C++ lambda operator ==

How do I compare two lambda functions in C++ (Visual Studio 2010)?

std::function<void ()> lambda1 = []() {};
std::function<void ()> lambda2 = []() {};
bool eq1 = (lambda1 == lambda1);
bool eq2 = (lambda1 != lambda2);

I get a compilation error claiming that operator == is inaccessible.

EDIT: I'm trying to compare the function instances. So lambda1 == lambda1 should return true, while lambda1 == lambda2 should return false.