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the line 109 show in asio allocation example

template <typename T>
class handler_allocator
{

...

private:
  template <typename> friend class handler_allocator; // line 109

...

}

I wonder what the template < typename > mean ? Is it template < typename T> in commaon way? By the way, the example could work.

foo
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    A class template with one type parameter is being made a friend; the parameter name is omitted. The point, it seems, is to make every instantiation of `handler_allocator` a friend of every other instantiation. – Igor Tandetnik Sep 07 '18 at 14:12
  • https://en.cppreference.com/w/cpp/language/friend#Template_friends – rsy56640 Sep 07 '18 at 14:38

1 Answers1

2

handler_allocator is a class template, as in, only upon concrete instantiation it would result in an actual class. Hence, these instantiations are multiple, so you end up with one class per each T. In case you want to have these different classes a friend of each other then this line would be the way to go.

I wonder what the template < typename > mean ? Is it template < typename T> ... ?

Not quite. If you'd add the T then it would result in invalid code as this T would shadow the first T. You could add any other named identifier instead (let's say S) but it works without it either way as there's no use for it in this snippet.

Geezer
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