Why can `auto b = v[0];` on a `std::vector ` be surprising?

It deduces a bitfield that cannot be used inside an if-statement, so the comparison silently returns false.

It deduces a proxy `std::vector ::reference` object (a bit reference), not `bool`, so `v[0]` behaves differently from other vectors.

vector is not allowed by the standard at all and will not compile in any conforming compiler implementation.

It returns a pointer to a single bit that is not addressable, so dereferencing it results in a compile error.

Multiple ChoiceIntermediate 45s

vector<bool> proxy reference

Why can `auto b = v[0];` on a `std::vector<bool>` be surprising?

Options

1It deduces a bitfield that cannot be used inside an if-statement, so the comparison silently returns false.
2It deduces a proxy `std::vector<bool>::reference` object (a bit reference), not `bool`, so `v[0]` behaves differently from other vectors.
3vector<bool> is not allowed by the standard at all and will not compile in any conforming compiler implementation.
4It returns a pointer to a single bit that is not addressable, so dereferencing it results in a compile error.

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