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Diffstat (limited to 'src/common/static_ptr.h')
| -rw-r--r-- | src/common/static_ptr.h | 341 |
1 files changed, 341 insertions, 0 deletions
diff --git a/src/common/static_ptr.h b/src/common/static_ptr.h new file mode 100644 index 000000000..31df4cf0a --- /dev/null +++ b/src/common/static_ptr.h @@ -0,0 +1,341 @@ +// -*- mode:C++; tab-width:8; c-basic-offset:2; indent-tabs-mode:t -*- +// vim: ts=8 sw=2 smarttab +/* + * Ceph - scalable distributed file system + * + * Copyright (C) 2017 Red Hat, Inc. + * + * This is free software; you can redistribute it and/or + * modify it under the terms of the GNU Lesser General Public + * License version 2.1, as published by the Free Software + * Foundation. See file COPYING. + * + */ + +#pragma once + +#include <cstddef> +#include <utility> +#include <type_traits> + +namespace ceph { +// `static_ptr` +// =========== +// +// It would be really nice if polymorphism didn't require a bunch of +// mucking about with the heap. So let's build something where we +// don't have to do that. +// +namespace _mem { + +// This, an operator function, is one of the canonical ways to do type +// erasure in C++ so long as all operations can be done with subsets +// of the same arguments (which is not true for function type erasure) +// it's a pretty good one. +enum class op { + move, destroy, size +}; +template<typename T> +static std::size_t op_fun(op oper, void* p1, void* p2) +{ + auto me = static_cast<T*>(p1); + + switch (oper) { + case op::move: + new (p2) T(std::move(*me)); + break; + + case op::destroy: + me->~T(); + break; + + case op::size: + return sizeof(T); + } + return 0; +} +} +// The thing itself! +// +// The default value for Size may be wrong in almost all cases. You +// can change it to your heart's content. The upside is that you'll +// just get a compile error and you can bump it up. +// +// I *recommend* having a size constant in header files (or perhaps a +// using declaration, e.g. +// ``` +// using StaticFoo = static_ptr<Foo, sizeof(Blah)>` +// ``` +// in some header file that can be used multiple places) so that when +// you create a new derived class with a larger size, you only have to +// change it in one place. +// +template<typename Base, std::size_t Size = sizeof(Base)> +class static_ptr { + template<typename U, std::size_t S> + friend class static_ptr; + + // Refuse to be set to anything with whose type we are + // incompatible. Also never try to eat anything bigger than you are. + // + template<typename T, std::size_t S> + constexpr static int create_ward() noexcept { + static_assert(std::is_void_v<Base> || + std::is_base_of_v<Base, std::decay_t<T>>, + "Value to store must be a derivative of the base."); + static_assert(S <= Size, "Value too large."); + static_assert(std::is_void_v<Base> || !std::is_const<Base>{} || + std::is_const_v<T>, + "Cannot assign const pointer to non-const pointer."); + return 0; + } + // Here we can store anything that has the same signature, which is + // relevant to the multiple-versions for move/copy support that I + // mentioned above. + // + size_t (*operate)(_mem::op, void*, void*); + + // This is mutable so that get and the dereference operators can be + // const. Since we're modeling a pointer, we should preserve the + // difference in semantics between a pointer-to-const and a const + // pointer. + // + mutable typename std::aligned_storage<Size>::type buf; + +public: + using element_type = Base; + using pointer = Base*; + + // Empty + static_ptr() noexcept : operate(nullptr) {} + static_ptr(std::nullptr_t) noexcept : operate(nullptr) {} + static_ptr& operator =(std::nullptr_t) noexcept { + reset(); + return *this; + } + ~static_ptr() noexcept { + reset(); + } + + // Since other pointer-ish types have it + void reset() noexcept { + if (operate) { + operate(_mem::op::destroy, &buf, nullptr); + operate = nullptr; + } + } + + // Set from another static pointer. + // + // Since the templated versions don't count for overriding the defaults + static_ptr(static_ptr&& rhs) + noexcept(std::is_nothrow_move_constructible_v<Base>) : operate(rhs.operate) { + if (operate) { + operate(_mem::op::move, &rhs.buf, &buf); + } + } + + template<typename U, std::size_t S> + static_ptr(static_ptr<U, S>&& rhs) + noexcept(std::is_nothrow_move_constructible_v<U>) : operate(rhs.operate) { + create_ward<U, S>(); + if (operate) { + operate(_mem::op::move, &rhs.buf, &buf); + } + } + + static_ptr& operator =(static_ptr&& rhs) + noexcept(std::is_nothrow_move_constructible_v<Base>) { + reset(); + if (rhs) { + operate = rhs.operate; + operate(_mem::op::move, &rhs.buf, &buf); + } + return *this; + } + + template<typename U, std::size_t S> + static_ptr& operator =(static_ptr<U, S>&& rhs) + noexcept(std::is_nothrow_move_constructible_v<U>) { + create_ward<U, S>(); + reset(); + if (rhs) { + operate = rhs.operate; + operate(_mem::op::move, &rhs.buf, &buf); + } + return *this; + } + + + bool operator ==(std::nullptr_t) const { + return !operate; + } + + // In-place construction! + // + // This is basically what you want, and I didn't include value + // construction because in-place construction renders it + // unnecessary. Also it doesn't fit the pointer idiom as well. + // + template<typename T, typename... Args> + static_ptr(std::in_place_type_t<T>, Args&& ...args) + noexcept(std::is_nothrow_constructible_v<T, Args...>) + : operate(&_mem::op_fun<T>){ + static_assert((!std::is_nothrow_copy_constructible_v<Base> || + std::is_nothrow_copy_constructible_v<T>) && + (!std::is_nothrow_move_constructible_v<Base> || + std::is_nothrow_move_constructible_v<T>), + "If declared type of static_ptr is nothrow " + "move/copy constructible, then any " + "type assigned to it must be as well. " + "You can use reinterpret_pointer_cast " + "to get around this limit, but don't " + "come crying to me when the C++ " + "runtime calls terminate()."); + create_ward<T, sizeof(T)>(); + new (&buf) T(std::forward<Args>(args)...); + } + + // I occasionally get tempted to make an overload of the assignment + // operator that takes a tuple as its right-hand side to provide + // arguments. + // + template<typename T, typename... Args> + void emplace(Args&& ...args) + noexcept(std::is_nothrow_constructible_v<T, Args...>) { + create_ward<T, sizeof(T)>(); + reset(); + operate = &_mem::op_fun<T>; + new (&buf) T(std::forward<Args>(args)...); + } + + // Access! + Base* get() const noexcept { + return operate ? reinterpret_cast<Base*>(&buf) : nullptr; + } + template<typename U = Base> + std::enable_if_t<!std::is_void_v<U>, Base*> operator->() const noexcept { + return get(); + } + template<typename U = Base> + std::enable_if_t<!std::is_void_v<U>, Base&> operator *() const noexcept { + return *get(); + } + operator bool() const noexcept { + return !!operate; + } + + // Big wall of friendship + // + template<typename U, std::size_t Z, typename T, std::size_t S> + friend static_ptr<U, Z> static_pointer_cast(const static_ptr<T, S>& p); + template<typename U, std::size_t Z, typename T, std::size_t S> + friend static_ptr<U, Z> static_pointer_cast(static_ptr<T, S>&& p); + + template<typename U, std::size_t Z, typename T, std::size_t S> + friend static_ptr<U, Z> dynamic_pointer_cast(const static_ptr<T, S>& p); + template<typename U, std::size_t Z, typename T, std::size_t S> + friend