in template class T1, class T2>, what is the meaning of ?

Question

I found some example of template in Modern C++ Design by A. Alexenderscu
where author used following lines

template
<
class T,
template <class> class CheckingPolicy  // <---- Please explain this line
>
class SmartPtr : public CheckingPolicy<T>
{
...
template
<
class T1,
template <class> class CP1,
>
SmartPtr(const SmartPtr<T1, CP1>& other)
    : pointee_(other.pointee_), CheckingPolicy<T>(other)
{ ... }
};

i do not understand meaning of in marked line. Please explain that line

It is a template template parameter. `` means that it has one type parameter. — Constructor, Apr 16 '14 at 11:26
can you explain it more ... what one type parameter. Mean same type as type as class T ( declared above ) — Jai, Apr 16 '14 at 11:28
In this code example `SmartPtr` class template has one type parameter `T` and one template parameter `CheckingPolicy`. `CheckingPolicy` template template parameter itself has one type parameter: `template class CheckingPolicy`. — Constructor, Apr 16 '14 at 11:32
@Constructor Yes. There's no point in answering a question in comments, in fact it's actively discouraged. It only makes sense if writing a proper answer would take significantly more time than a simple comment, and you happen not to have that time. But here, the answer does not require much beyond what you put in the two comments already. — Angew is no longer proud of SO, Apr 16 '14 at 11:51
Check http://stackoverflow.com/questions/6484484/template-template-parameters please — 101010, Apr 16 '14 at 12:01
@Angew Thank you for your explanation. I post it as an answer. — Constructor, Apr 16 '14 at 12:05

score 2 · Accepted Answer · answered Apr 16 '14 at 12:04

In this code example SmartPtr class template has one type parameter T and one template parameter CheckingPolicy. CheckingPolicy template template parameter itself has one type parameter: template <class> class CheckingPolicy. I recommend you to format template code which is unclear for you in the following way to make it more understandable:

template
    <
        class T, // type parameter of a SmartPtr class template
        template
            <
                class // type parameter of a template parameter CheckingPolicy
            >
        class CheckingPolicy // template parameter of a SmartPtr class template
    >
class SmartPtr // class template

Simply put, the name of the template is ignored in `template` instead of `template`. Similar to functions `void foo(int)` instead of `void foo(int bar)` — balki, Apr 16 '14 at 12:08
@balki Thank you for your valuable remark. But there are some cases in which it would not be ignored. — Constructor, Apr 16 '14 at 12:11

in template class T1, class T2>, what is the meaning of ?

1 Answers1