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// Copyright (C) 2023 The Qt Company Ltd.
// SPDX-License-Identifier: LicenseRef-Qt-Commercial OR LGPL-3.0-only OR GPL-2.0-only OR GPL-3.0-only
#ifndef QUNIQUEHANDLE_P_H
#define QUNIQUEHANDLE_P_H
//
// W A R N I N G
// -------------
//
// This file is not part of the Qt API. It exists purely as an
// implementation detail. This header file may change from version to
// version without notice, or even be removed.
//
// We mean it.
//
#include <QtCore/qtconfigmacros.h>
#include <QtCore/qassert.h>
#include <memory>
QT_BEGIN_NAMESPACE
/*! \internal QUniqueHandle is a general purpose RAII wrapper intended
for interfacing with resource-allocating C-style APIs, for example
operating system APIs, database engine APIs, or any other scenario
where resources are allocated and released, and where pointer
semantics does not seem a perfect fit.
QUniqueHandle does not support copying, because it is intended to
maintain ownership of resources that can not be copied. This makes
it safer to use than naked handle types, since ownership is
maintained by design.
The underlying handle object is described using a client supplied
HandleTraits object that is implemented per resource type. The
traits struct must describe two properties of a handle:
1) What value is considered invalid
2) How to close a resource.
Example 1:
struct InvalidHandleTraits {
using Type = HANDLE;
static Type invalidValue() {
return INVALID_HANDLE_VALUE;
}
static bool close(Type handle) {
return CloseHandle(handle) != 0;
}
}
using FileHandle = QUniqueHandle<InvalidHandleTraits>;
Usage:
// Takes ownership of returned handle.
FileHandle handle{ CreateFile(...) };
if (!handle.isValid()) {
qDebug() << GetLastError()
return;
}
...
Example 2:
struct SqLiteTraits {
using Type = sqlite3*;
static Type invalidValue() {
return nullptr;
}
static bool close(Type handle) {
sqlite3_close(handle);
return true;
}
}
using DbHandle = QUniqueHandle<SqLiteTraits>;
Usage:
DbHandle h;
// Take ownership of returned handle.
int result = sqlite3_open(":memory:", &h);
...
NOTE: The QUniqueHandle assumes that closing a resource is
guaranteed to succeed, and provides no support for handling failure
to close a resource. It is therefore only recommended for use cases
where failure to close a resource is either not an error, or an
unrecoverable error.
*/
// clang-format off
template <typename HandleTraits>
class QUniqueHandle
{
public:
using Type = typename HandleTraits::Type;
QUniqueHandle() = default;
explicit QUniqueHandle(const Type &handle) noexcept
: m_handle{ handle }
{}
QUniqueHandle(QUniqueHandle &&other) noexcept
: m_handle{ other.release() }
{}
~QUniqueHandle() noexcept
{
close();
}
QUniqueHandle& operator=(QUniqueHandle &&rhs) noexcept
{
if (this != std::addressof(rhs))
reset(rhs.release());
return *this;
}
QUniqueHandle(const QUniqueHandle &) = delete;
QUniqueHandle &operator=(const QUniqueHandle &) = delete;
[[nodiscard]] bool isValid() const noexcept
{
return m_handle != HandleTraits::invalidValue();
}
[[nodiscard]] explicit operator bool() const noexcept
{
return isValid();
}
[[nodiscard]] Type get() const noexcept
{
return m_handle;
}
void reset(const Type& handle) noexcept
{
if (handle == m_handle)
return;
close();
m_handle = handle;
}
[[nodiscard]] Type release() noexcept
{
Type handle = m_handle;
m_handle = HandleTraits::invalidValue();
return handle;
}
[[nodiscard]] Type *operator&() noexcept // NOLINT(google-runtime-operator)
{
Q_ASSERT(!isValid());
return &m_handle;
}
void close() noexcept
{
if (!isValid())
return;
const bool success = HandleTraits::close(m_handle);
Q_ASSERT(success);
m_handle = HandleTraits::invalidValue();
}
[[nodiscard]] friend bool operator==(const QUniqueHandle &lhs, const QUniqueHandle &rhs) noexcept
{
return lhs.get() == rhs.get();
}
[[nodiscard]] friend bool operator!=(const QUniqueHandle &lhs, const QUniqueHandle &rhs) noexcept
{
return lhs.get() != rhs.get();
}
[[nodiscard]] friend bool operator<(const QUniqueHandle &lhs, const QUniqueHandle &rhs) noexcept
{
return lhs.get() < rhs.get();
}
[[nodiscard]] friend bool operator<=(const QUniqueHandle &lhs, const QUniqueHandle &rhs) noexcept
{
return lhs.get() <= rhs.get();
}
[[nodiscard]] friend bool operator>(const QUniqueHandle &lhs, const QUniqueHandle &rhs) noexcept
{
return lhs.get() > rhs.get();
}
[[nodiscard]] friend bool operator>=(const QUniqueHandle &lhs, const QUniqueHandle &rhs) noexcept
{
return lhs.get() >= rhs.get();
}
private:
Type m_handle{ HandleTraits::invalidValue() };
};
// clang-format on
QT_END_NAMESPACE
#endif
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