mongocxx cursor and rvalue references

Question

I was looking to the mongocxx query exemples and I don't get whats the point of using auto&& over auto& here.

auto cursor = db["restaurants"].find({}, opts);
        for (auto&& doc : cursor) {
            std::cout << bsoncxx::to_json(doc) << std::endl;
}

In the documentation, they use it this way:

mongocxx::cursor cursor = collection.find(document{} << finalize);
for(auto doc : cursor) {
  std::cout << bsoncxx::to_json(doc) << "\n";
}

I want to use for(auto& doc : cursor)

What is the best practice here, and why ?

score 2 · Accepted Answer · answered Nov 04 '16 at 17:04

In this bit:

for (auto&& doc : cursor)
...

The "range expression" in "range for" can return a temporary.

It is a "best practice" to use rvalue reference here (when using auto).

Have a look at this: http://en.cppreference.com/w/cpp/language/range-for

Quote:

If range_expression returns a temporary, its lifetime is extended until the end of the loop, as indicated by binding to the rvalue reference __range, but beware that the lifetime of any temporary within range_expression is not extended.

And this:

http://www.artima.com/cppsource/rvalue.html

Quote:

An rvalue reference behaves just like an lvalue reference except that it can bind to a temporary (an rvalue), whereas you can not bind a (non const) lvalue reference to an rvalue.

mongocxx cursor and rvalue references

1 Answers1