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

As per the Standard, if a lambda doesn't capture any variable, then it is implicitly convertible to function pointer.

Based on that, I came up with is_stateless<> meta-function which tells you whether a lambda is stateless or not.

#include 

template 
struct helper : helper
{};

template 
struct helper 
{
    static const bool value = std::is_convertible::value;
};

template
struct is_stateless
{
    static const bool value = helper::value;
};

And here is the test code:

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

    std::cout<::value<< "\n";
    std::cout<::value<< "\n";
    std::cout<::value<< "\n";
}

Output:

false
true
false

Online Demo.