Does type order in std::tuple arguments have any effects?

Question

Say I want to store three types in a tuple : int, float and std::vector<double>

If I leave aside matters of subsequent interface, does this

tuple<int, float, vector<int>> t;

have any differences from this

tuple<vector<int>, int, float> t;

Due to the implementation of tuple as a class of variadic bases, I'm expecting a different layout for the produced classes, but does it matter in any way ? Also are there any optimization considerations to take into account, when placing types in a tuple (eg put the largest first etc) ?

Answer 1

The standard doesn't place any restrictions on the actual layout of the types. The only things the order influences are the results of std::get<N>, std::tuple_element<N, T> and so on.

I know that libstdc++ and Visual C++ lay out the types in reverse order of the order given; libc++ lays out the types in the order given. This essentially means that there is no portable way to pick an order that always produces the best layout.

Other orders are possible, though. An implementation is allowed to implement tuple with a layout that always produces minimal size but still preserves the same semantics for std::get<N> and so on. I don't know of any standard library implementation that does this, though.

Answer 2

The standard does not specify an implementation for std::tuple. However it guarantees that std::tuple<A,B,C> shall be a different type than for example std::tuple<B,A,C>. std::tuple is an ordered list of types.

boost::fusion provides a data type for a set style container of types, for cases where the order is not important: boost::fusion::set<>