C++ tuples: time complexity for get(i)

Question

I want to access element at position i of a tuple( https://en.cppreference.com/w/cpp/utility/tuple/get ) in c++.\ If for instance I have:\

std::tuple <int, int, int> t{1, 2, 3};
int x = std::get<2>(t);

Is it constant time to access an element or linear?

Answer 1

The two following blocks of code are functionally equivalent:

std::tuple<int, int, int> t{1, 2, 3};
int x = std::get<2>(t);

struct __type_for_tuple_iii {int __1; int __2; int __3;};
__type_for_tuple_iii t{1, 2, 3};
int x = t._2;

NB This remains true no matter how many members the tuple has.

As you can see, std::get<>(std::tuple) doesn't really do much actual "work" per-se. More specifically, almost all of the work it does is guaranteed to be feasible during compilation. All that's left at runtime is a simple memory offset.

So you might as well be asking: "What is the complexity of accessing a struct's member?".

That answer to which is: O(1) , no matter how big the struct is, so the same goes for tuples and std::get<> .