Boost MPL at template overload missing?

Question

I'm trying to do a POC of policy based design using the boost-mpl's map container. (code here)

I pass my policies around using an mpl::map, which is empty by default :

typedef boost::mpl::map<> DefaultPolicy;

To get the policy I tried the following:

typedef typename boost::mpl::at<TPolicy, LogPK, DefaultLogP>::type LoggingPolicy;

Instead, I'm getting the following error with g++ 4.81 (and an equivalent one with clang++ 3.3):

main.cpp:49:61: error: wrong number of template arguments (3, should be 2)
    typedef typename boost::mpl::at<TPolicy, LogPK, DefaultLogP>::type LoggingPolicy;
                                                               ^

The boost documentation mentions a three-arguments overload for the at template. It looks like it's not located in #include <boost/mpl/at.hpp>. I even searched this overload throught the boost code, without success. The only template I found is the one with two arguments. I failed at googling this issue ("at" is too common).

Is it a boost documentation bug, or has anyone found out how to use this mpl::at overload ?

facepalm. I've finally found the trac issue of this (at least) three years old documentation bug. — madidier
– madidier, Commented Jul 8, 2013 at 21:37

Barry · Accepted Answer · 2013-08-30 23:37:49Z

2

There's only the 2-type version. The documentation is unfortunately wrong and has been forever. But you can always roll your own

template <typename Seq, typename Key, typename Def>
struct at_def
: mpl::eval_if<
    typename mpl::has_key<Seq, Key>::type,
    mpl::at<Seq, Key>,
    mpl::identity<Def>
    >
{ }

answered Aug 30, 2013 at 23:37

Barry

312k32 gold badges734 silver badges1.1k bronze badges

Sign up to request clarification or add additional context in comments.

Collectives™ on Stack Overflow

Boost MPL at template overload missing?

1 Answer 1

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

Comments

Your Answer

Sign up or log in

Post as a guest

Related