C++: Inherit class from template parameter

Question

I recently saw the following C++ code-snippet

template 
class A : public B
{
   ...
};

and I am wondering in which setting such

Answer 1

A place where I use this style was where I need to implement a generic graph library which is both easy to use and also easy to maintain After a while I came with this design :

ABstract class for GraphContainer,Edge,Node :

template < class T1,class T2>
class  GraphAbstractContainer
{
public:
    using Node = T1;
    using Edge = T2;
    virtual std::list getConnectedNodes(const Node& node)const = 0;
    virtual Node addNode(const Node&) = 0;
    //...
};

class  GraphAbstracthNode
{
public:
    virtual uint32_t getId() const = 0;
    virtual void setID(uint32_t id)=0;
    //..
};

template
class  GraphAbstractEdge
{
public:
    using Node = T;
    //GraphAbstractEdge(){}
    virtual Node  firstNode() const = 0;
    virtual Node   secondNode() const = 0;
    virtual void  setFirstNode(const Node& node)  = 0;
    virtual void  setSecondNode(const Node& node) = 0;
    //...

};

Then I add Adj_List and Adj Matrix implementation by inheriting directly from template parametrs .

for example My Adj List classess looks some thing like this :

template>
class  GraphAdjListContainer : public T1
{
public:
    using Node = typename T1::Node;
    using Edge = typename T1::Edge;

    //return connected Nodes
    virtual std::list getConnectedNodes(const Node& node) const
    {
        //..
    }
    //..
  };

};

template
class  GraphAdjNode : public T
{
public:
    //implementing abstract class methods...
};

template
class  GraphAdjEdge : public T
{
public:
   //...

};

And also My Graph class inherit directly from template too :

template>
    class   Graph :public  GraphContainer
    {
    public:
        using Node = typename GraphContainer::Node;
        using Edge = typename GraphContainer::Edge;
         //...

}

An advantage for this design pattern is you can simply change the whole class underlying's stuffs by just inherit from abstract classes and fill the template parametrs.

for example I define Trie data structure by simply doing this :

class TrieNode :public GraphAdjNode
{
public:
    //...
    std::string word_;
};

class Trie 
{
public:
    using Graph = Graph < ecv::GraphAdjListContainer>>;
    using Node =  Graph::Node;
    using Edge =  Graph::Edge;
    void addWord(wstring word);
    //...
private:
    Graph graph_;
}