Most of the work in defining a concrete visitor is writing the code for the actual work that has to happen for every visitable object. There is not a lot of boilerplate code in a specific visitor class. But sometimes we may not want to declare the class itself. Think about lambda expressions—anything that can be done with a lambda expression can also be done with an explicitly declared callable class because lambdas are (anonymous) callable classes. Nonetheless, we find lambda expressions very useful for writing one-off callable objects. Similarly, we may want to write a visitor without explicitly naming it—a lambda Visitor. We would want it to look something like this:
auto v(lambda_visitor<PetVisitor>( [](Cat* c) { std::cout ...
