It's a lambda expression ("closure"/"anonymous function"/…) that doesn't capture any outside state (see e.g. http://en.cppreference.com/w/cpp/language/lambda ht

It's a lambda expression ("closure"/"anonymous function"/…) that doesn't capture any outside state (see e.g. http://en.cppreference.com/w/cpp/language/lambda http://en.cppreference.com/w/cpp/language/lambda).