Small shared two way pointer

Author: danpoe

src/util/small_shared_two_way_ptr.h

@@ -0,0 +1,287 @@
+/*******************************************************************\
+
+Module: Small shared two-way pointer
+
+Author: Daniel Poetzl
+
+\*******************************************************************/
+
+#ifndef CPROVER_UTIL_SMALL_SHARED_TWO_WAY_PTR_H
+#define CPROVER_UTIL_SMALL_SHARED_TWO_WAY_PTR_H
+
+#include <type_traits>
+#include <limits>
+#include <utility>
+
+#include "invariant.h"
+
+template <typename Num>
+class small_shared_two_way_pointeet;
+
+/// This class is similar to small_shared_ptrt and boost's intrusive_ptr. Like
+/// those, it stores the use count with the pointed-to object instead of in a
+/// separate control block. Additionally, it uses the MSB of the use count to
+/// indicate the type of the managed object (which is either of type U or V,
+/// with both being derived from T).
+///
+/// A possible use case is the implementation of data structures with sharing
+/// that consist of two different types of objects (such as a tree with internal
+/// nodes and leaf nodes). Storing the type with the use count avoids having to
+/// keep a separate `type` member or using `typeid` or `dynamic_cast`. Moreover,
+/// since the shared pointer is aware of the concrete type of the object being
+/// stored, it can delete it without requiring a virtual destructor or custom
+/// delete function (like std::shared_ptr).
+template <typename U, typename V>
+class small_shared_two_way_ptrt final
+{
+public:
+  typedef decltype(std::declval<U>().use_count()) use_countt;
+
+  typedef small_shared_two_way_pointeet<use_countt> pointeet;
+
+  static_assert(std::is_base_of<pointeet, U>::value, "");
+  static_assert(std::is_base_of<pointeet, V>::value, "");
+
+  small_shared_two_way_ptrt() = default;
+
+  explicit small_shared_two_way_ptrt(U *u) : p(u)
+  {
+    PRECONDITION(u != nullptr);
+    PRECONDITION(u->use_count() == 0);
+
+    p->set_derived1();
+    p->increment_use_count();
+  }
+
+  explicit small_shared_two_way_ptrt(V *v) : p(v)
+  {
+    PRECONDITION(v != nullptr);
+    PRECONDITION(v->use_count() == 0);
+
+    p->set_derived2();
+    p->increment_use_count();
+  }
+
+  small_shared_two_way_ptrt(const small_shared_two_way_ptrt &rhs) : p(rhs.p)
+  {
+    PRECONDITION(is_same(rhs));
+
+    if(p)
+    {
+      p->increment_use_count();
+    }
+  }
+
+  small_shared_two_way_ptrt(small_shared_two_way_ptrt &&rhs)
+  {
+    PRECONDITION(is_same(rhs));
+
+    swap(rhs);
+  }
+
+  small_shared_two_way_ptrt &operator=(const small_shared_two_way_ptrt &rhs)
+  {
+    PRECONDITION(is_same(rhs));
+
+    small_shared_two_way_ptrt copy(rhs);
+    swap(copy);
+    return *this;
+  }
+
+  small_shared_two_way_ptrt &operator=(small_shared_two_way_ptrt &&rhs)
+  {
+    PRECONDITION(is_same(rhs));
+
+    swap(rhs);
+    return *this;
+  }
+
+  ~small_shared_two_way_ptrt()
+  {
+    if(!p)
+    {
+      return;
+    }
+
+    auto use_count = p->use_count();
+
+    if(use_count == 1)
+    {
+      if(p->is_derived1())
+      {
+        U *u = static_cast<U *>(p);
+        delete u;
+      }
+      else
+      {
+        V *v = static_cast<V *>(p);
+        delete v;
+      }
+    }
+    else
+    {
+      p->decrement_use_count();
+    }
+  }
+
+  void swap(small_shared_two_way_ptrt &rhs)
+  {
+    PRECONDITION(is_same(rhs));
+
+    std::swap(p, rhs.p);
+  }
+
+  use_countt use_count() const
+  {
+    return p ? p->use_count() : 0;
+  }
+
+  bool is_derived1() const
+  {
+    return p == nullptr || p->is_derived1();
+  }
+
+  bool is_derived2() const
+  {
+    return p == nullptr || p->is_derived2();
+  }
+
+  pointeet *get() const
+  {
+    return p;
+  }
+
+  U *get_derived1() const
+  {
+    PRECONDITION(is_derived1());
+
+    return static_cast<U *>(p);
+  }
+
+  V *get_derived2() const
+  {
+    PRECONDITION(is_derived2());
+
+    return static_cast<V *>(p);
+  }
+
+  bool is_same(const small_shared_two_way_ptrt &other) const
+  {
+    const bool b1 = p == nullptr;
+    const bool b2 = other.p == nullptr;
+
+    if(b1 || b2)
+      return true;
+
+    return p->is_same(*other.p);
+  }
+
+  explicit operator bool() const
+  {
+    return p != nullptr;
+  }
+
+private:
+  pointeet *p = nullptr;
+};
+
+template <typename U, typename V, typename... Ts>
+small_shared_two_way_ptrt<U, V> make_shared_derived1(Ts &&... ts)
+{
+  return small_shared_two_way_ptrt<U, V>(new U(std::forward<Ts>(ts)...));
+}
+
+template <typename U, typename V, typename... Ts>
+small_shared_two_way_ptrt<U, V> make_shared_derived2(Ts &&... ts)
+{
+  return small_shared_two_way_ptrt<U, V>(new V(std::forward<Ts>(ts)...));
+}
+
+template <typename U, typename V>
+bool operator==(
+  const small_shared_two_way_ptrt<U, V> &lhs,
+  const small_shared_two_way_ptrt<U, V> &rhs)
+{
+  return lhs.get() == rhs.get();
+}
+
+template <typename U, typename V>
+bool operator!=(
+  const small_shared_two_way_ptrt<U, V> &lhs,
+  const small_shared_two_way_ptrt<U, V> &rhs)
+{
+  return lhs.get() != rhs.get();
+}
+
+template <typename Num>
+class small_shared_two_way_pointeet
+{
+public:
+  static_assert(std::is_unsigned<Num>::value, "");
+
+  static constexpr int bit_idx = std::numeric_limits<Num>::digits - 1;
+  static constexpr Num mask = ~(1 << bit_idx);
+
+  small_shared_two_way_pointeet() = default;
+
+  // The use count shall be unaffected
+  small_shared_two_way_pointeet(const small_shared_two_way_pointeet &rhs)
+  {
+  }
+
+  // The use count shall be unaffected
+  small_shared_two_way_pointeet &
+  operator=(const small_shared_two_way_pointeet &rhs)
+  {
+    return *this;
+  }
+
+  Num use_count() const
+  {
+    return use_count_ & mask;
+  }
+
+  void increment_use_count()
+  {
+    PRECONDITION((use_count_ & mask) < mask);
+
+    use_count_++;
+  }
+
+  void decrement_use_count()
+  {
+    PRECONDITION((use_count_ & mask) > 0);
+
+    use_count_--;
+  }
+
+  void set_derived1()
+  {
+    use_count_ &= mask;
+  }
+
+  void set_derived2()
+  {
+    use_count_ |= ~mask;
+  }
+
+  bool is_derived1() const
+  {
+    return !(use_count_ & ~mask);
+  }
+
+  bool is_derived2() const
+  {
+    return use_count_ & ~mask;
+  }
+
+  bool is_same(const small_shared_two_way_pointeet &other) const
+  {
+    return !((use_count_ ^ other.use_count_) & ~mask);
+  }
+
+private:
+  Num use_count_ = 0;
+};
+
+#endif

