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I'm having difficulty understanding the syntax of C++ Template Template parameters. I understand why they are useful, as per the excellent description here, I just find their syntax hard to get to understand. Two examples taken from the above website (there are others):
template <typename T, template <typename> class Cont>
class Stack;
and
template <template <typename,typename> class Cont>
class Wrapper3;
Clearly generalizing such declarations is impossible without some understanding of the rationale behind this syntax. Memorizing is harder and does not seem to be of much help.
Edit: I realize that my attempt at a question came across like an observation. What I'm asking for is help on how to interprete the Template Template parameter syntax in everyday speak. I can do this with the C++ syntax and the all the other programming languages that I've learned. However I'm having difficulty "explaining" the syntax of C++ Template Template parameters to myself. I've gotten a book, "C++ templates : the complete guide" by David Vandevoorde and Nicolai M. Josuttis, and while its a nice book, it hasn't been of much help to me in understanding this syntax which I'm sure many will agree is at best quirky.
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A class template starts with the keyword template followed by template parameter(s) inside <> which is followed by the class declaration. template <class T> class className { private: T var; ... .. ... public: T functionName(T arg); ... .. ... };
A template parameter is a special kind of parameter that can be used to pass a type as argument: just like regular function parameters can be used to pass values to a function, template parameters allow to pass also types to a function.
If there is just a single template parameter, I name it T (or U,V for nested templates). When there are multiple parameters and the use is not immediately obvious then I use descriptive names prefixed with T. For example, TKey, TValue, TIdentifiier, etc ...
What is the correct syntax of defining function template/template functions? Explanation: Starts with keyword template and then <class VAR>, then use VAR as type anywhere in the function below. 7.
I am not sure what is your question exactly, but here is the explanation for the two examples you gave.
template <typename T, template <typename> class Cont>
class Stack;
Stack is a class template with two template parameters. The first parameter, T can be any type (including built-in types, user-defined types, template instantiations and so on). The second parameter, Cont, must be a class template taking one parameter. The parameter is unnamed because it would not make much sense (the parameter is never bound to anything).
template <template <typename,typename> class Cont>
class Wrapper3;
Wrapper3 is a class template with a single parameter, Cont. Cont must be a class template with two parameters.
The syntax to define a template template parameter is the same as the one to define a class template (template <typename [param1], typename [param2], ...> class Name), so I don't really understand what is your problem.
However, I agree that the syntax can become a bit awkward when you start "nesting" template template parameters:
// class template whose parameter must be a class template whose parameter
// must be a class template
template <template <template <typename> class > class C >
struct Wow {};
Doesn't happen that often, though...
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