c – 通过指向非多态类型的基类获取已分配内存的地址

struct Base
{
  std::string a;
};

struct Derived : Base
{
  std::string b;
};

Base* p = custom_new<Derived>();
custom_delete(p);

在这个例子中,custom_delete实际上会释放正确的地址(static_cast< void *>(static_cast< Derived *>(p))== static_cast< void *>(p)),但是行obj- T()将调用Base的析构函数,这意味着b字段被泄露.

所以不要这样做

不要从custom_new返回一个原始指针,而是返回绑定到T类型的对象,并知道如何删除它.例如：

template <class T> struct CustomDeleter
{
  void operator()(T* object) const
  {
    object->~T();
    custom_free(object); 
  }
};

template <typename T> using CustomPtr = std::unique_ptr<T,CustomDeleter<T>>;

template <typename T,typename... Args> CustomPtr<T> custom_new(Args&&... args)
{
  void* ptr = custom_malloc(sizeof(T));
  try
  {
    return CustomPtr<T>{ new(ptr) T(std::forward<Args>(args)...) };
  }
  catch (...)
  {
    custom_free(ptr);
    throw;
  }
}

现在不可能不小心释放错误的地址并调用错误的析构函数,因为调用custom_free的唯一代码知道它正在删除的事物的完整类型.

注意：请注意unique_ptr :: reset(pointer)方法.这种方法在使用自定义删除器时非常危险,因为onus是在调用者上提供以正确方式分配的指针.如果使用无效指针调用该方法,编译器将无法帮助.

通过基指针

可能是你想要将一个基指针传递给一个函数,并赋予该函数释放对象的责任.在这种情况下,您需要使用类型擦除来从消费者隐藏对象的类型,同时在内部保留其最常派生类型的知识.最简单的方法是使用std :: shared_ptr.例如：

struct Base
{
  int a;
};

struct Derived : Base
{
  int b;
};

CustomPtr<Derived> unique_derived = custom_new<Derived>();

std::shared_ptr<Base> shared_base = std::shared_ptr<Derived>{ std::move(unique_derived) };

现在,您可以自由传递shared_base,当最终引用发布时,完整的Derived对象将被销毁,其正确的地址传递给custom_free.如果你不喜欢shared_ptr的语义,那么创建一个带有unique_ptr语义的类型擦除指针是非常简单的.

注意：这种方法的一个缺点是,shared_ptr需要为其控件块单独分配(不会使用custom_malloc).再多一点工作,你可以解决这个问题.您需要创建一个包装custom_malloc和custom_free的自定义分配器,然后使用std :: allocate_shared创建对象.

完整的工作实例

#include <memory>
#include <iostream>

void* custom_malloc(size_t size)
{
  void* mem = ::operator new(size);
  std::cout << "allocated object at " << mem << std::endl;
  return mem;
}

void custom_free(void* mem)
{
  std::cout << "freeing memory at " << mem << std::endl;
  ::operator delete(mem);
}

template <class T> struct CustomDeleter
{
  void operator()(T* object) const
  {
    object->~T();
    custom_free(object); 
  }
};

template <typename T> using CustomPtr = std::unique_ptr<T,typename... Args> CustomPtr<T> custom_new(Args&&... args)
{
  void* ptr = custom_malloc(sizeof(T));
  try
  {      
    return CustomPtr<T>{ new(ptr) T(std::forward<Args>(args)...) };
  }
  catch (...)
  {
    custom_free(ptr);
    throw;
  }
}

struct Base
{
  int a;
  ~Base()
  {
    std::cout << "destroying Base" << std::endl;
  }
};

struct Derived : Base
{
  int b;
  ~Derived()
  {
    std::cout << "detroying Derived" << std::endl;
  }
};

int main()
{
  // Since custom_new has returned a unique_ptr with a deleter bound to the
  // type Derived,we cannot accidentally free the wrong thing.
  CustomPtr<Derived> unique_derived = custom_new<Derived>();

  // If we want to get a pointer to the base class while retaining the ability
  // to correctly delete the object,we can use type erasure.  std::shared_ptr
  // will do the trick,but it's easy enough to write a similar class without
  // the sharing semantics.
  std::shared_ptr<Base> shared_base = std::shared_ptr<Derived>{ std::move(unique_derived) };

  // Notice that when we release the shared_base pointer,we destroy the complete
  // object.
  shared_base.reset();
}