structured bindings: when something looks like a reference and behaves similarly to a reference, but it's not a reference

问题

Yesterday I\'ve seen an interesting question here on SO about structured binding.
We can sum up it as it follows. Consider the example code below:

#include <tuple>
#include <type_traits>

int main() {
    auto tup = std::make_tuple(1, 2);
    auto & [ a, b ] = tup;
    // the following line won\'t compile for a isn\'t a reference
    // static_assert(std::is_reference_v<decltype(a)>);
}

In this case decltype(a) is int (probably) because of this bullet (working draft):

if e is an unparenthesized id-expression naming a structured binding [...], decltype(e) is the referenced type as given in the specification of the structured binding declaration

Here is a snippet on wandbox provided by @Curious in the comments for those that are interested. It shows that actually a isn\'t a reference, nothing more.
So far so good for the original question, OP asked why it was int instead of int & and the standard says that looked like an acceptable answer.

Anyway, I\'d like to know why the committee decided so. At the end of the day, a refers to an element in the tuple and I can modify that element through a. In other terms, the declaration of a looks like the one of a reference, it behaves similarly to a reference but it\'s not a reference.

I can live with this, but I\'d like to know what are the reasons behind that. Why decltype(a) cannot be simply int &? Is there a meaningful reason that a profane can understand?

回答1:

I wrote this yesterday:

decltype(x), where x is a structured binding, names the referenced type of that structured binding. In the tuple-like case, this is the type returned by std::tuple_element, which may not be a reference even though the structured binding itself is in fact always a reference in this case. This effectively emulates the behavior of binding to a struct whose non-static data members have the types returned by tuple_element, with the referenceness of the binding itself being a mere implementation detail.

回答2:

This topic has been covered before (look in the structured-bindings tag) and the behavior you're talking about is even addressed in the second answer. However, the rationale is spelled out in p0144r2 section 3.5:

Should the syntax be extended to allow const/&-qualifying individual names' types?

For example:

auto [&x, const y, const& z] = f(); // NOT proposed

We think the answer should be no. This is a simple feature to store a value and bind names to its components, not to declare multiple variables. Allowing such qualification would be feature creep, extending the feature to be something different, namely a way to declare multiple variables.

If we do want to declare multiple variables, we already have a way to spell it:

 auto val    = f();
 T& x        = get<0>(val);
 T2 const y  = get<1>(val);
 T3 const& z = get<2>(val);

来源：https://stackoverflow.com/questions/44695684/structured-bindings-when-something-looks-like-a-reference-and-behaves-similarly