[ACCEPTED]-C++: Inherit class from template parameter-design-patterns

It's often used to realize static polymorphism.

Use cases are:

• Policy-based design
• Curiously recurring template pattern
• Barton–Nackman trick

In 5 general you have the benefits from dynamic 4 polymorphism, without the extra runtime 3 costs of virtual functions. But it's only 2 useful if the concrete type can be determined 1 at compile time.

Sounds like a good candidate for a wrapper 3 class:

class base {
public:
  virtual void f() = 0;
};

class d1 : public base {
public:
  virtual void f() override { ... };
};

class d2 : public base {
public:
  virtual void f() override { ... };
};

template <typename T>
class wrapper : public T {
public:
  virtual void f() override {
    pre_op();
    T::f();
    post_op();
  }

private:
  void pre_op() { ... }
  void post_op() { ... }
}

int main(int, char**) {
  wrapper<d1> w1;
}

For example, the wrapper class can 2 provide synchronized access to the derived 1 classes.

It is used frequently in the so called "policy-based" design, i.e. you 13 add characteristics to a base class by composition 12 with desired derived classes, see "Modern 11 C++ Design: Generic Programming and Design 10 Patterns Applied" by Andrei Alexandrescu. The 9 instantiated template class from which you 8 derive is called the "policy". Such 7 a design is sometimes better than inheritance, as 6 it allows to combine policies and avoid 5 a combinatorial explosion inevitable in 4 the inheritance-based model.

See for example 3 the following simple code, where RED and BLUE are 2 drawing policies for a Pen:

#include <iostream>
#include <string>

struct RED
{
    std::string getColor()
    {
        return "RED";
    }
};

struct BLUE
{
    std::string getColor()
    {
        return "BLUE";
    }
};

template <typename PolicyClass>
class Pencil: public PolicyClass
{
public:
    void Draw()
    {
        std::cout << "I draw with the color " << PolicyClass::getColor() << std::endl; 
    }
};


int main()
{   
    Pencil<RED> red_pencil; // Drawing with RED
    red_pencil.Draw();
    Pencil<BLUE> blue_pencil; // Different behaviour now
    blue_pencil.Draw();

    return 0;
}

Can read a bit more 1 here: http://en.wikipedia.org/wiki/Policy-based_design

A place where I use this style was where 14 I need to implement a generic graph library 13 which is both easy to use and also easy 12 to maintain After a while I came with this 11 design :

ABstract class for GraphContainer,Edge,Node 10 :

template < class T1,class T2>
class  GraphAbstractContainer
{
public:
    using Node = T1;
    using Edge = T2;
    virtual std::list<Node> 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 T>
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 9 by inheriting directly from template parametrs .

for example My Adj List classess looks 8 some thing like this :

template<class T1 = GraphAbstractContainer<GraphAdjNode,
                   GraphAdjEdge>>
class  GraphAdjListContainer : public T1
{
public:
    using Node = typename T1::Node;
    using Edge = typename T1::Edge;

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

};

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

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

};

And also My Graph 7 class inherit directly from template too 6 :

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

}

An advantage for this design pattern is 5 you can simply change the whole class underlying's 4 stuffs by just inherit from abstract classes 3 and fill the template parametrs.

for example 2 I define Trie data structure by simply 1 doing this :

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

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