Lambda implicit capture fails with variable declared from structured binding

Question

With the following code, I get a compile error C2065 'a': undeclared identifier (using visual studio 2017):

[] {
    auto [a, b] = [] {return std::make_tuple(1, 2); }();
    auto r = [&] {return a; }(); //error C2065
}();

However, the following code compiles:

[] {
    int a, b;
    std::tie(a, b) = [] {return std::make_tuple(1, 2); }();
    auto r = [&] {return a; }();
}();

I thought that the two samples were equivalent. Is it a compiler bug or am I missing something ?

Answer 1

Core issue 2313 changed the standard so that structured bindings are never names of variables, making them never capturable.

P0588R1's reformulation of lambda capture wording makes this prohibition explicit:

If a lambda-expression [...] captures a structured binding (explicitly or implicitly), the program is ill-formed.

Note that this wording is supposedly a placeholder while the committee figures out exactly how such captures should work.

_{Previous answer kept for historical reasons:}

---

This technically should compile, but there's a bug in the standard here.

The standard says that lambdas can only capture variables. And it says that a non-tuple-like structured binding declaration doesn't introduce variables. It introduces names, but those names aren't names of variables.

A tuple-like structured binding declaration, on the other hand, does introduce variables. a and b in auto [a, b] = std::make_tuple(1, 2); are actual reference-typed variables. So they can be captured by a lambda.

Obviously this is not a sane state of affairs, and the committee knows this, so a fix should be forthcoming (though there appears be some disagreement over exactly how capturing a structured binding should work).

Answer 2

You can use init-capture like this, as suggested in https://burnicki.pl/en/2021/04/19/capture-structured-bindings.html. The variable is captured by reference, so there is no overhead, and no need to deal with pointers.

auto [a, b] = [] { return std::make_tuple(1, 2); }();
auto r = [&a = a] { return a; }();

Using the same name both for structured binding and for its reference can be misleading, but actually it means

auto r = [&a_ref = a] { return a_ref; }();

As far as I know, it works on all compilers starting from C++17 and above, including clang.

Answer 3

A possible workaround is to use a lambda capture with the initializer. The following code compiles fine in Visual Studio 2017 15.5.

[] {
    auto[a, b] = [] {return std::make_tuple(1, 2); }();
    auto r = [a = a] {return a; }();
}();

Answer 4

Now lambda could capture structured binding since c++20, see this.