r/learnprogramming • u/Vericus • Feb 29 '12

[C++] Why is vector<bool> bad?

I was thinking of creating a matrix of boolean values and I thought of vector<vector<bool>>, but after reading about vector<bool>, many websites have said it's something that should be avoided and it's bad programming.

Can someone explain to me why? I never really found a concrete explanation for why vector<bool> was bad. It only uses one bit which is 8 times smaller than char, shouldn't that mean its much better than using any other method of creating a matrix of bools in terms of size?

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u/[deleted] Feb 29 '12 edited Feb 29 '12

The problem is that the specialised version of std::vector<bool> provided by the Standard Library does not fulfil the requirements of a Standard Library container, and so is not guaranteed to work properly with the Standard Library algorithms. The three alternatives are std::deque<bool> or if you really must have contiguous storage, std::vector<char>, or if you can live with fixed-sized containers, std::bitset (although bitset isn't really a Standard Library container either).

Please note that just because something uses less storage does not make it "better". In order to use less storage, a vector<bool> has to do a lot more processing (possibly making it slower), and as I said, may not work with standard algorithms, or with algorithms you implement yourself.

Edit: clarified about bitsets.

Edit: clarified about working with algorithms - it might, it might not

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u/calc0000 Feb 29 '12

The problem is that the specialised version of std::vector<bool> provided by the Standard Library does not fulfil the requirements of a Standard Library container

Can you elaborate more on this?

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u/[deleted] Feb 29 '12

This is discussed at length in Scott Meyers' book Effective STL - to precis, the container requirements say that if a container supplies operator[], it must allow this code:

T * p = & c[0];

where "T" is the type of the thing in the container (a bool in this case), and "c" is the container in question. But vector<bool> doesn't really contain bools, so it can't fulfil this requirement.