proxy-iterators and auto&&

3/4/2025 / 3 minutes to read / Tags: cpp

I regularly use range-based for loops (e.g. auto& e : elements) for direct access to each element in a container. However I recently came across the following issue:

std::vector<bool> vec{true, false, true, true};
for (auto& v : vec)
{
  // Modify v below
  v = !v;
  std::cout << v << " ";
}
// error: cannot bind non-const lvalue reference of type ‘std::_Bit_reference&’ to an rvalue of type ‘std::_Bit_iterator::reference’

It turns out that std::vector<bool> is a specialized template that packs boolean values into bits rather than storing each as a separate bool. Because individual bits cannot be referenced directly, std::vector<bool> provides a proxy type called std::vector<bool>::reference instead of a real bool&. auto& here expects a real reference, which is not provided. The workaround for this is to use the universal reference auto&& to bind correctly to the proxy reference :

for (auto&& v : vec)
{
  // Modify v below
  v = !v;
  std::cout << v << " ";
}