metric_value.hpp

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#pragma once
 
/// @file userver/utils/statistics/metric_value.hpp
/// @brief @copybrief utils::statistics::MetricValue
 
#include <cstdint>
#include <type_traits>
#include <variant>
 
#include <userver/utils/statistics/histogram_view.hpp>
#include <userver/utils/statistics/rate.hpp>
 
USERVER_NAMESPACE_BEGIN
 
namespace utils::statistics {
 
/// @brief The value of a metric.
///
/// Cheap to copy, expected to be passed around by value.
class MetricValue final {
 public:
  using RawType = std::variant<std::int64_t, double, Rate, HistogramView>;
 
  // trivially copyable
  MetricValue(const MetricValue&) = default;
  MetricValue& operator=(const MetricValue&) = default;
 
  bool operator==(const MetricValue& other) const noexcept {
    return value_ == other.value_;
  }
 
  bool operator!=(const MetricValue& other) const noexcept {
    return value_ != other.value_;
  }
 
  /// @brief Retrieve the value of an integer metric.
  /// @throws std::exception on type mismatch.
  std::int64_t AsInt() const { return std::get<std::int64_t>(value_); }
 
  /// @brief Retrieve the value of a floating-point metric.
  /// @throws std::exception on type mismatch.
  double AsFloat() const { return std::get<double>(value_); }
 
  /// @brief Retrieve the value of a Rate metric.
  /// @throws std::exception on type mismatch.
  Rate AsRate() const { return std::get<Rate>(value_); }
 
  /// @brief Returns whether metric is Rate metric.
  bool IsRate() const noexcept { return std::holds_alternative<Rate>(value_); }
 
  /// @brief Retrieve the value of a HistogramView metric.
  /// @throws std::exception on type mismatch.
  HistogramView AsHistogram() const { return std::get<HistogramView>(value_); }
 
  /// @brief Returns whether metric is HistogramView metric.
  bool IsHistogram() const noexcept {
    return std::holds_alternative<HistogramView>(value_);
  }
 
  /// @brief Calls @p visitor with either a `std::int64_t` or a `double` value.
  /// @returns Whatever @p visitor returns.
  template <typename VisitorFunc>
  decltype(auto) Visit(VisitorFunc visitor) const {
    return std::visit(visitor, value_);
  }
 
  /// @cond
  MetricValue() noexcept : value_(std::int64_t{0}) {}
 
  explicit MetricValue(RawType value) noexcept : value_(value) {}
  /// @endcond
 
 private:
  RawType value_;
};
 
}  // namespace utils::statistics
 
USERVER_NAMESPACE_END