[Bug libstdc++/48635] New: [C++0x] unique_ptr<T, D&> moves the deleter instead of copying it
daniel.kruegler at googlemail dot com
gcc-bugzilla@gcc.gnu.org
Fri Apr 15 21:01:00 GMT 2011
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=48635
Summary: [C++0x] unique_ptr<T, D&> moves the deleter instead of
copying it
Product: gcc
Version: 4.7.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: libstdc++
AssignedTo: unassigned@gcc.gnu.org
ReportedBy: daniel.kruegler@googlemail.com
The following program using 4.7.0 20110409 (experimental) should print "copy",
but it prints "move" instead:
//-------------
#include <memory>
#include <utility>
#include <iostream>
struct Deleter {
Deleter() = default;
Deleter(const Deleter&) = default;
Deleter(Deleter&&) = default;
Deleter& operator=(const Deleter&) {
std::cout << "copy" << std::endl;
return *this;
}
Deleter& operator=(Deleter&&) {
std::cout << "move" << std::endl;
return *this;
}
template<class T>
void operator()(T* p) const
{
delete p;
}
};
int main() {
Deleter d1, d2;
std::unique_ptr<int, Deleter&> p1(new int, d1), p2(nullptr, d2);
p2 = std::move(p1);
}
//-------------
The reason for the failure is, that the library implementation of the
move-assignment operator of std::unique_ptr (and of it's templated variant)
uses std::move to transfer the deleter from the source to the target, but as of
[unique.ptr.single.asgn] p. 2 and p. 6 it shall use std::forward instead. Doing
it correctly will ensure that the deleter is copied and not moved in this case,
which is an intended design aim of std::unique_ptr with deleters that are
lvalue-references.
The necessary fix is to replace the usage of std::move at the two places by
std::forward<deleter_type>.
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