Unpacking nested tuples in C++

Question

std::tie provides a convenient way to unpack the contents of a tuple in C++ into separately defined variables, like the sample below illustrates

int a, b, c, d, e, f;

auto tup1 = std::make_tuple(1, 2, 3);
std::tie(a, b, c) = tup1;

However, if we have a nested tuple like below

auto tup2 = std::make_tuple(1, 2, 3, std::make_tuple(4, 5, 6));

attempting to compile the code

std::tie(a, b, c, std::tie(d, e, f)) = tup2;

fails with an error

/tmp/tuple.cpp:10: error: invalid initialization of non-const reference of type ‘std::tuple<int&, int&, int&>&’ from an rvalue of type ‘std::tuple<int&, int&, int&>’
  std::tie(a, b, c, std::tie(d, e, f)) = tup2;
                            ^

Is there an idiomatic way to unpack a tuple of tuples in C++?

Possible duplicate http://stackoverflow.com/questions/21298732/can-we-do-deep-tie-with-a-c1y-stdtie-like-function — n. m. could be an AI, Nov 08 '15 at 18:36
You can make a [temporary tuple for nesting](http://ideone.com/s3YLG9), though it's not as nice looking. — Cornstalks, Nov 08 '15 at 18:37

score 4 · Accepted Answer · answered Nov 08 '15 at 18:35

4

When you know that there is no risk, you can convert an rvalue reference to an lvalue one through the following helper function :

template <class T>
constexpr T &lvalue(T &&v) {
    return v;
}

You can then use it thusly :

std::tie(a, b, c, lvalue(std::tie(d, e, f))) = tup2;

In your case there is indeed no problem in doing that, since the inner tuple only has to be alive for the duration of the statement, and it is (exactly) the case.

answered Nov 08 '15 at 18:35

Quentin

62,093
7
131
191

Is it possible to define a composite function `ltie`, where calling `ltie(a, b, c)` is equivalent to calling `lvalue(std::tie(a, b, c))` in order to simplify the code to `ltie(a, b, c, ltie(d, e, f)) = tup2`; – D R Nov 08 '15 at 19:05
1

Unfortunately it isn't. If you bury the temporary tuple into a function, it will then be local to that function, and won't live long enough. You could make it a macro instead. – Quentin Nov 08 '15 at 19:12
4

Or just use `forward_as_tuple`. – T.C. Nov 08 '15 at 19:54

Unpacking nested tuples in C++

1 Answers1