x, y) :m_x(x), m_y(y) {}
   };
    
   std::vector<Entity> entities;

   To add something to vector we use push_back() method. It creates an object and then copies it into vector.

   #include <vector>
    
   struct Entity{
       float m_x, m_y;
       Entity(float x, float y) :m_x(x), m_y(y) {}
   };
    
   std::vector<Entity> entities;
   entities.push_back(Entity(1, 2));

   To erase from vector we use erase() method

   #include <vector>
    
   struct Entity{
       float m_x, m_y;
       Entity(float x, float y) :m_x(x), m_y(y) {}
   };
    
   std::vector<Entity> entities;
   entities.push_back(Entity(1, 2));
   entities.push_back(Entity(2, 2));
    
   entities.erase(entities.begin() + 1); // it will erase second element

   If you know space you will need you can allocate space using reserve() method. It will boost performance because it won't need to resize vector.

   #include <vector>
    
   struct Entity{
       float m_x, m_y;
       Entity(float x, float y) :m_x(x), m_y(y) {}
   };
    
   std::vector<Entity> entities;
   entities.reserve(2); // you reserve space for 2 entities objects
   entities.push_back(Entity(1, 2));
   entities.push_back(Entity(2, 2));

   emplace_back() method constructs object in place in memory where vector is. No copy will be made like with push_back() method. You dont pass constructor for object but you pass parameter list for the constructor.

   #include <vector>
    
   struct Entity{
       float m_x, m_y;
       Entity(float x, float y) :m_x(x), m_y(y) {}
   };
    
   std::vector<Entity> entities;
   entities.emplace_back(1, 2);