How to deal with "non-const reference cannot bind to bit-field" error?

I encounter this error while using emplace_back to construct a bit field structure in a vector:

struct Foo
{
  Foo(uint32_t foo1): foo1(foo1) {}
  uint32_t foo1 : 4;
};


int main()
{
  vector<Foo> fooVector;
  Foo foo = Foo(10);

  fooVector.emplace_back(foo.foo1); // Error: non-const reference cannot bind to bit-field
}

I have found that I can avoid this error by either using push_back (but I prefer emplace_back for performance reasons), or by modifying the concerned line like so:

fooVector.emplace_back(uint32_t(foo.foo1)); // Don't raise error

Is it the proper way to deal with it?
Why does this solution work?

>Solution :

This is the definition of std::vector::emplace_back:

template< class... Args >
reference emplace_back( Args&&... args );

Args&& in this case gets deduced to uint32_t&, and, as the error says, a bit-field (Foo::foo1) cannot be bound to this type since it is a non-const reference.

In general, you cannot have a reference or a pointer to a bit-field because it has no address. A const reference works because it creates a temporary that is copy initialized with the value of the bit-field, and binds to that temporary instead.

You can indeed do an intermediate cast, like in your example or more explicitly like this:

fooVector.emplace_back(static_cast<uint32_t>(foo.foo1));

However, I am questioning why you want a 8-bit bitfield of a uint32_t instead of just using a uint8_t.