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Re: Move semantics in stringstream

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Am 26.02.2012 17:04, schrieb Jason McKesson:
[..]
>
> In C++11, we have move semantics. So the copying is completely
> unnecessary from a language point of view. Sadly, the API was not
> properly updated to match.
>
> What we need is for std::stringbuf and the corresponding functions in
> the stringstream classes to be able to have a string moved into them.
> Also, they should be able to have their contents removed without a copy.


I agree.

> So it would look something like this:
>
> std::stringstream out_data{std::move(some_string)};


Presumably this is a function that implementors could easy provide
(and probably will).

> out_data<< ...;
> std::string my_data = out_data.move_str();


I generally agree with the usefulness of that.

> Where move_str() will actually steal the string from the stream. No
> string data is ever copied here. move_str should properly invalidate the
> stream, as if the user had moved from the stream itself.


My initial reaction was that this may be too strong (because the
function name may indicate a less harmful operation), but after some
further reflections I tend to agree.

> The current constructor of std::stringbuf, and the associated
> stringstream classes, take a `const&` to a basic_string. They should
> take their string argument by value (thus copying when movement is not
> allowed) and simply move it to where it needs to go. If that is not
> possible due to backwards compatibility issues, then it can take an
> r-value reference instead of a value.


There is no reason to enforce implementations to break their ABI. IMO
adding constructors that accept a basic_string<>&& should suffice. We
may want the same for the str() setter function. There are similar
examples in the standard that follow this signature "strategy", e.g.
the container adaptors.

Related to that one could argue that basic_regex should provide
similar string-based move operations.

Greetings from Bremen,

Daniel Krügler




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Move semantics in stringstream Jason McKesson<jmckesson@gmail.com> - 2012-02-26 08:04 -0800
  Re: Move semantics in stringstream Daniel Krügler <daniel.kruegler@googlemail.com> - 2012-03-04 14:23 -0800

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