memory_resource.h

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/// @file
///
/// @brief Class that wraps resources in a polymorphic manner.
 
#ifndef MYSQL_ALLOCATORS_MEMORY_RESOURCE_H
#define MYSQL_ALLOCATORS_MEMORY_RESOURCE_H
 
#include <cstdlib>     // malloc
#include <functional>  // function
 
/// @addtogroup GroupLibsMysqlAllocators
/// @{
 
namespace mysql::allocators {
 
/// Polymorphism-free memory resource class with custom allocator and
/// deallocator functions.
///
/// This is used as the "back-end" for Allocator objects.  The
/// allocator and deallocator functions are member objects of type
/// std::function, specified as constructor arguments.
///
/// Example: a class that needs to allocate memory, and should have a
/// way to use a specified PSI key, can be implemented as follows:
///
/// - For each type the class needs to allocate, it has one member
///   object of type Allocator.  The constructor takes a
///   Memory_resource parameter, which defaults to a
///   default-constructed Memory_resource.  The constructor passes the
///   Memory_resource to the Allocator constructors.  The class calls
///   the Allocator member functions for its memory management.
///
/// - API clients that don't need a PSI key don't need to pass a
///   Memory_resource to the constructor; they use the default value
///   for the parameter.  API clients that need a PSI key, pass a
///   Memory_resource object where the allocate/deallocate functions
///   are PSI-enabled.  Such a Memory_resource object can be obtained
///   from the factory function @c psi_memory_resource in
///   sql/psi_memory_resource.h.
///
/// Notes on design choices:
///
/// We avoid using polymorphism to configure the behavior of the
/// allocate/deallocate member functions.  Classes that use
/// polymorphic memory resources would have to take a reference or
/// pointer to the memory resource as argument, so either the class
/// can't own the memory resource or it has to be dynamically
/// allocated.  If the class can't own the memory resource, it
/// complicates the usage since the caller has to ensure the
/// Memory_resource outlives the class that uses it.  If the
/// Memory_resource has to be allocated dynamically, that allocation
/// cannot use a custom Memory_resource, which may defeat the purpose
/// of using custom Memory_resources to track all allocations.
///
/// We also avoid static polymorphism, since every user of a
/// statically polymorphic Memory_resource would have to be a
/// template, which has two problems: first, two classes with the same
/// behavior but different allocation strategies would be different
/// types.  Second, any class that allocates memory and has a
/// requirement that the API client can configure the Memory_resource,
/// has to be templated and therefore be defined in a header.
///
/// With the current solution, the caller just creates a
/// Memory_resource object and passes it to the allocator.
class Memory_resource {
 public:
  using Size_t = std::size_t;
  using Ptr_t = void *;
  using Allocator_t = std::function<Ptr_t(Size_t)>;
  using Deallocator_t = std::function<void(Ptr_t)>;
 
  /// Construct a new Memory_resource that uses the given allocator
  /// and deallocator.
  ///
  /// @param allocator The allocator, e.g. std::malloc or my_malloc.
  ///
  /// @param deallocator The deallocator, e.g. std::free or my_free.
  Memory_resource(const Allocator_t &allocator,
                  const Deallocator_t &deallocator)
      : m_allocator(allocator), m_deallocator(deallocator) {}
 
  /// Construct a new Memory_resource that uses std::malloc and std::free.
  Memory_resource() : Memory_resource(std::malloc, std::free) {}
 
  /// Allocate memory using the provided allocator.
  ///
  /// @param n The size.
  ///
  /// @return The new memory.
  [[nodiscard]] void *allocate(Size_t n) const { return m_allocator(n); }
 
  /// Deallocate memory using the provided deallocator.
  ///
  /// @param p The pointer.
  void deallocate(Ptr_t p) const { m_deallocator(p); }
 
  /// Return the deallocator.
  [[nodiscard]] Deallocator_t get_deallocator() const { return m_deallocator; }
 
  /// Return const reference to a defult-constructed Memory_resource.
  ///
  /// This memory resource is live until program termination.
  [[nodiscard]] static const Memory_resource &get_default() {
    static Memory_resource instance;
    return instance;
  }
 
 private:
  /// The allocator object.
  Allocator_t m_allocator;
 
  /// The deallocator object.
  Deallocator_t m_deallocator;
};  // class Memory_resource
 
/// Return object.get_memory_resource(), if that function exists. Otherwise,
/// returns a default-constructed Memory_resource.
///
/// This may be useful to implement get_memory_resource() in class templates,
/// when the returned object is retrieved from a member object of a template
/// type that may or may not have a get_memory_resource() member function.
[[nodiscard]] decltype(auto) get_memory_resource_or_default(
    const auto &object) {
  if constexpr (requires { object.get_memory_resource(); }) {
    return object.get_memory_resource();
  } else {
    return Memory_resource::get_default();
  }
}
 
}  // namespace mysql::allocators
 
/// @}
 
#endif  // MYSQL_ALLOCATORS_MEMORY_RESOURCE_H