Re: [Boost-users] Odd Warning in transform_iterator.hpp ?

On 7/28/2011 4:10 AM, Nathan Ridge wrote:

...

I'm using the current Boost library on Microsoft Visual Studio 2005. I'm getting a warning on line 121 of transform_iterator.hpp, complaining "returning address of local variable or temporary". The line in question is the body of this function:

typename super_t::reference dereference() const { return m_f(*this->base()); }

My m_f supplied to the template is ordinary enough; it returns a value.

Is your m_f a function object that uses result_of protocol to declare its return value?

It may be that while its operator() returns a value, its result<> is telling transform_iterator that it returns a reference.

For example, if its result<> looks like this:

template<typename> struct result; template struct result { typedef T type; };

then transform iterator will deduce its return type to be a reference:

typedef typename ia_dflt_help< Reference , result_of >::type reference;

and it will use this as the return type of operator*. Hence the warning.

Instead, you need to declare result<> like this:

template<typename> struct result; template struct result { typedef T type; };

(and possibly add a non-const reference version if necessary).

Regards, Nate.

Many thanks for the explanation. Perhaps that should be added to http://www.boost.org/doc/libs/1_47_0/libs/utility/utility.htm#result_of, or replace the struct result in the instructions!