box.hpp

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#pragma once
 
/// @file userver/utils/box.hpp
/// @brief @copybrief utils::Box
 
#include <concepts>
#include <memory>
#include <type_traits>
#include <utility>
 
#include <userver/formats/parse/to.hpp>
#include <userver/formats/serialize/to.hpp>
#include <userver/logging/log_helper_fwd.hpp>
#include <userver/utils/assert.hpp>
 
USERVER_NAMESPACE_BEGIN
 
namespace utils {
 
template <typename T>
class Box;
 
namespace impl {
 
template <typename T>
struct IsBox : std::false_type {};
 
template <typename... Args>
struct IsBox<Box<Args...>> : std::true_type {};
 
}  // namespace impl
 
/// @brief Remote storage for a single item. Implemented as a unique pointer
/// that is never `null`, except when moved from.
///
/// Has the semantics of non-optional `T`.
/// Copies the content on copy, compares by the contained value.
///
/// Use in the following cases:
/// - to create recursive types while maintaining value semantics;
/// - to hide the implementation of a class in cpp;
/// - to prevent the large size or alignment of a field from inflating the size
///   or alignment of an object.
///
/// Use utils::UniqueRef instead:
/// - to add a non-movable field to a movable object;
/// - to own an object of a polymorphic base class.
///
/// Usage example:
/// @snippet universal/src/utils/box_test.cpp  sample
template <typename T>
class Box {
public:
    /// Allocate a default-constructed value.
    // Would like to use SFINAE here, but std::optional<Box> requests tests for
    // default construction eagerly, which errors out for a forward-declared T.
    Box()
        : data_(std::make_unique<T>())
    {}
 
    /// Allocate a `T`, copying or moving @a arg.
    ///
    /// `explicit` if and only if `T`'s constructor is `explicit`.
    /// Split into `explicit` / `explicit(false)` overloads due to limitations of clang-16.
    template <typename U = T>
    // Protect against hiding special constructors.
    requires(!std::same_as<std::remove_cvref_t<U>, Box>) &&
            // Prevent infinite recursion from constructible_from in the next check.
            (!impl::IsBox<std::remove_cvref_t<U>>::value) &&
            // Prevent infinite recursion.
            (std::same_as<std::remove_cvref_t<U>, T> || !std::is_constructible_v<T, Box>) &&
            // Normal requirement.
            std::is_convertible_v<U, T>
    explicit(false) Box(U&& arg)
        : data_(std::make_unique<T>(std::forward<U>(arg)))
    {}
 
    /// @overload
    template <typename U = T>
    // Protect against hiding special constructors.
    requires(!std::same_as<std::remove_cvref_t<U>, Box>) &&
            // Prevent infinite recursion from constructible_from in the next check.
            (!impl::IsBox<std::remove_cvref_t<U>>::value) &&
            // Prevent infinite recursion.
            (std::same_as<std::remove_cvref_t<U>, T> || !std::is_constructible_v<T, Box>) &&
            // Normal requirement.
            std::is_constructible_v<T, U> && (!std::is_convertible_v<U, T>)
    explicit Box(U&& arg)
        : data_(std::make_unique<T>(std::forward<U>(arg)))
    {}
 
    /// Allocate the value, emplacing it with the given @a args.
    template <typename... Args>
    requires(sizeof...(Args) >= 2) && std::is_constructible_v<T, Args...>
    explicit Box(Args&&... args)
        : data_(std::make_unique<T>(std::forward<Args>(args)...))
    {}
 
    /// Allocate the value as constructed by the given @a factory.
    /// Allows to save an extra move of the contained value.
    template <typename Factory>
    static Box MakeWithFactory(Factory&& factory) {
        return Box(EmplaceFactory{}, std::forward<Factory>(factory));
    }
 
    Box(Box&& other) noexcept = default;
    Box& operator=(Box&& other) noexcept = default;
 
    Box(const Box& other)
        : data_(std::make_unique<T>(*other))
    {}
 
    Box& operator=(const Box& other) {
        *this = Box{other};
        return *this;
    }
 
    /// Assigns-through to the contained value.
    template <typename U = T>
    requires(!std::same_as<std::remove_cvref_t<U>, Box>) && std::is_assignable_v<T&, U>
    Box& operator=(U&& other) {
        if (data_) {
            *data_ = std::forward<U>(other);
        } else {
            data_ = std::make_unique<T>(std::forward<U>(other));
        }
        return *this;
    }
 
    // Box is always engaged, unless moved-from. Just call *box.
    /*implicit*/ operator bool() const = delete;
 
    T* operator->() noexcept { return Get(); }
    const T* operator->() const noexcept { return Get(); }
 
    T& operator*() noexcept { return *Get(); }
    const T& operator*() const noexcept { return *Get(); }
 
    bool operator==(const Box& other) const { return **this == *other; }
 
    bool operator!=(const Box& other) const { return **this != *other; }
 
    bool operator<(const Box& other) const { return **this < *other; }
 
    bool operator>(const Box& other) const { return **this > *other; }
 
    bool operator<=(const Box& other) const { return **this <= *other; }
 
    bool operator>=(const Box& other) const { return **this >= *other; }
 
private:
    struct EmplaceFactory final {};
 
    template <typename Factory>
    explicit Box(EmplaceFactory, Factory&& factory)
        : data_(new T(std::forward<Factory>(factory)()))
    {}
 
    T* Get() noexcept {
        UASSERT_MSG(data_, "Accessing a moved-from Box");
        return data_.get();
    }
 
    const T* Get() const noexcept {
        UASSERT_MSG(data_, "Accessing a moved-from Box");
        return data_.get();
    }
 
    std::unique_ptr<T> data_;
};
 
template <typename Value, typename T>
Box<T> Parse(const Value& value, formats::parse::To<Box<T>>) {
    return Box<T>::MakeWithFactory([&value] { return value.template As<T>(); });
}
 
template <typename Value, typename T>
Value Serialize(const Box<T>& value, formats::serialize::To<Value>) {
    return Serialize(*value, formats::serialize::To<Value>{});
}
 
template <typename StringBuilder, typename T>
void WriteToStream(const Box<T>& value, StringBuilder& sw) {
    WriteToStream(*value, sw);
}
 
template <typename T>
logging::LogHelper& operator<<(logging::LogHelper& lh, const Box<T>& box) {
    lh << *box;
    return lh;
}
 
}  // namespace utils
 
USERVER_NAMESPACE_END