What does ::type in c++ declaration mean?

Question

The following code is taken from cpp-netlib but I've seen similar declarations elsewhere.

template <class Handler>
struct server : server_base<tags::http_server, Handler>::type 
{
    typedef typename server_base<tags::http_server, Handler>::type server_base;
    typedef server_options<tags::http_server, Handler> options;

    explicit server(options const &options) : server_base(options) {}
};

Please can someone explain what the significance is of using ::type in the declaration. Searching with the word 'type' in the search box is throwing up far too many unrelated results to find an answer.

TIA

score 4 · Accepted Answer · edited May 23 '17 at 12:14

4

It's a workaround since C++98 (and also C++03) does not allow to template typedefs. If you look into server_base class, you'll see a typedef called type, whose type depends on the template parameters of server_base. FYI, in C++11 you can use alias declarations. See C++ template typedef

edited May 23 '17 at 12:14

Community

1
1

answered Mar 16 '15 at 08:12

Antonio

19,451
13
99
197

I've never heard of C++99, only C++98 and C99 – phuclv Mar 16 '15 at 08:30
@LưuVĩnhPhúc :) Yes, sorry, I mixed up with C99 – Antonio Mar 16 '15 at 08:33

score 2 · Answer 2 · answered Mar 16 '15 at 08:08

2

It is probably a typedef inside a templated class/struct that defines some type. In order to see what it really is either look in sever_base template definition or if using Visual Studio just hover mouse over type - it should show the underlying type as a tooltip

answered Mar 16 '15 at 08:08

Adrian Lis

647
4
20

Reason for downvoting? – Adrian Lis Mar 16 '15 at 08:11
I don't know who downvoted because this _is_ a correct answer to the question. – emvee Mar 16 '15 at 08:13
I did not downvote. But not everybody uses Visual Studio and this question is not tagged as a Visual Studio question. – Antonio Mar 16 '15 at 08:26

score 1 · Answer 3 · answered Mar 16 '15 at 08:28

Most of the time, it is a tool for simplifying to access/expose a inner type. Instead of writing a long declaration of a template class, you can just access the type by ::type. This way is widely used in C++ standard library, for example std::integral_constant:

#include <type_traits>

typedef std::integral_constant<int, 2> two_t;

two_t::type my_constant; // <--- simplified

What does ::type in c++ declaration mean?

3 Answers3