This idea - of providing a handle that encapsulates both access to a synchronized object and the necessary locking - is not new: see Enforcing Correct Mutex Usage with Synchronized Values.
I liked Rook's idea to use a unique_ptr
with a custom deleter for the handle, so I came up with one:
template <typename BasicLockable>
class unlock_deleter {
std::unique_lock<BasicLockable> lock_;
public:
unlock_deleter(BasicLockable& mtx) : lock_{mtx} {}
unlock_deleter(BasicLockable& mtx, std::adopt_lock_t a) noexcept : lock_{mtx, a} {}
template <typename T>
void operator () (T*) const noexcept {
// no-op
}
};
template <typename T, typename M = std::mutex>
using locked_ptr = std::unique_ptr<T, unlock_deleter<M>>;
which is used in the implementation of a template class synchronized
that wraps an object and a mutex:
template <typename T, typename M = std::mutex>
class synchronized {
T item_;
mutable M mtx_;
// Implement Copy/Move construction
template <typename Other, typename N>
synchronized(Other&& other, const std::lock_guard<N>&) :
item_{std::forward<Other>(other).item_} {}
// Implement Copy/Move assignment
template <typename Other>
void assign(Other&& other) {
std::lock(mtx_, other.mtx_);
std::lock_guard<M> _{mtx_, std::adopt_lock};
std::lock_guard<decltype(other.mtx_)> _o{other.mtx_, std::adopt_lock};
item_ = std::forward<Other>(other).item_;
}
public:
synchronized() = default;
synchronized(const synchronized& other) :
synchronized(other, std::lock_guard<M>(other.mtx_)) {}
template <typename N>
synchronized(const synchronized<T, N>& other) :
synchronized(other, std::lock_guard<N>(other.mtx_)) {}
synchronized(synchronized&& other) :
synchronized(std::move(other), std::lock_guard<M>(other.mtx_)) {}
template <typename N>
synchronized(synchronized<T, N>&& other) :
synchronized(std::move(other), std::lock_guard<N>(other.mtx_)) {}
synchronized& operator = (const synchronized& other) & {
if (&other != this) {
assign(other);
}
return *this;
}
template <typename N>
synchronized& operator = (const synchronized<T, N>& other) & {
assign(other);
return *this;
}
synchronized& operator = (synchronized&& other) & {
if (&other != this) {
assign(std::move(other));
}
return *this;
}
template <typename N>
synchronized& operator = (synchronized<T, N>&& other) & {
assign(std::move(other));
return *this;
}
template <typename N>
void swap(synchronized<T, N>& other) {
if (static_cast<void*>(&other) != static_cast<void*>(this)) {
std::lock(mtx_, other.mtx_);
std::lock_guard<M> _{mtx_, std::adopt_lock};
std::lock_guard<N> _o{other.mtx_, std::adopt_lock};
using std::swap;
swap(item_, other.item_);
}
}
locked_ptr<T, M> data() & {
return locked_ptr<T, M>{&item_, mtx_};
}
locked_ptr<const T, M> data() const& {
return locked_ptr<const T, M>{&item_, mtx_};
}
};
template <typename T, typename M, typename N>
void swap(synchronized<T, M>& a, synchronized<T, N>& b) {
a.swap(b);
}
Taking care to synchronize copies/moves/swaps properly as well. Here's your sample program live at Coliru.