operator== for std::function compares a std::function with a null pointer, as far as I can tell the standard does not provide any details as to why.
Although, this boost FAQ entry, Why can’t I compare boost::function objects with operator== or operator!=? provides a rationale and as far as I can tell should be applicable to std::function as well. Quoting the FAQ:
Comparison between boost::function objects cannot be implemented “well”, and therefore will not be implemented. […]
it then outlines requested solutions similar to Preet’s and goes on to say:
The problem occurs when the type of the function objects stored by both f and g doesn’t have an operator==[…]
and explains why this has to has to be dealt with in either the assignment operator or constructor and then goes on to say:
All of these problems translate into failures in the boost::function constructors or assignment operator, even if the user never invokes operator==. We can’t do that to users.
Found a standards rationale in Accessing the target of a tr1::function object, which is pretty old but is consistent with the boost FAQ and says:
operator== is unimplementable for tr1::function within the C++ language, because we do not have a reliable way to detect if a given type T is Equality Comparable without user assistance.