On 3/21/21 11:48 PM, Gavin Lambert via Boost wrote:
On 22/03/2021 5:48 pm, Robert Ramey wrote:
I have a really dumb question about this. Consider the count_if example. I understand how example works. But I can't figure out where the parameter _1 gets filled in. So I think that the count_if example should be completed with a small bit of code which uses the newly count_if function.
The _1 parameter is filled in by std::count_if itself (in this case, using each element of the passed iterators). That's the point of a lambda, so I'm not quite sure what you're unclear on. This is not creating a "count_if" function, if that's what you thought; it is a complete example including both definition and usage.
Having said that, I do think the docs could benefit by having some "compare and contrast" examples -- i.e. showing how each of those examples would have been implemented using C++11 lambdas or std::bind instead (and thus demonstrate the syntactic benefits of the library).
OK - I see this now. I knew is something dumb. I haven't needed lambdas much in the work I've done. I think the following as an example #include <boost/lambda2.hpp> #include <algorithm> #include <vector> #include <iostream> void main(){ std::vector x = {0,1,3,4,234,2,234,55,23}; std::cout << "using standard lambda - number of even values is: " << "std::count_if( x.begin(), x.end, bool [](x::value_type z){ return z % 2 == 0 } ); << '\n'; // using "boost::lambda2" std::cout << "using "boost::lambda2 - number of even values is: " << "std::count_if( x.begin(), x.end, boost::_1 % 2 == 0 ); << '\n'; } // explain why lambda2 would be preferred in this case. That is, what improvement does it provide? And of course it would provide a perfect place to include a short description of the motivation for such a library. Robert Ramey
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