[Boost-users] [BLL] bind(data_member_ptr,_1) functor won't accept temporaries as arguments

In our last exciting episode Eelis van der Weegen wrote:

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When Lambda's bind() is used to create a functor for dereferencing a stored data member pointer given an instance pointer, it seems it is not possible to pass the resulting functor a temporary instance pointer. For example:

#include #include

struct S { int i; };

S * f ();

void g (S * p) { using namespace boost::lambda;

bind(&S::i, _1)(p); // ok bind(&S::i, _1)(p + 0); // not ok, "no match for call ..." bind(&S::i, _1)(f()); // not ok, "no match for call ..." }

Does this restriction have a purpose? If not, is this something that needs to be (and can be) fixed?

Lambda functors cannot take a non-const rvalue as an argument. There isn't a pretty fix for it at the momemnt, but there is some work going on in the standards committee regarding the so called "forwarding problem" which would allow a fix. So in a few years... :) Now, there are several workarounds: Use make_const helper function: bind(&S::i, _1)(make_const(p + 0)); or do: S* p2 = p + 0; bind(&S::i, _1)(p2); and more, check 5.9.2. Rvalues as actual arguments to lambda functors in lambda docs. Cheers, Jaakko