mem_root_allocator.h

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/* Copyright (c) 2014, 2024, Oracle and/or its affiliates.
 
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#ifndef MEM_ROOT_ALLOCATOR_INCLUDED
#define MEM_ROOT_ALLOCATOR_INCLUDED
 
#include <assert.h>
#include <limits>
#include <new>
#include <utility>  // std::forward
 
#include "my_alloc.h"
 
/**
  Mem_root_allocator is a C++ STL memory allocator based on MEM_ROOT.
 
  No deallocation is done by this allocator. Calling the constructor
  and destructor on the supplied MEM_ROOT is the responsibility of
  the caller. Do *not* call Clear() or ~MEM_ROOT until the destructor
  of any objects using this allocator has completed. This includes iterators.
 
  Example of use:
  vector<int, Mem_root_allocator<int> > v((Mem_root_allocator<int>(&mem_root)));
 
  @note allocate() throws std::bad_alloc() similarly to the default
  STL memory allocator. This is necessary - STL functions which allocate
  memory expect it. Otherwise these functions will try to use the memory,
  leading to seg faults if memory allocation was not successful.
 
  @note This allocator cannot be used for std::basic_string with RHEL 6/7
  because of this bug:
  https://bugzilla.redhat.com/show_bug.cgi?id=1546704
  "Define _GLIBCXX_USE_CXX11_ABI gets ignored by gcc in devtoolset-7"
 
  @note C++98 says that STL implementors can assume that allocator objects
  of the same type always compare equal. This will only be the case for
  two Mem_root_allocators that use the same MEM_ROOT. Care should be taken
  when this is not the case. Especially:
  - Using list::splice() on two lists with allocators using two different
    MEM_ROOTs causes undefined behavior. Most implementations seem to give
    runtime errors in such cases.
  - swap() on two collections with allocators using two different MEM_ROOTs
    is not well defined. At least some implementations also swap allocators,
    but this should not be depended on.
*/
 
template <class T>
class Mem_root_allocator {
  // This cannot be const if we want to be able to swap.
  MEM_ROOT *m_memroot;
 
 public:
  typedef T value_type;
  typedef size_t size_type;
  typedef ptrdiff_t difference_type;
 
  typedef T *pointer;
  typedef const T *const_pointer;
 
  typedef T &reference;
  typedef const T &const_reference;
 
  pointer address(reference r) const { return &r; }
  const_pointer address(const_reference r) const { return &r; }
 
  explicit Mem_root_allocator(MEM_ROOT *memroot) : m_memroot(memroot) {}
 
  explicit Mem_root_allocator() : m_memroot(nullptr) {}
 
  template <class U>
  Mem_root_allocator(const Mem_root_allocator<U> &other)
      : m_memroot(other.memroot()) {}
 
  template <class U>
  Mem_root_allocator &operator=(const Mem_root_allocator<U> &other
                                [[maybe_unused]]) {
    assert(m_memroot == other.memroot());  // Don't swap memroot.
  }
 
  pointer allocate(size_type n, const_pointer hint [[maybe_unused]] = nullptr) {
    if (n == 0) return nullptr;
    if (n > max_size()) throw std::bad_alloc();
 
    pointer p = static_cast<pointer>(m_memroot->Alloc(n * sizeof(T)));
    if (p == nullptr) throw std::bad_alloc();
    return p;
  }
 
  void deallocate(pointer, size_type) {}
 
  template <class U, class... Args>
  void construct(U *p, Args &&... args) {
    assert(p != nullptr);
    try {
      ::new ((void *)p) U(std::forward<Args>(args)...);
    } catch (...) {
      assert(false);  // Constructor should not throw an exception.
    }
  }
 
  void destroy(pointer p) {
    assert(p != nullptr);
    try {
      p->~T();
    } catch (...) {
      assert(false);  // Destructor should not throw an exception
    }
  }
 
  size_type max_size() const {
    return std::numeric_limits<size_t>::max() / sizeof(T);
  }
 
  template <class U>
  struct rebind {
    typedef Mem_root_allocator<U> other;
  };
 
  MEM_ROOT *memroot() const { return m_memroot; }
};
 
template <class T>
bool operator==(const Mem_root_allocator<T> &a1,
                const Mem_root_allocator<T> &a2) {
  return a1.memroot() == a2.memroot();
}
 
template <class T>
bool operator!=(const Mem_root_allocator<T> &a1,
                const Mem_root_allocator<T> &a2) {
  return a1.memroot() != a2.memroot();
}
 
#endif  // MEM_ROOT_ALLOCATOR_INCLUDED