On Fri, Feb 17, 2012 at 1:30 AM, Jeffrey Lee Hellrung, Jr. <jeffrey.hellrung@gmail.com> wrote:
On Thu, Feb 16, 2012 at 3:06 PM, Robert Dailey <rcdailey@gmail.com> wrote:
On Thu, Feb 16, 2012 at 5:01 PM, Jeffrey Lee Hellrung, Jr. < jeffrey.hellrung@gmail.com> wrote:
On Thu, Feb 16, 2012 at 2:40 PM, Robert Dailey <rcdailey@gmail.com> wrote:
I'm using C++03 and would love some way to apply move semantics to a std::string. Can boost facilitate this in some way without using smart pointers? Consider an example like below (This is a contrived example, so assume NRVO does not apply):
std::string GetInformation( int someNumber ) { // We use 'someNumber' to look up a string std::string myValue( "hello world" ); // Create the string here return myValue; }
Above, the return value would result in a copy, I'd like to move it instead.
Well I think you can always make NRVO apply if you're always returning the same variable, which is always possible: just swap what you would otherwise return into that common variable.
In general, though, no, I don't think you can cleanly introduce (emulated) move semantics non-intrusively in C++03.
Just to be clear, what "common variable" are you referring to?
Thanks for your answers.
std::string foo() { std::string result; /*...*/ // would like to "return x;", but instead... result.swap(x); return result; /*...*/ // would like to "return y;", but instead... result.swap(y); return result; }
- Jeff
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Or, if you're willing to count on your compiler's brains, you could avoid the common variable: template <typename T> inline T swapped(T& x) { T res; swap(x, res); return res; } string f() { [...] // res.swap(x); // return res; return swapped(x); } Note that in either case, this only avoids the copy from the return-statement expression to the return-value. What happens with the returned string-temporary in the context of the caller is a different issue. If you use it to construct another string, RVO will usually kick in. However, anything else is likely to introduce a superfluous copy, or something similiar. For example, assigning it to a string, passing it to a function taking a 'const string&', and so on... There has been a discussion not so long ago about rvalue emulation in C++03, which seems to have faded away. I proposed a method there for non-intrusive rvalue-ref emulation, with an example of how it can be used to avoid excessive temporaries for string concatenation. Depending on what you're actually trying to acheive, you might find this interesting: [http://lists.boost.org/Archives/boost/2012/02/190102.php] -- Dan