fast_pimpl.hpp

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#pragma once
 
/// @file userver/utils/fast_pimpl.hpp
/// @brief @copybrief utils::FastPimpl
 
#include <cstddef>
#include <memory>
#include <new>
#include <type_traits>
#include <utility>
 
#include <userver/compiler/impl/lifetime.hpp>
 
USERVER_NAMESPACE_BEGIN
 
namespace utils {
 
/// @brief Helper constant to use with FastPimpl
inline constexpr bool kStrictMatch = true;
 
/// @ingroup userver_universal userver_containers
///
/// @brief Implements pimpl idiom without dynamic memory allocation.
///
/// FastPimpl doesn't require either memory allocation or indirect memory
/// access. But you have to manually set object size when you instantiate
/// FastPimpl.
///
/// ## Example usage:
/// Take your class with pimpl via smart pointer and
/// replace the smart pointer with utils::FastPimpl<Impl, Size, Alignment>
/// @snippet utils/widget_fast_pimpl_test.hpp  FastPimpl - header
///
/// If the Size and Alignment are unknown - just put a random ones and
/// the compiler would show the right ones in the error message:
/// @code
/// In instantiation of 'void FastPimpl<T, Size, Alignment>::Validate()
/// [with int ActualSize = 1; int ActualAlignment = 8; T = sample::Widget;
/// int Size = 8; int Alignment = 8]'
/// @endcode
///
/// Change the initialization in source file to not allocate for pimpl
/// @snippet utils/widget_fast_pimpl_test.cpp  FastPimpl - source
///
/// Done! Now you can use the header without exposing the implementation
/// details:
/// @snippet utils/fast_pimpl_test.cpp  FastPimpl - usage
template <class T, std::size_t Size, std::size_t Alignment, bool Strict = false>
class FastPimpl final {
public:
    // NOLINTNEXTLINE(cppcoreguidelines-pro-type-member-init,performance-noexcept-move-constructor)
    FastPimpl(FastPimpl&& v) noexcept(noexcept(T(std::declval<T>())))
        : FastPimpl(std::move(*v))
    {}
 
    // NOLINTNEXTLINE(cppcoreguidelines-pro-type-member-init)
    FastPimpl(const FastPimpl& v) noexcept(noexcept(T(std::declval<const T&>())))
        : FastPimpl(*v)
    {}
 
    // NOLINTNEXTLINE(bugprone-unhandled-self-assignment,cert-oop54-cpp)
    FastPimpl& operator=(const FastPimpl& rhs) noexcept(noexcept(std::declval<T&>() = std::declval<const T&>())) {
        *AsHeld() = *rhs;
        return *this;
    }
 
    FastPimpl& operator=(FastPimpl&& rhs) noexcept(
        // NOLINTNEXTLINE(performance-noexcept-move-constructor)
        noexcept(std::declval<T&>() = std::declval<T>())
    ) {
        *AsHeld() = std::move(*rhs);
        return *this;
    }
 
    template <typename... Args>
    // NOLINTNEXTLINE(cppcoreguidelines-pro-type-member-init)
    explicit FastPimpl(Args&&... args) noexcept(noexcept(T(std::declval<Args>()...))) {
        ::new (AsHeld()) T(std::forward<Args>(args)...);
    }
 
    T* operator->() noexcept USERVER_IMPL_LIFETIME_BOUND { return AsHeld(); }
 
    const T* operator->() const noexcept USERVER_IMPL_LIFETIME_BOUND { return AsHeld(); }
 
    T& operator*() noexcept USERVER_IMPL_LIFETIME_BOUND { return *AsHeld(); }
 
    const T& operator*() const noexcept USERVER_IMPL_LIFETIME_BOUND { return *AsHeld(); }
 
    ~FastPimpl() noexcept {
        Validate<sizeof(T), alignof(T), noexcept(std::declval<T*>()->~T())>();
        std::destroy_at(AsHeld());
    }
 
private:
    // Use a template to make actual sizes visible in the compiler error message.
    template <std::size_t ActualSize, std::size_t ActualAlignment, bool ActualNoexcept>
    static void Validate() noexcept {
        static_assert(Size >= ActualSize, "invalid Size: Size >= sizeof(T) failed");
        static_assert(!Strict || Size == ActualSize, "invalid Size: Size == sizeof(T) failed");
 
        static_assert(Alignment % ActualAlignment == 0, "invalid Alignment: Alignment % alignof(T) == 0 failed");
        static_assert(!Strict || Alignment == ActualAlignment, "invalid Alignment: Alignment == alignof(T) failed");
 
        static_assert(ActualNoexcept, "Destructor of FastPimpl is marked as noexcept, the ~T() is not");
    }
 
    alignas(Alignment) std::byte storage_[Size];
 
    T* AsHeld() noexcept { return reinterpret_cast<T*>(&storage_); }
 
    const T* AsHeld() const noexcept { return reinterpret_cast<const T*>(&storage_); }
};
 
}  // namespace utils
 
USERVER_NAMESPACE_END