Why does the = operator work on structs without having been defined?

Let's look at a simple example:

struct some_struct {
   std::string str;
   int a, b, c;
}

some_struct abc, abc_copy;
abc.str = "some text";
abc.a = 1;
abc.b = 2;
abc.c = 3;

abc_copy = abc;

Then abc_copy is an exact copy of abc.. how is it possible without defining the = operator?

(This took me by surprise when working on some code..)