static_ptr<U, Z> dynamic_pointer_cast(static_ptr<T, S>&& p); + + template<typename U, std::size_t Z, typename T, std::size_t S> + friend static_ptr<U, Z> const_pointer_cast(const static_ptr<T, S>& p); + template<typename U, std::size_t Z, typename T, std::size_t S> + friend static_ptr<U, Z> const_pointer_cast(static_ptr<T, S>&& p); + + template<typename U, std::size_t Z, typename T, std::size_t S> + friend static_ptr<U, Z> reinterpret_pointer_cast(const static_ptr<T, S>& p); + template<typename U, std::size_t Z, typename T, std::size_t S> + friend static_ptr<U, Z> reinterpret_pointer_cast(static_ptr<T, S>&& p); + + template<typename U, std::size_t Z, typename T, std::size_t S> + friend static_ptr<U, Z> resize_pointer_cast(const static_ptr<T, S>& p); + template<typename U, std::size_t Z, typename T, std::size_t S> + friend static_ptr<U, Z> resize_pointer_cast(static_ptr<T, S>&& p); +}; + +// These are all modeled after the same ones for shared pointer. +// +// Also I'm annoyed that the standard library doesn't have +// *_pointer_cast overloads for a move-only unique pointer. It's a +// nice idiom. Having to release and reconstruct is obnoxious. +// +template<typename U, std::size_t Z, typename T, std::size_t S> +static_ptr<U, Z> static_pointer_cast(static_ptr<T, S>&& p) { + static_assert(Z >= S, + "Value too large."); + static_ptr<U, Z> r; + if (static_cast<U*>(p.get())) { + p.operate(_mem::op::move, &p.buf, &r.buf); + r.operate = p.operate; + } + return r; +} + +// Here the conditional is actually important and ensures we have the +// same behavior as dynamic_cast. +// +template<typename U, std::size_t Z, typename T, std::size_t S> +static_ptr<U, Z> dynamic_pointer_cast(static_ptr<T, S>&& p) { + static_assert(Z >= S, + "Value too large."); + static_ptr<U, Z> r; + if (dynamic_cast<U*>(p.get())) { + p.operate(_mem::op::move, &p.buf, &r.buf); + r.operate = p.operate; + } + return r; +} + +template<typename U, std::size_t Z, typename T, std::size_t S> +static_ptr<U, Z> const_pointer_cast(static_ptr<T, S>&& p) { + static_assert(Z >= S, + "Value too large."); + static_ptr<U, Z> r; + if (const_cast<U*>(p.get())) { + p.operate(_mem::op::move, &p.buf, &r.buf); + r.operate = p.operate; + } + return r; +} + +// I'm not sure if anyone will ever use this. I can imagine situations +// where they might. It works, though! +// +template<typename U, std::size_t Z, typename T, std::size_t S> +static_ptr<U, Z> reinterpret_pointer_cast(static_ptr<T, S>&& p) { + static_assert(Z >= S, + "Value too large."); + static_ptr<U, Z> r; + p.operate(_mem::op::move, &p.buf, &r.buf); + r.operate = p.operate; + return r; +} + +// This is the only way to move from a bigger static pointer into a +// smaller static pointer. The size of the total data stored in the +// pointer is checked at runtime and if the destination size is large +// enough, we copy it over. +// +// I follow cast semantics. Since this is a pointer-like type, it +// returns a null value rather than throwing. +template<typename U, std::size_t Z, typename T, std::size_t S> +static_ptr<U, Z> resize_pointer_cast(static_ptr<T, S>&& p) { + static_assert(std::is_same_v<U, T>, + "resize_pointer_cast only changes size, not type."); + static_ptr<U, Z> r; + if (Z >= p.operate(_mem::op::size, &p.buf, nullptr)) { + p.operate(_mem::op::move, &p.buf, &r.buf); + r.operate = p.operate; + } + return r; +} + +// Since `make_unique` and `make_shared` exist, we should follow their +// lead. +// +template<typename Base, typename Derived = Base, + std::size_t Size = sizeof(Derived), typename... Args> +static_ptr<Base, Size> make_static(Args&& ...args) { + return { std::in_place_type<Derived>, std::forward<Args>(args)... }; +} +} |