Implementation C++14 make_integer_sequence

Question

I tried to implement the C++14 alias template make_integer_sequence, which simplifies the creation of the class template integer_sequence.

templ         


        

Answer 1

You are missing a -1 here:

typedef typename mpl::if_< T(0) == N,  
              mpl::identity< integer_sequence >,
              make_helper< T, N, N-1,I...> 
           >::type;

in particular:

typedef typename mpl::if_< T(0) == N,  
              mpl::identity< integer_sequence >,
              make_helper< T, N-1, N-1,I...> 
           >::type;

Next, the first branch shouldn't be integer_sequence, but rather integer_sequence.

typedef typename mpl::if_< T(0) == N,  
              mpl::identity< integer_sequence >,
              make_helper< T, N-1, N-1,I...> 
           >::type;

which should be enough to get your original code to compile.

In general, when writing serious template metaprogramming, your main goal should be to keep the depth of template instantiation down. A way to think about this problem is imagining you have an infinite-thread computer: each independent calculation should be shuffled off onto its own thread, then shuffled together at the end. You have a few operations that take O(1) depth, like ... expansion: exploit them.

Usually, pulling of logarithmic depth is enough, because with a 900 depth, that allows 2^900 sized structures, and something else breaks first. (To be fair, what is more likely to happen is 90 different layers of 2^10 sized structures).