How to use std::find algorithm on a vector of boolean elements

Question

I'm getting this std::_Bit_const_iterator error when trying to find an element in a std::vector<bool> using std::find. Can somebody explain what is it that I'm doing wrong.

#include <iostream> #include <vector> #include <iterator> #include <algorithm> int32_t main(int32_t argc, char* argv[]) { std::vector<std::vector<bool>> bin_mat; bin_mat = { { true, false, false }, { false, false, true }, { true, false, false } }; for(const auto& row : bin_mat) { const std::vector<bool>::iterator row_itr = std::find(row.begin(), row.end(), true); std::size_t column_index = std::distance(row_itr, row.begin()); std::cout << column_index << std::endl; } return EXIT_SUCCESS; }

This is the error I'm getting conversion from ‘std::_Bit_const_iterator’ to non-scalar type ‘const iterator {aka const std::_Bit_iterator}’ requested const std::vector<bool>::iterator row_itr = std::find(row.begin(), row.end(), true);

Unrelated, the call to std::distance is odd. I think you mean to swap the arguments. — cigien
– cigien, Commented Sep 13, 2020 at 16:12
Then you need to swap the arguments, you want the distance from begin to row_itr not the other way around. — cigien
– cigien, Commented Sep 13, 2020 at 16:18

Igor Tandetnik · Accepted Answer · 2020-09-13 16:11:37Z

Since row is a const std::vector<bool>, the return value of std::find is of type std::vector<bool>::const_iterator, and is not convertible to std::vector<bool>::iterator.

Note that const iterator is not at all the same thing as const_iterator - the former means the iterator itself cannot change (e.g. can't be pointed to a different element); the latter means the element pointed to cannot be changed via this iterator.

Just make it auto row_itr = std::find(...) and let the compiler figure out the right type.

463035818_is_not_an_ai · Accepted Answer · 2020-09-13 16:25:03Z

You are trying to get a iterator to a const vector. Use a const_iterator and the code is fine:

for(const auto& row : bin_mat) { std::vector<bool>::const_iterator row_itr = std::find(row.begin(), row.end(), true); // ^^ ----------- !!! // ... }

You can, but you do not have to make the row_itr itself constant, but you can only get a const_iterator from a const vector (it is const auto& row). You cannot modify the container element via a const_iterator, while a cosnt iterator cannot be assigned a different iterator after initialization. They are different types and you cannot turn a const_iterator to a iterator, that would break const-correctness. Thats basically what the error message is trying to say, leaked implementation details aside it says:

conversion from ‘const_iterator’ to ‘const iterator’ requested

This is similar to pointers to constant ( -> const_iterator) vs constant pointer ( -> const iterator).

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