How it works...

Structured bindings are always applied with the same pattern:

auto [var1, var2, ...] = <pair, tuple, struct, or array expression>;

• The list of variables var1, var2, ... must exactly match the number of variables contained by the expression being assigned from.
• The <pair, tuple, struct, or array expression> must be one of the following:
  • An std::pair.
  • An std::tuple.
  • A struct. All members must be non-static and defined in the same base class. The first declared member is assigned to the first variable, the second member to the second variable, and so on.
  • An array of fixed size.
• The type can be auto, const auto, const auto&, and even auto&&.

If we write too many or not enough variables between the square brackets, the compiler will error out, telling us about our mistake:

std::tuple<int, float, long> tup {1, 2.0, 3};
auto [a, b] = tup; // Does not work

This example obviously tries to stuff a tuple variable with three members into only two variables. The compiler immediately chokes on this and tells us about our mistake:

error: type 'std::tuple<int, float, long>' decomposes into 3 elements, but only 2 names were provided
auto [a, b] = tup;