Range-for-loops and std::vector

Question

Why does this code work

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

And this doesn\'t?

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Answer 1

std::vector does not obey the standard container rules.

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

The loop in the invalid code

#include 
#include 

int main() {
  std::vector 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.