C++: What does template<class> mean?

Question

I'm trying to understand some C++ code. I'm an experienced Java programmer trying to learn C++. I've already read some exhaustive articles on templates, but no one of them answered me what does the following template specification mean.

template<
    template<template<class> class, class> class VisualOdometryTT, 
    template<class> class NodeBuilderTT,
    class PoseGraphT> 
class VORosInterface{ ... };

The part I don't understand is template<class> where I think some type specification is missing. But the code compiles without problems.

Answer 1

Using NodeBuilderTT as an example because it's easier:

NodeBuilderTT is a template parameter that is itself a template having one parameter -- let's call that Z.

You could have chosen to formally name Z and the code would compile just the same:

template<class Z> class NodeBuilderTT

So far this is quite similar to declaring function arguments:

void foo(int x) {}   // works
void foo(int)   {}   // also works

However, with the functions you would typically use the name x inside the function body. With templates you cannot use Z in the definition of VORosInterface, so there is absolutely no point in naming it and it is idiomatic to write

template<class> class NodeBuilderTT

My thanks to K-ballo for helping set the record straight here.