Test if a lambda is stateless?

Question

How would I go about testing if a lambda is stateless, that is, if it captures anything or not? My guess would be using overload resolution with a function pointer overload, or

Answer 1

Boost.TypeTraits is_stateless seems to do the job for whatever reason without much drama:

#include
#include
int main(){
  auto l1 = [a](){ return 1; };
  auto l2 = [](){ return 2; };
  auto l3 = [&a](){ return 2; };

  assert( boost::is_stateless::value == false );
  assert( boost::is_stateless::value == true );
  assert( boost::is_stateless::value == false );
}

boost::is_stateless is simple the combination of other conditions, it can be expressed in terms of standard type traits I suppose:

::boost::is_stateless = 
::boost::has_trivial_constructor::value
&& ::boost::has_trivial_copy::value
&& ::boost::has_trivial_destructor::value
&& ::boost::is_class::value
&& ::boost::is_empty::value

http://www.boost.org/doc/libs/1_60_0/libs/type_traits/doc/html/boost_typetraits/reference/is_stateless.html

Check my other answer based on sizeof: https://stackoverflow.com/a/34873353/225186