Revert "Replacing old-style C++ allocator with a polymorphic memory r…

…esource (PMR) (#653)"

This reverts commit 49bbb52.

demo_nodes_cpp/src/topics/allocator_tutorial.cpp

@@ -15,7 +15,6 @@
 #include <chrono>
 #include <list>
 #include <memory>
-#include <memory_resource>
 #include <string>
 #include <utility>
 
@@ -29,34 +28,69 @@ using namespace std::chrono_literals;
 // For demonstration purposes only, not necessary for allocator_traits
 static uint32_t num_allocs = 0;
 static uint32_t num_deallocs = 0;
-// A very simple custom memory resource. Counts calls to do_allocate and do_deallocate.
-class CustomMemoryResource : public std::pmr::memory_resource
+// A very simple custom allocator. Counts calls to allocate and deallocate.
+template<typename T = void>
+struct MyAllocator
 {
-private:
-  void * do_allocate(std::size_t bytes, std::size_t alignment) override
+public:
+  using value_type = T;
+  using size_type = std::size_t;
+  using pointer = T *;
+  using const_pointer = const T *;
+  using difference_type = typename std::pointer_traits<pointer>::difference_type;
+
+  MyAllocator() noexcept
   {
+  }
+
+  ~MyAllocator() noexcept {}
+
+  template<typename U>
+  MyAllocator(const MyAllocator<U> &) noexcept
+  {
+  }
+
+  T * allocate(size_t size, const void * = 0)
+  {
+    if (size == 0) {
+      return nullptr;
+    }
     num_allocs++;
-    (void)alignment;
-    return std::malloc(bytes);
+    return static_cast<T *>(std::malloc(size * sizeof(T)));
   }
 
-  void do_deallocate(
-    void * p, std::size_t bytes,
-    std::size_t alignment) override
+  void deallocate(T * ptr, size_t size)
   {
+    (void)size;
+    if (!ptr) {
+      return;
+    }
     num_deallocs++;
-    (void)bytes;
-    (void)alignment;
-    std::free(p);
+    std::free(ptr);
   }
 
-  bool do_is_equal(
-    const std::pmr::memory_resource & other) const noexcept override
+  template<typename U>
+  struct rebind
   {
-    return this == &other;
-  }
+    typedef MyAllocator<U> other;
+  };
 };
 
+template<typename T, typename U>
+constexpr bool operator==(
+  const MyAllocator<T> &,
+  const MyAllocator<U> &) noexcept
+{
+  return true;
+}
+
+template<typename T, typename U>
+constexpr bool operator!=(
+  const MyAllocator<T> &,
+  const MyAllocator<U> &) noexcept
+{
+  return false;
+}
 
 // Override global new and delete to count calls during execution.
 
@@ -96,7 +130,7 @@ void operator delete(void * ptr) noexcept
 int main(int argc, char ** argv)
 {
   using rclcpp::memory_strategies::allocator_memory_strategy::AllocatorMemoryStrategy;
-  using Alloc = std::pmr::polymorphic_allocator<void>;
+  using Alloc = MyAllocator<void>;
   using MessageAllocTraits =
     rclcpp::allocator::AllocRebind<std_msgs::msg::UInt32, Alloc>;
   using MessageAlloc = MessageAllocTraits::allocator_type;
@@ -150,8 +184,7 @@ int main(int argc, char ** argv)
     };
 
   // Create a custom allocator and pass the allocator to the publisher and subscriber.
-  CustomMemoryResource mem_resource{};
-  auto alloc = std::make_shared<Alloc>(&mem_resource);
+  auto alloc = std::make_shared<Alloc>();
   rclcpp::PublisherOptionsWithAllocator<Alloc> publisher_options;
   publisher_options.allocator = alloc;
   auto publisher = node->create_publisher<std_msgs::msg::UInt32>(