unit/Makefile

@@ -54,6 +54,7 @@ SRC += unit_tests.cpp \
        catch_example.cpp \
        java_bytecode/java_virtual_functions/virtual_functions.cpp \
        java_bytecode/java_bytecode_parse_generics/parse_generic_superclasses.cpp \
+       small_shared_two_way_ptr.cpp
        # Empty last line
 
 INCLUDES= -I ../src/ -I.

unit/small_shared_two_way_ptr.cpp

@@ -0,0 +1,99 @@
+#include <util/small_shared_two_way_ptr.h>
+#include <testing-utils/catch.hpp>
+
+class D1 : public small_shared_two_way_pointeet<unsigned>
+{
+public:
+  D1() = default;
+
+  D1(int i) : d1(i)
+  {
+  }
+
+  int d1;
+};
+
+class D2 : public small_shared_two_way_pointeet<unsigned>
+{
+public:
+  D2() = default;
+
+  D2(int i) : d2(i)
+  {
+  }
+
+  int d2;
+};
+
+TEST_CASE("Small shared two-way pointer")
+{
+  typedef small_shared_two_way_ptrt<D1, D2> spt;
+
+  SECTION("Types")
+  {
+    spt sp1;
+    spt sp2(new D1());
+    spt sp3(new D2());
+
+    REQUIRE(sp1.is_same(sp1));
+    REQUIRE(sp2.is_same(sp2));
+    REQUIRE(sp3.is_same(sp3));
+
+    REQUIRE(sp1.is_same(sp2));
+    REQUIRE(sp1.is_same(sp3));
+
+    REQUIRE(!sp2.is_same(sp3));
+
+    REQUIRE(sp1.is_derived1());
+    REQUIRE(sp1.is_derived2());
+
+    REQUIRE(sp2.is_derived1());
+    REQUIRE(!sp2.is_derived2());
+
+    REQUIRE(sp3.is_derived2());
+    REQUIRE(!sp3.is_derived1());
+  }
+
+  SECTION("Basic")
+  {
+    spt sp1;
+    REQUIRE(sp1.use_count() == 0);
+
+    const D1 *p;
+
+    p = sp1.get_derived1();
+    REQUIRE(p == nullptr);
+
+    spt sp2(new D1());
+    REQUIRE(sp2.use_count() == 1);
+
+    p = sp2.get_derived1();
+    REQUIRE(p != nullptr);
+
+    spt sp3(sp2);
+    REQUIRE(sp3.is_derived1());
+    REQUIRE(sp2.get_derived1() == sp3.get_derived1());
+    REQUIRE(sp2.use_count() == 2);
+    REQUIRE(sp3.use_count() == 2);
+
+    sp1 = sp2;
+    REQUIRE(sp1.is_derived1());
+    REQUIRE(sp1.get_derived1() == sp2.get_derived1());
+    REQUIRE(sp1.use_count() == 3);
+    REQUIRE(sp2.use_count() == 3);
+    REQUIRE(sp3.use_count() == 3);
+  }
+
+  SECTION("Creation")
+  {
+    spt sp1 = make_shared_derived1<D1, D2>();
+    spt sp2 = make_shared_derived2<D1, D2>();
+
+    REQUIRE(!sp1.is_same(sp2));
+
+    sp1 = make_shared_derived1<D1, D2>(0);
+    sp2 = make_shared_derived2<D1, D2>(0);
+
+    REQUIRE(!sp1.is_same(sp2));
+  }
+}