升压图是否切割了我的shared_ptr

Question

我正在使用boost::shared_ptr<Obj>作为边缘属性的boost图的实现。我有两个类Obj和Obj2这样：

class Obj{
    public:
    Obj(){};
    virtual func(){std::cout << "Obj func " << std::endl};
};

class Obj2: public Obj{
    public:
    Obj2(){};
    virtual func(){std::cout << "Obj2 func " << std::endl};
};

我使用（简化）函数在图中添加边，例如：

void addEdge(Vertex index, Vertex index2, const boost::shared_ptr<Obj>& edgeAttribute){
    out = boost::add_edge(index, index2, edgeAttribute, graph).first;
}

我打电话使用：

Obj2* obj = new Obj2();
boost::shared_ptr<Obj2> obj_ptr(obj);
addEdge(index, index2, obj_ptr);

然而，在我的代码中稍后通过执行edgeAttr = graph[edge]然后调用函数edgeAttr->func()来获取edge属性时，我调用Obj的函数而不是Obj2的函数。

据我所知，这意味着我的对象被切片。这个简短的例子是我在使用boost：shared_ptr和BGL时应该切片我的对象，还是我自己的其他一些实现问题就像初始化一样？

Answer 1

没有图模型不切片任何属性（注意，“属性”实际上称为捆绑属性）。

事实上，它永远不会复制Obj或Obj2，因为它只是复制shared_ptr<>导致共享所有权，而不是复制属性。

让我们展示它是如何工作的

Live On Coliru

#include <boost/graph/adjacency_list.hpp>
#include <boost/shared_ptr.hpp>
#include <boost/make_shared.hpp>
#include <iostream>

class Obj {
  public:
    Obj(){};
    virtual void func(std::ostream& os) { os << "Obj  func\n"; };
};

class Obj2 : public Obj {
  public:
    Obj2(){};
    virtual void func(std::ostream& os) { os << "Obj2 func\n"; };
};
// I'm adding edges in the graph using a (simplified) function such as:

using Graph = boost::adjacency_list<boost::vecS, boost::vecS,
      boost::directedS, boost::no_property, boost::shared_ptr<Obj> >;
using Vertex = Graph::vertex_descriptor;

struct Program {
    Program() : _graph(10) {}

    void addEdge(Vertex index, Vertex index2, const boost::shared_ptr<Obj> &edgeAttribute) {
        auto out = boost::add_edge(index, index2, edgeAttribute, _graph).first;
    }

    void sample_edges() {
        Obj2 *obj = new Obj2();
        boost::shared_ptr<Obj2> obj_ptr(obj);

        addEdge(1, 2, boost::make_shared<Obj2>());
        addEdge(2, 3, boost::make_shared<Obj >());
        addEdge(3, 4, boost::make_shared<Obj2>());
        addEdge(4, 5, boost::make_shared<Obj >());
        addEdge(5, 6, boost::make_shared<Obj2>());
        addEdge(6, 7, boost::make_shared<Obj >());
    }

    void debug_dump() const {
        for (auto ed : boost::make_iterator_range(boost::edges(_graph))) {
            _graph[ed]->func(std::cout << "Edge " << ed << ": ");
        }
    }

  private:
    Graph _graph;
};

int main() {
    std::cout << "Demo edges:\n";
    Program demo;
    demo.sample_edges();
    demo.debug_dump();

    std::cout << "Copied edges:\n";
    // copy the whole shebang
    Program clone = demo;
    clone.debug_dump();
}

打印：

Demo edges:
Edge (1,2): Obj2 func
Edge (2,3): Obj  func
Edge (3,4): Obj2 func
Edge (4,5): Obj  func
Edge (5,6): Obj2 func
Edge (6,7): Obj  func
Copied edges:
Edge (1,2): Obj2 func
Edge (2,3): Obj  func
Edge (3,4): Obj2 func
Edge (4,5): Obj  func
Edge (5,6): Obj2 func
Edge (6,7): Obj  func