Yes, std::function is a fairly inefficient class. It's useful for storing a lambda but not so much for passing it around. You can pass around lambdas as a template argument (requires all callees to be templates) or as a function pointer if there are no captures. If you have captures a helper class such as llvm::function_ref[1] might come in handy, which acts like a function pointer but also allows captures to be passed around.
[1] https://github.com/llvm-mirror/llvm/blob/master/include/llvm/ADT/STLExtras.h#L80 On Mon, Oct 24, 2016 at 8:32 PM, Jens Alfke via cfe-users <cfe-users@lists.llvm.org> wrote: > I’ve started using C++11 lambdas in my code. A few of them are in > performance-sensitive areas, so I was looking at (optimized) assembly code of > some of my methods to see what goes on under the hood. > > I don’t entirely understand the code, but the code on the calling side (i.e. > the part that creates the lambda and passes it as a function parameter) it > looks larger and slower than similar code using Clang blocks. It looks as > though there’s overhead in converting the lambda itself into a std::function > object … especially because I see a call to a std::function method that makes > a heap allocation. :'( The equivalent block-based code just fills out a small > struct on the stack and passes its address. > > Is there a more efficient way to pass a lambda to a function in C++? As far > as I can tell, a lambda is of an anonymous type that can’t be named*, so the > only way to pass it as a parameter is to wrap it in a std::function, which is > comparatively expensive. > > (My code has to be cross-platform, but I’m thinking of writing some macros > that can expand into either lambda or block expressions, depending on the > compiler. That way it’ll at least be more efficient when built with Clang.) > > —Jens > > PS: This is with Xcode 8, clang-800.0.38 > > * Ironically, my copy of the Tao Te Ching just arrived from Amazon while I > was writing this email, and it starts right off with the famous statement > “The way you can go isn’t the true way. The name you can say isn’t the true > name.” > _______________________________________________ > cfe-users mailing list > cfe-users@lists.llvm.org > http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-users _______________________________________________ cfe-users mailing list cfe-users@lists.llvm.org http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-users
