A helper type: I dislike using int
for it.
struct do_in_order { template<class T>do_in_order(T&&){}};
Add up sizes:'
template<class V>
std::size_t sum_size( std::size_t& s, V&& v ) {return s+= v.size(); }
Concat. Returns type to be ignored:
template<class V>
do_in_order concat_helper( V& lhs, V const& rhs ) { lhs.insert( lhs.end(), rhs.begin(), rhs.end() ); return {}; }
Micro optimization, and lets you concat vectors of move only types:
template<class V>
do_in_order concat_helper( V& lhs, V && rhs ) { lhs.insert( lhs.end(), std::make_move_iterator(rhs.begin()), std::make_move_iterator(rhs.end()) ); return{}; }
actual function. Above stuff should be in a details namespace:
template< typename T, typename A, typename... Vs >
std::vector<T,A> concat( std::vector<T,A> lhs, Vs&&...vs ){
std::size s=lhs.size();
do_in_order _0[]={ sum_size(s,vs)..., 0 };
lhs.reserve(s);
do_in_order _1[]={ concat_helper( lhs, std::forward<Vs>(vs) )..., 0 };
return std::move(lhs); // rvo blocked
}
apologies for any typos.