c++ - 模板成员变量

Question

考虑以下两个类:

class LunchBox
{
  public:
    std::vector<Apple> m_apples;
};

和

class ClassRoom
{
  public:
    std::vector<Student> m_students;
};

类的相似之处在于它们都包含对象的成员变量 vector ；但是，它们的不同之处在于 vector 的对象不同并且成员变量具有不同的名称。

我想编写一个模板，将 LunchBox 或 ClassRoom 作为模板参数(或其他一些参数)和相同类型的现有对象(类似到 std::shared_ptr)。该模板将返回一个对象，该对象添加了一个 getNthElement(int i); 成员函数以改进对方法的访问。用法如下:

// lunchBox is a previously initialized LunchBox
// object with apples already pushed into m_apples
auto lunchBoxWithAccessor = MyTemplate<LunchBox>(lunchBox);
auto apple3 = lunchBoxWithAccessor.getNthElement(3);

我想在不为每个类编写模板特化的情况下执行此操作(这可能需要指定要以某种方式操作的成员变量)。最好，我不想修改 LunchBox 或 ClassRoom 类。 是否可以编写这样的模板？

Answer 1

您可以最大限度地减少必须为每个类编写的代码量——它不必是模板特化，也不必是整个类。

class LunchBox
{
  public:
    std::vector<Apple> m_apples;
};

class ClassRoom
{
  public:
    std::vector<Student> m_students;
};

// you need one function per type, to provide the member name
auto& get_associated_vector( Student& s ) { return s.m_apples; }
auto& get_associated_vector( ClassRoom& r ) { return r.m_students; }

// and then the decorator is generic
template<typename T>
class accessor_decorator
{
     T& peer;
public:
     auto& getNthElement( int i ) { return get_associated_vector(peer).at(i); }

     auto& takeRandomElement( int i ) { ... }

     // many more ways to manipulate the associated vector

     auto operator->() { return &peer; }
};

LunchBox lunchBox{};
accessor_decorator<LunchBox> lunchBoxWithAccessor{lunchBox};
auto apple3 = lunchBoxWithAccessor.getNthElement(3);

理想情况下，简单的辅助函数重载应该与类型位于相同的命名空间中，以使参数相关的查找工作(也称为 Koenig 查找)。

如果您愿意，也可以在构造时指定成员:

template<typename T, typename TMemberCollection>
struct accessor_decorator
{
     // public to make aggregate initialization work
     // can be private if constructor is written
     T& peer;
     TMemberCollection const member;

public:
     auto& getNthElement( int i ) { return (peer.*member).at(i); }

     auto& takeRandomElement( int i ) { ... }

     // many more ways to manipulate the associated vector

     auto operator->() { return &peer; }
};

template<typename T, typename TMemberCollection>
auto make_accessor_decorator(T& object, TMemberCollection T::*member)
     -> accessor_decorator<T, decltype(member)>
{
    return { object, member };
}

LunchBox lunchBox{};
auto lunchBoxWithAccessor = make_accessor_decorator(lunchBox, &LunchBox::m_apples);
auto apple3 = lunchBoxWithAccessor.getNthElement(3);