connection.hpp

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#pragma once
 
#include <cassert>
#include <memory>
#include <type_traits>
 
namespace opcua::detail {
 
/**
 * Helper base class to derive a wrapper (e.g. `Server&`) from its underlying connection.
 * The single connection pointer member is the same as in the parent connection class. A custom
 * deleter is provided to prevent circular dependencies and double deletion.
 * Because the connection pointer is the first and single member of the wrapper class, the
 * connection pointer and the wrapper class are pointer-interconvertible (standard-layout required).
 */
template <typename WrapperType>
class ConnectionBase {
public:
    static_assert(sizeof(WrapperType) == sizeof(std::unique_ptr<ConnectionBase>));
 
    // unique_ptr<T> is standard layout but clang disagrees:
    // https://github.com/llvm/llvm-project/issues/53021
    // static_assert(std::is_standard_layout_v<WrapperType>);
 
    ConnectionBase() noexcept
        : connectionPtr_(this) {}
 
    ~ConnectionBase() = default;
 
    ConnectionBase(const ConnectionBase&) = delete;
    ConnectionBase(ConnectionBase&&) noexcept = delete;
    ConnectionBase& operator=(const ConnectionBase&) = delete;
    ConnectionBase& operator=(ConnectionBase&&) noexcept = delete;
 
    [[nodiscard]] WrapperType* wrapperPtr() noexcept {
        assert(connectionPtr_ != nullptr);
        // NOLINTNEXTLINE(bugprone-casting-through-void)
        return static_cast<WrapperType*>(static_cast<void*>(&connectionPtr_));
    };
 
    [[nodiscard]] WrapperType& wrapper() noexcept {
        return *wrapperPtr();
    }
 
private:
    struct NoDeleter {
        constexpr void operator()(ConnectionBase* /* unused */) noexcept {};
    };
 
    std::unique_ptr<ConnectionBase, NoDeleter> connectionPtr_;
};
 
}  // namespace opcua::detail