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

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

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

#include 
#include 

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

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

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


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

    return 0;
}

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