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

Sounds like a good candidate for a wrapper 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 
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 w1;
}

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