Why the [] operator in sorted_vector_map?

I do not understand what the operator [] does in sorted_vector_map.

• Specifically when the key does not exist, what value is added to the data structure?
• What is value_type(key, mapped_type())?
• Is it a constructor call to std::pair by default?
• What is mapped_type()?
• Is it also a constructor call?

mapped_type& operator[](const key_type& key) {
    iterator it = lower_bound(key);
    if (it == end() || key_comp()(key, it->first)) {
      return insert(it, value_type(key, mapped_type()))->second;
    }
    return it->second;
}

Code is from the following link...

https://github.com/facebook/folly/blob/master/folly/sorted_vector_types.h#L1097

Container is the template argument that defines what container is used by sorted_vector_map to store the key-value pairs and defaults to a std::vector(std::vector<std::pair<Key, Value>, Allocator>>)

value_type is Container::value_type (typedef typename Container::value_type value_type;) which (for the default template argument) is std::pair<Key, Value> (see std::vector Member types)

mapped_type is Value (typedef Value mapped_type;) so the type of the value stored in the sorted_vector_map

What is value_type(key, mapped_type())?
What is mapped_type()?
Is it also a constructor call?

So value_type(key, mapped_type()) creates a std::pair with key as first, and a default constructed Value (mapped_type()) as second.

Is it a constructor call to std::pair by default?

yes

template <
    class Key,
    class Value, // <<===============
    class Compare = std::less<Key>,
    class Allocator = std::allocator<std::pair<Key, Value>>,
    class GrowthPolicy = void,
    class Container = std::vector<std::pair<Key, Value>, Allocator>>   // <<===============
class sorted_vector_map : detail::growth_policy_wrapper<GrowthPolicy> {
  detail::growth_policy_wrapper<GrowthPolicy>& get_growth_policy() {
    return *this;
  }

  template <typename K, typename V, typename C = Compare>
  using if_is_transparent =
      _t<detail::sorted_vector_enable_if_is_transparent<void, C, K, V>>;

  struct EBO;

 public:
  typedef Key key_type;
  typedef Value mapped_type; // <<===============
  typedef typename Container::value_type value_type; // <<===============
  typedef Compare key_compare;
  typedef Allocator allocator_type;
  typedef Container container_type;