structured bindings with std::minmax and rvalues

Question

I ran into a rather subtle bug when using std::minmax with structured bindings. It appears that passed rvalues will not always be copied as one might expect. Or

Answer 1

What's important to note in auto [amin, amax] is that the auto, auto& and so forth are applied on the made up object e that is initialized with the return value of std::minmax, which is a pair. It's essentially this:

auto e = std::minmax(3, 6);

auto&& amin = std::get<0>(e);
auto&& amax = std::get<1>(e);

The actual types of amin and amax are references that refer to whatever std::get<0> and std::get<1> return for that pair object. And they themselves return references to objects long gone!

When you use std::tie, you are doing assignment to existing objects (passed by reference). The rvalues don't need to live longer than the assignment expressions in which they come into being.

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As a work around, you can use something like this (not production quality) function:

template
auto as_value(std::pair in) {
    using U1 = std::decay_t;
    using U2 = std::decay_t;
    return std::pair(in);
}

It ensures the pair holds value types. When used like this:

auto [amin, amax] = as_value(std::minmax(3, 6));

We now get a copy made, and the structured bindings refer to those copies.