String.h

#ifndef STRING_H
#define STRING_H

#include <initializer_list>
#include <memory>
#include <string>

class String {
public:
  typedef size_t size_type;
  typedef char *iterator;
  typedef const char *const_iterator;
  typedef char &reference;
  typedef const char &const_reference;

  String() : first_elem(nullptr), first_free(nullptr), cap(nullptr) { }
  String(const_iterator);
  String(std::initializer_list<char>);

  ~String();
  String(const String &);
  String &operator=(const String &);
  String(String &&) noexcept;
  String &operator=(String &&) noexcept;

  String &operator=(const_iterator);

  void push_back(const_reference);
  void pop_back();

  void reserve(size_type);
  void resize(size_type, const_reference = char());
  // default value for parameter put in declaration

  bool empty() const { return cbegin() == cend(); }
  size_type size() const { return first_free - first_elem; }
  size_type capacity() const { return cap - first_elem; }

  iterator begin() { return first_elem; }
  iterator end() { return first_free; }
  const_iterator begin() const { return first_elem; }
  const_iterator end() const { return first_free; }
  const_iterator cbegin() const { return begin(); }
  const_iterator cend() const { return end(); }

  std::string str() const;

private:
  static std::allocator<char> alloc;

  iterator first_elem;
  iterator first_free;
  iterator cap;
  // These three pointer should be either all `nullptr`s or all pointing to the
  // same memory block. (Class invariant)

  void reallocate(size_type);
  void free();
};

#endif