items.hpp

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#pragma once
 
/// @file userver/formats/common/items.hpp
/// @brief @copybrief formats::common::Items()
/// @ingroup userver_universal
 
#include <cstddef>
#include <iterator>
#include <string>
#include <type_traits>
 
USERVER_NAMESPACE_BEGIN
 
namespace formats::common {
 
/// @brief Wrapper for handy python-like iteration over a map.
///
/// See formats::common::Items() for usage example
template <typename Value>
class ItemsWrapper final {
 public:
  class Iterator {
   public:
    using RawIterator = decltype(std::declval<Value>().begin());
 
    struct ItValue {
      std::string key;
      typename RawIterator::reference value;
    };
 
    using iterator_category = std::forward_iterator_tag;
    using difference_type = std::ptrdiff_t;
    using value_type = ItValue;
    using reference = ItValue;
 
    explicit Iterator(RawIterator it) : it_(it) {}
    Iterator(const Iterator& other) = default;
    Iterator(Iterator&& other) noexcept = default;
 
    Iterator& operator=(const Iterator& other) = default;
    Iterator& operator=(Iterator&& other) noexcept = default;
 
    ItValue operator*() const { return {it_.GetName(), *it_}; }
 
    Iterator operator++(int) {
      auto it = *this;
      return {it++};
    }
 
    Iterator& operator++() {
      ++it_;
      return *this;
    }
 
    bool operator==(const Iterator& other) const { return it_ == other.it_; }
 
    bool operator!=(const Iterator& other) const { return !(*this == other); }
 
   private:
    RawIterator it_;
  };
 
  ItemsWrapper(Value&& value) : value_(static_cast<Value&&>(value)) {}
 
  auto begin() const { return cbegin(); }
  auto end() const { return cend(); }
  auto cbegin() const { return Iterator(value_.begin()); }
  auto cend() const { return Iterator(value_.end()); }
 
 private:
  Value value_;
};
 
/// @brief Wrapper for handy python-like iteration over a map
///
/// @code
///   for (const auto& [name, value]: Items(map)) ...
/// @endcode
template <typename Value>
ItemsWrapper<Value> Items(Value&& value) {
  // when passed an lvalue, store by reference
  // when passed an rvalue, store by value
  return ItemsWrapper<Value>(static_cast<Value&&>(value));
}
 
}  // namespace formats::common
 
USERVER_NAMESPACE_END