Re: [Boost-users] [Iterators] Change in transform_iterator from 1.31 to1.32?

Nat Goodspeed wrote:

We have a class that contains a std::map. This class wants to provide its clients with iterators over just the values in the map, hiding the fact that there's a map under the covers at all. transform_iterator seems like just the ticket, and with Boost 1.31 we used these methods:

typedef std::map MapType;

typedef boost::transform_iterator< boost::function, MapType::const_iterator

...

const_iterator;

const_iterator begin() const { return boost::make_transform_iterator(mymap.begin(), boost::bind(&MapType::value_type::second, _1)); }

const_iterator end() const { return boost::make_transform_iterator(mymap.end(), boost::bind(&MapType::value_type::second, _1)); }

The above compiled cleanly, except when it caused a compiler ICE, and the instances that compile seem to run okay.

Now we're trying to upgrade to Boost 1.32 -- I know, a little late in the 1.32 lifespan. The same code now produces an ominous warning (full text at end of message):

c:/videobranch\boost\boost\function\function_template.hpp(111) : warning C4172: returning address of local variable or temporary

When it finally gets around to mentioning a line number in our own source code, the indicated line is the return statement in the above begin() method.

The change that's breaking your code is (unfortunately) in boost::bind, not in transform_iterator. In 1.31, boost::bind(&MapType::value_type::second, _1) returns a reference. In 1.32, the same construct returns by value. You can change boost::function to boost::function to return by value, or change boost::bind(&MapType::value_type::second, _1) to boost::bind(&MapType::value_type::second, _1) to return by reference. boost::bind was changed to return by value because the old behavior suffered from a similar reference-to-temporary problem with nested binds (that didn't even produce a warning.)