Basic <algorithm> (reverse_copy) doesn't work with QString

Following code returns empty string, why?

Code:

---

QString reverseQString( const QString & str )
{
    QString rev;
    reverse_copy( str.begin(), str.end(), rev.begin() );
    return rev;
}

---

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Added after 27 minutes:

Is the problem that reverse_copy in fact needs functionality of std::back_inserter, not just an iterator?

Hello,

the local object rev is an empty string. std::reverse_copy expects that the destination container (in your case rev) already has allocated memory to store the reverse copy. To achieve this, add

Code:

---

rev.resize(str.size());

---

before calling reverse_copy.

See also http://www.cplusplus.com/reference/a.../reverse_copy/
Best regards
ars

My guess would be that QString::begin() returns QString::end() when called on a null QString and one cannot assign to end().
I.e. QString::begin() of a null QString might not qualify as an OutputIterator.

You could try with a std::back_inserter.

Cheers,
_

Quote:

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Originally Posted by ars

Hello,

the local object rev is an empty string. std::reverse_copy expects that the destination container (in your case rev) already has allocated memory to store the reverse copy. To achieve this, add

Code:

---

rev.resize(str.size());

---

before calling reverse_copy.

See also http://www.cplusplus.com/reference/a.../reverse_copy/
Best regards
ars

---

It then works, thanks

Using std::back_inserter makes use of a sequence of QString::push_back() calls, which result in multiple memory reallocations (depending on the memory allocation strategy implemented in QString::push_back()). In the scenario posted by OP we already know the size of the result, so I expect better performance using a single memory reallocation than a sequence of reallocations. Of course this will not give measurable differences for small containers/strings.

Best regards
ars