shared_readable_ptr.hpp

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#pragma once
 
/// @file userver/utils/shared_readable_ptr.hpp
/// @brief @copybrief utils::SharedReadablePtr
 
#include <memory>
#include <type_traits>
 
#include <userver/compiler/impl/lifetime.hpp>
 
USERVER_NAMESPACE_BEGIN
 
namespace utils {
 
/// @ingroup userver_universal userver_containers
///
/// @brief `std::shared_ptr<const T>` wrapper that makes sure that the pointer
/// is stored before dereferencing. Protects from dangling references.
///
/// Example of dangling:
/// @code
///   // BAD! Result of `cache.Get()` may be destroyed after the invocation.
///   const auto& snapshot = *cache.Get();
///   Use(snapshot);
/// @endcode
///
/// Such code is may work fine 99.9% of that time and, and such bugs are not detectable by most tests.
/// This is because typically the cache data will be held by the cache itself longer than the runtime
/// of the current handler (or whatever) that uses the data.
/// However, 0.1% of the time there will be a crash, because at that exact time the cache will update itself, replacing
/// the data snapshot and dropping the old shared pointer, which will turn out to be the only one.
///
/// The correct way to handle shared pointers:
/// @code
///   // Stores the shared pointer.
///   const auto snapshot = cache.Get();
///   // We only have the right to use `*snapshot` while we hold `snapshot` itself.
///   Use(*snapshot);
/// @endcode
template <typename T>
class SharedReadablePtr final {
    static_assert(!std::is_const_v<T>, "SharedReadablePtr already adds `const` to `T`");
    static_assert(!std::is_reference_v<T>, "SharedReadablePtr does not work with references");
 
public:
    using Base = std::shared_ptr<const T>;
    using MutableBase = std::shared_ptr<T>;
    using Weak = typename Base::weak_type;
    using Unique = std::unique_ptr<const T>;
    using element_type = T;
 
    SharedReadablePtr(const SharedReadablePtr& ptr) noexcept = default;
    SharedReadablePtr(SharedReadablePtr&& ptr) noexcept = default;
 
    constexpr SharedReadablePtr(std::nullptr_t) noexcept : base_(nullptr) {}
 
    SharedReadablePtr& operator=(const SharedReadablePtr& ptr) noexcept = default;
    SharedReadablePtr& operator=(SharedReadablePtr&& ptr) noexcept = default;
 
    SharedReadablePtr(const Base& ptr) noexcept : base_(ptr) {}
 
    SharedReadablePtr(Base&& ptr) noexcept : base_(std::move(ptr)) {}
 
    SharedReadablePtr(const MutableBase& ptr) noexcept : base_(ptr) {}
 
    SharedReadablePtr(MutableBase&& ptr) noexcept : base_(std::move(ptr)) {}
 
    SharedReadablePtr(Unique&& ptr) noexcept : base_(std::move(ptr)) {}
 
    SharedReadablePtr& operator=(const Base& ptr) noexcept {
        base_ = ptr;
        return *this;
    }
 
    SharedReadablePtr& operator=(Base&& ptr) noexcept {
        base_ = std::move(ptr);
        return *this;
    }
 
    SharedReadablePtr& operator=(const MutableBase& ptr) noexcept {
        base_ = ptr;
        return *this;
    }
 
    SharedReadablePtr& operator=(MutableBase&& ptr) noexcept {
        base_ = std::move(ptr);
        return *this;
    }
 
    SharedReadablePtr& operator=(std::nullptr_t) noexcept {
        Reset();
        return *this;
    }
 
    const T* Get() const& noexcept USERVER_IMPL_LIFETIME_BOUND { return base_.get(); }
 
    const T& operator*() const& noexcept USERVER_IMPL_LIFETIME_BOUND { return *base_; }
 
    const T& operator*() && { ReportMisuse(); }
 
    const T* operator->() const& noexcept USERVER_IMPL_LIFETIME_BOUND { return base_.get(); }
 
    const T* operator->() && { ReportMisuse(); }
 
    operator const Base&() const& noexcept USERVER_IMPL_LIFETIME_BOUND { return base_; }
 
    operator const Base&() && { ReportMisuse(); }
 
    operator Weak() const& noexcept USERVER_IMPL_LIFETIME_BOUND { return base_; }
 
    operator Weak() && { ReportMisuse(); }
 
    explicit operator bool() const noexcept { return !!base_; }
 
    bool operator==(const SharedReadablePtr<T>& other) const { return base_ == other.base_; }
 
    bool operator!=(const SharedReadablePtr<T>& other) const { return !(*this == other); }
 
    void Reset() noexcept { base_.reset(); }
 
private:
    [[noreturn]] static void ReportMisuse() { static_assert(!sizeof(T), "keep the pointer before using, please"); }
 
    Base base_;
};
 
template <typename T>
bool operator==(const SharedReadablePtr<T>& ptr, std::nullptr_t) {
    return !ptr;
}
 
template <typename T>
bool operator==(std::nullptr_t, const SharedReadablePtr<T>& ptr) {
    return !ptr;
}
 
template <typename T>
bool operator!=(const SharedReadablePtr<T>& ptr, std::nullptr_t) {
    return !(ptr == nullptr);
}
 
template <typename T>
bool operator!=(std::nullptr_t, const SharedReadablePtr<T>& ptr) {
    return !(nullptr == ptr);
}
 
template <typename T, typename... Args>
SharedReadablePtr<T> MakeSharedReadable(Args&&... args) {
    return SharedReadablePtr<T>{std::make_shared<T>(std::forward<Args>(args)...)};
}
 
}  // namespace utils
 
USERVER_NAMESPACE_END