how to complement the values of std::vector using range for loop taking element by reference

Question

I need to complement the values of std::vector<bool>. So I thought of using range for loop taking elements by reference. But the compiler is giving me the below error

error: cannot bind non-const lvalue reference of type 'std::_Bit_reference&' to an rvalue of type 'std::_Bit_iterator::reference'
   13 |     for (auto& bit : rep)

This is my sample code

#include <iostream>
#include <vector>

int main() {
    std::vector<bool> rep;
    rep.push_back(true);
    rep.push_back(false);
    rep.push_back(true);
    rep.push_back(false);
    rep.push_back(true);
    rep.push_back(false);

    for (auto& bit : rep)
        bit = !bit;
}

This was probably a trick question, if you found it as challenge somewhere. — Sebastian, Feb 06 '22 at 06:14
Does this answer your question? [Why does std::vector have no .data()?](https://stackoverflow.com/questions/46115669/why-does-stdvectorbool-have-no-data) — Sebastian, Feb 06 '22 at 06:16
I would recommend to use `std::bitset` if possible. I think `std::vector` is design mistake that they have to keep. — Slava, Feb 06 '22 at 06:38

apple apple · Accepted Answer · 2022-02-06T06:31:18.437

3

you can bind them (the returned proxy object) to rvalue reference

for (auto&& bit : rep)
   bit = !bit;

godbolt

edited Feb 06 '22 at 06:31

answered Feb 06 '22 at 06:17

apple apple

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But would inverting the bits work as intended? – Sebastian Feb 06 '22 at 06:19
1

@Sebastian yes it'd work https://godbolt.org/z/fWdaKddh6 – apple apple Feb 06 '22 at 06:20
actually you can probably also catch the proxy object by value. (I'm not sure standard require them to be copyable though) – apple apple Feb 06 '22 at 06:22

Ch3steR · Answer 2 · 2022-02-06T07:03:19.570

You could also use std::vector<bool>::flip to toggle every value in the vector<bool>.

std::vector<bool> rep{true, false, true, false, true, false};

for(bool v: rep)
    std::cout << v; 
std::cout << '\n';
rep.flip();
for(bool v: rep)
    std::cout << v;

Output:

101010
010101

std::vector<bool>::reference is the proxy type stored underneath which also has flip function std::vector<bool>::reference::flip.

for(auto&& v: rep)
    v.flip();

how to complement the values of std::vector using range for loop taking element by reference

2 Answers2