Re: [boost] [utility] string_ref construction from rvalue reference to string

On 03/08/2014 09:27 a.m., Andrey Davydov wrote:

On Sat, Aug 2, 2014 at 2:34 AM, Agustín K-ballo Bergé

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wrote:

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You wouldn't generally use `string_ref`s as automatic variables or members.

I can't confirm or refute this statement using code of Boost, because `boost::string_ref` has been used a little so far. But results of "grep StringRef" inside LLVM code show that StringRef's are used often as automatic variables. And in addition to explicitly defined class members there are another dangerous `string_ref` use cases, namely lambda capturing value and `bind` argument.

LLVM has some memory management that could be considered extreme, and it does things to avoid even a single allocation (for good reason). Nevertheless, you'll note that I said "wouldn't generally", and not "can't", "won't" or "dangerous". There's nothing specially dangerous about `string_ref` to temporaries, or as members. In order to use `string_ref`, even for the simplest of use cases, you have to guarantee that the referred content will outlive it. If you use a `string_ref` as a function parameter, the language will guarantee that for you, regardless of the value category of the argument. Otherwise, it's "dangerous" as in you have think what you are doing. I took your idea a step further, and implemented your proposed change to `StringRef`. I got hundreds of compilation errors. Most of them stemmed from construction of `StringRef` from temporaries, not explicitly as in your example, but as arguments to functions. There where other sources of errors, like comparisons between `StringRef` and a temporary.

It's possible to add `string_ref(reference_wrapper<const std::string>)` constructor to allow people write `print(boost::cref(getStr()))` instead of `print(getStr().c_str())`. The first version is more efficient and less ugly IMO. I want to emphasize, that it should be `boost::cref` not `std:: cref`, because `std::cref(std::string &&) = deleted`.

Not as of 1.56.

It seems to me, that prohibition of constructing `reference_wrapper` from temporary in C++11 is one more argument in favor of my proposition to prohibit `string_ref` construction from temporary string.

A `reference_wrapper` is a wrapper for an lvalue reference. The motivation to prohibit binding to an rvalue is a different one. Regards, -- Agustín K-ballo Bergé.- http://talesofcpp.fusionfenix.com