Why auto deduces a reference object instead of bool for std::bitset::operator []

Question

I have the following code

std::bitset<32> bs{21};
auto ref_obj = bs[0];
auto &another_ref = bs[0];
bool bool_obj = bs[0];

The type of ref_obj is not bool . But another_ref has the same type as ref_obj . std::bitset::operator[] has 3 overloads listed at http://en.cppreference.com/w/cpp/utility/bitset/operator_at

But I could not figure out why it is so.

Answer 1

According to the C++11 standard, if your bitset is not const (and yours isn't), then a reference type is returned:

§20.6.2 bitset operations:

constexpr bool operator[](size_t pos) const; // for b[i];
reference operator[](size_t pos); // for b[i];

Answer 2

The minimum addressable type in C++ is a byte which is presumably to be at least 8 bits [intro.memory/1] . Hence there is no way to return a reference to a single bit (which is less than 1 byte). So when you need a reference to that, a proxy that does some black magic is returned.

Your object isn't const , hence it is assumed you may want to modify the element through the via the [] operator. You can cast it to a const reference and access the const qualified member overload.