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#pragma once
#include <memory>
namespace mbgl {
/**
* `Mutable<T>` is a non-nullable uniquely owning reference to a `T`. It can be efficiently converted
* to `Immutable<T>`.
*
* The lifecycle of `Mutable<T>` and `Immutable<T>` is as follows:
*
* 1. Create a `Mutable<T>` using `makeMutable(...)`
* 2. Mutate it freely
* 3. When you're ready to freeze its state and enable safe cross-thread sharing, move assign or
* move construct it to `Immutable<T>`
*
* The reason that `Mutable<T>` exists, rather than simply using a `std::unique_ptr<T>`, is to take advantage
* of the underlying single-allocation optimization provided by `std::make_shared`.
*/
template <class T>
class Mutable {
public:
Mutable(Mutable&&) = default;
Mutable& operator=(Mutable&&) = default;
Mutable(const Mutable&) = delete;
Mutable& operator=(const Mutable&) = delete;
T* get() { return ptr.get(); }
T* operator->() { return ptr.get(); }
T& operator*() { return *ptr; }
private:
Mutable(std::shared_ptr<T>&& s)
: ptr(std::move(s)) {}
std::shared_ptr<T> ptr;
template <class S> friend class Immutable;
template <class S, class... Args> friend Mutable<S> makeMutable(Args&&...);
template <class S, class U> friend Mutable<S> staticMutableCast(const Mutable<U>&);
};
template <class T, class... Args>
Mutable<T> makeMutable(Args&&... args) {
return Mutable<T>(std::make_shared<T>(std::forward<Args>(args)...));
}
template <class S, class U>
Mutable<S> staticMutableCast(const Mutable<U>& u) {
return Mutable<S>(std::static_pointer_cast<S>(u.ptr));
}
/**
* `Immutable<T>` is a non-nullable shared reference to a `const T`. Construction requires
* a transfer of unique ownership from a `Mutable<T>`; once constructed it has the same behavior
* as `std::shared_ptr<const T>` but with better indication of intent.
*
* Normally one should not share state between threads because it's difficult to verify the
* absence of read/write data races. `Immutable` provides a guarantee that no writes are
* possible, and instances therefore can be freely transferred and shared between threads.
*/
template <class T>
class Immutable {
public:
template <class S>
Immutable(Mutable<S>&& s)
: ptr(std::const_pointer_cast<const S>(std::move(s.ptr))) {}
template <class S>
Immutable(Immutable<S> s)
: ptr(std::move(s.ptr)) {}
Immutable(Immutable&&) = default;
Immutable(const Immutable&) = default;
template <class S>
Immutable& operator=(Mutable<S>&& s) {
ptr = std::const_pointer_cast<const S>(std::move(s.ptr));
return *this;
}
Immutable& operator=(Immutable&&) = default;
Immutable& operator=(const Immutable&) = default;
const T* get() const { return ptr.get(); }
const T* operator->() const { return ptr.get(); }
const T& operator*() const { return *ptr; }
friend bool operator==(const Immutable<T>& lhs, const Immutable<T>& rhs) {
return lhs.ptr == rhs.ptr;
}
friend bool operator!=(const Immutable<T>& lhs, const Immutable<T>& rhs) {
return lhs.ptr != rhs.ptr;
}
private:
Immutable(std::shared_ptr<const T>&& s)
: ptr(std::move(s)) {}
std::shared_ptr<const T> ptr;
template <class S> friend class Immutable;
template <class S, class U> friend Immutable<S> staticImmutableCast(const Immutable<U>&);
};
template <class S, class U>
Immutable<S> staticImmutableCast(const Immutable<U>& u) {
return Immutable<S>(std::static_pointer_cast<const S>(u.ptr));
}
/**
* Constrained mutation of an immutable reference. Makes a temporarily-mutable copy of the
* input Immutable using the inner type's copy constructor, runs the given callable on the
* mutable copy, and then freezes the copy and reassigns it to the input reference.
*
* Note that other Immutables referring to the same inner instance are not affected; they
* continue to referencing the original immutable instance.
*/
template <class T, class Fn>
void mutate(Immutable<T>& immutable, Fn&& fn) {
Mutable<T> mut = makeMutable<T>(*immutable);
std::forward<Fn>(fn)(*mut);
immutable = std::move(mut);
}
} // namespace mbgl
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