Question

Consider the case when “whole” objects with move semantics enabled are returned from functions, as with std::basic_string<>:

std::wstring build_report() const
{
    std::wstring report;
    ...

    return report;
}

Can I then realistically be expected to make the “best” choice whether to use the returned string with move semantics, as in

const std::wstring report(std::move(build_report()));

or if I should rely on (N)RVO to take place with

const std::wstring report(build_report());

or even bind a const reference to the temporary with

const std::wstring& report(build_report());

What scheme is there to make a deterministic choice of these options, if any?

EDIT 1: Note that the usage of std::wstring above is just an example of a move semantics enabled type. It just as well be swapped for your arbitrary_large_structure. 🙂

EDIT 2: I checked the generated assembly when running a speed-optmized release build in VS 2010 of the following:

std::wstring build_report(const std::wstring& title, const std::wstring& content)
{
    std::wstring report;
    report.append(title);
    report.append(content);

    return report;
}

const std::wstring title1(L"title1");
const std::wstring content1(L"content1");

const std::wstring title2(L"title2");
const std::wstring content2(L"content2");

const std::wstring title3(L"title3");
const std::wstring content3(L"content3");

int _tmain(int argc, _TCHAR* argv[])
{
    const std::wstring  report1(std::move(build_report(title1, content1)));
    const std::wstring  report2(build_report(title2, content2));
    const std::wstring& report3(build_report(title3, content3));

    ...

    return 0;
}

The 2 most interesting outcomes:

• Explicitly calling std::move for report1 to use the move constructor triples the instruction count.
• As noted by James McNellis in his answer below, report2 and report3 does indeed generate identical assembly with 3 times fewer instructions than explicitly calling std::move.

Answer 1

std::move(build_report()) is wholly unnecessary: build_report() is already an rvalue expression (it is a call of a function that returns an object by value), so the std::wstring move constructor will be used if it has one (it does).

Plus, when you return a local variable, it gets moved if it is of a type that has a move constructor, so no copies will be made, period.

There shouldn’t be any functional difference between declaring report as an object or as a const-reference; in both cases you end up with an object (either the named report object or an unnamed object to which the report reference can be bound).