C++ reference can be assignable?

Question

I've been messing with references wrapped in container classes. Why is the following code legal and appear to behave correctly?

#include <iostream>

class Foo
{
public:
  Foo( int i ) : i_( i ) {}
  int i_;
};

class FooWrapper
{
public:
  FooWrapper( Foo& foo ) : foo_( foo ) {}
  Foo& foo_;
};

int main( int argc, char* argv[] )
{
  Foo foo( 42 );
  FooWrapper fw( foo );
  FooWrapper fw2 = fw;
  std::cout << fw2.foo_.i_ << std::endl;
  return 0;
}

In the absence of an explicit operator=, I believe C++ does a memberwise copy. So when I do FooWrapper fw2 = fw;, is this not two operations: (1) create FooWrapper fw with a default ctor followed by (2) assignment from fw to fw2? I'm sure this isn't happening, because a reference can't be created uninitialized, so is the creation of fw2 actually treated as a copy construction?

I'm sometimes unclear on the roles of copy construction vs. assignment; they seem to bleed into each other sometimes, as in this example, but there's probably some rule I'm unaware of. I'd be grateful for an explanation.

Answer 1

Despite the = in the syntax, FooWrapper fw2 = fw; copy constructs fw2 (using the copy constructor). Neither default construction nor assignment is involved at all.

And to answer the question in the title: no, references cannot be assigned. If you wrote code that attempted to actually default construct or assign a FooWrapper, such as:

FooWrapper fw2;
fw2 = fw;

...this would fail. Officially, only a "diagnostic" is required. Unofficially, every compiler I know of would/will refuse to compile it.