c++ - 使用带 boost::shared_ptr 的基类在 map 中查找

Question

我正在寻找一种使用基类在 map 中查找元素的方法(下面的代码只是一个基本示例):

#include <map>
#include <boost/shared_ptr.hpp>

class Base {
public:
    Base(int v) : id(v) {};
    int id;  
};

class Derived : public Base {
public:
    Derived(int v) : Base(v) {};
};


int main()
{
    std::map<boost::shared_ptr<Derived>, double> m;
    m.insert(std::make_pair(boost::shared_ptr<Derived>(new Derived(1)), 10));
    m.insert(std::make_pair(boost::shared_ptr<Derived>(new Derived(2)), 20));

    auto b1 = boost::shared_ptr<Base>(new Base(1));
    m.find(b1);

    return 0;
}

基本上，我想比较 id 属性。编译器返回的错误如下:

main.cpp: In function 'int main()':
main.cpp:35:14: error: no matching function for call to 'std::map<boost::shared_ptr<Derived>, double>::find(boost::shared_ptr<Base>&)'
     m.find(b1);
              ^
main.cpp:35:14: note: candidates are:
In file included from /usr/include/c++/4.8/map:61:0,
                 from main.cpp:1:
/usr/include/c++/4.8/bits/stl_map.h:820:7: note: std::map<_Key, _Tp, _Compare, _Alloc>::iterator std::map<_Key, _Tp, _Compare, _Alloc>::find(const key_type&) [with _Key = boost::shared_ptr<Derived> _Tp = double; _Compare = std::less<boost::shared_ptr<Derived> > _Alloc = std::allocator<std::pair<const boost::shared_ptr<Derived>, double> > std::map<_Key, _Tp, _Compare, _Alloc>::iterator = std::_Rb_tree_iterator<std::pair<const boost::shared_ptr<Derived>, double> > std::map<_Key, _Tp, _Compare, _Alloc>::key_type = boost::shared_ptr<Derived>]
       find(const key_type& __x)
       ^
/usr/include/c++/4.8/bits/stl_map.h:820:7: note:   no known conversion for argument 1 from 'boost::shared_ptr<Base>' to 'const key_type& {aka const boost::shared_ptr<Derived>&}'
/usr/include/c++/4.8/bits/stl_map.h:835:7: note: std::map<_Key, _Tp, _Compare, _Alloc>::const_iterator std::map<_Key, _Tp, _Compare, _Alloc>::find(const key_type&) const [with _Key = boost::shared_ptr<Derived> _Tp = double; _Compare = std::less<boost::shared_ptr<Derived> > _Alloc = std::allocator<std::pair<const boost::shared_ptr<Derived>, double> > std::map<_Key, _Tp, _Compare, _Alloc>::const_iterator = std::_Rb_tree_const_iterator<std::pair<const boost::shared_ptr<Derived>, double> > std::map<_Key, _Tp, _Compare, _Alloc>::key_type = boost::shared_ptr<Derived>]
       find(const key_type& __x) const
       ^
/usr/include/c++/4.8/bits/stl_map.h:835:7: note:   no known conversion for argument 1 from 'boost::shared_ptr<Base>' to 'const key_type& {aka const boost::shared_ptr<Derived>&}'

Answer 1

如果您想使用您的 map 通过 id 进行查找，您需要传入一个适当的比较函数，以便 map 按 id 对其键进行排序而不是默认的 operator < (我相信，这是将所有权 block 地址与 boost::shared_ptr 参数进行比较)。

所以像这样改变 map :

struct Less_id
{
  bool operator() (const boost::shared_ptr<Derived> &lhs, const boost::shared_ptr<Derived> &rhs) const
  {
    return lhs->id < rhs->id;
  }
};

typedef std::map<boost::shared_ptr<Derived>, double, Less_id> Map;

Map m;

这将相应地对 map 进行排序，但仍然不允许通过 Base 进行查找指针。为此，您可以在 std::lower_bound 上面编写自己的函数。 :

Map::const_iterator find_base(const Map &map, const boost::shared_ptr<Base> &base)
{
  auto it = std::lower_bound(
    map.begin(), map.end(), base,
    [](const Map::value_type &lhs, const boost::shared_ptr<Base> &rhs)
    { return lhs.first->id < rhs->id; }
  );
  if (it != map.end() && it->first->id == base->id)
    return it;
  else
    return map.end();
}

std::lower_bound()用于保持对数复杂度 std::map::find()优惠。

Live example