Range-for-loops and std::vector<bool>

Question

Why does this code work

std::vector<int> intVector(10); for(auto& i : intVector)     std::cout << i; 

And this doesn't?

std::vector<bool> boolVector(10); for(auto& i : boolVector)     std::cout << i; 

In the latter case, I get an error

error: invalid initialization of non-const reference of type ‘std::_Bit_reference&’ from an rvalue of type ‘std::_Bit_iterator::reference {aka std::_Bit_reference}’

for(auto& i : boolVector) 

Answer 1

Because std::vector<bool> is not a container !

std::vector<T>'s iterators usually dereference to a T&, which you can bind to your own auto&.

std::vector<bool>, however, packs its bools together inside integers, so you need a proxy to do the bit-masking when accessing them. Thus, its iterators return a Proxy.
And since the returned Proxy is an prvalue (a temporary), it cannot bind to an lvalue reference such as auto&.

The solution : use auto&&, which will correctly collapse into an lvalue reference if given one, or bind and maintain the temporary alive if it's given a proxy.

Answer 2

std::vector<bool> does not obey the standard container rules.

In particular, its operator[] does not return bool&.

The loop in the invalid code

#include <vector> #include <iostream>  int main() {   std::vector<bool> boolVector(10);   for (auto& i: boolVector)       std::cout << i; } 

can be rewritten in any of three ways to iterate through the values:

1. (read-only)

   for (auto i: boolVector)     std::cout << i; 

2. (read-only, possibly inefficient)

   for (auto const& i: boolVector)       std::cout << i; 

3. (read/write)

   for (auto&& i: boolVector)     std::cout << i; 

The choice between the first and last is down to whether you need to modify the values in the vector, or just to read them.