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

An option could be to explicitly look a the size of the type, a stateless should in principle have the same size as other stateless types (I picked std::true_type for a reference type).

#include
#include

template
struct is_stateless_lambda : std::integral_constant{};

int main(){

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

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

I don't know how portable this solution is. In any case check my other solution: https://stackoverflow.com/a/34873139/225186