C++ object assignment to NULL

Question

I was looking at some code that uses Boost.Function and have a question about how code can be written to allow assignment to NULL. I tried to track down the corresponding Boost code, but was unable to. Basically, what makes this possible?

boost::function<void()> func;
func = NULL;

EDIT: The following doesn't compile for me though, so how do they prevent this too?

func = 1;

Answer 1

By operator overloading with pointer parameter. From boost sources:

#ifndef BOOST_NO_SFINAE
   self_type& operator=(clear_type*)
   {
     this->clear();
     return *this;
   }
#endif

This doesn't mean that "func" itself is NULL, indeed you can access its own functions. Following code compiles and doesn't crash.

TEST_F(CppTest, BoostFunctions) {
    boost::function<void()> func;
    func = NULL;
    ASSERT_TRUE(func==NULL);
    ASSERT_FALSE(func.has_trivial_copy_and_destroy());
}