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Document number: N3610

Date: 2013-03-15

Generic Lambda-capture Initializers, Supporting Capture-by-move

by Ville Voutilainen

Excerpt:

C++11 lambdas do not support capture-by-move. There has been at least two rejected NB comments on this for C++11, the first one was JP9 on CD1, the second was FI8 on FCD. FI8 refers to a Core reflector message by Roshan Naik, where Roshan explains that it would be very useful to be able to move-capture containers and other objects that are expensive to copy. Furthermore, move-only types like iostreams (especially stringstreams) and unique_ptrs can't be captured without wrapping them. This lack of cooperation between move semantics and lambdas was briefly discussed in the Evolution Working Group in Portland 2012 as one of potential extensions that would "complete C++11". Unfortunately, scheduling conflicts and lack of time has prevented producing a wording proposal for such an extension; this paper tries to explain the intended design.