std::vector<bool> guaranteed to default all entries to false?

Question

I want all of the entries in my std::vector to be false after I resize it to the desired container size. This appears to be the case on testing, but I can't seem to find any documentation that guarantees this to always be the case.

I realize I could set everything to false myself, but this seems inefficient if it is already guaranteed to default to false for every entry (it is a decent sized vector and will be created in thousands of places).

Is there any guarantee of this? Thanks in advance.

Answer 1

resize will default-insert elements, going by the (C++11) standard this calls:

allocator_traits<bool>::construct(m, p)

Where m is an allocator, and p points to the target address.

This in turn (assuming m is a standard allocator) calls

m.construct(p)

which in turn calls

::new((void *)p) bool()

So your bool ends up being value-initialized, which for bools means zero-initialization, which means false.

If m is a custom allocator and for some reason is missing construct(), it still ends up being value-initialized.

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As it has been pointed out std::vector<bool> does not use the standard allocator, still resize() is defined as

void resize(size_type sz, bool c = false);

so the result is the same.

Answer 2

Yes, a std::vector<bool> is initialized with zeroed bytes, which are false (the default value for initialized bool-s) on most machines.

See the note on std::vector

Answer 3

In my opinion, if you use:

vec.resize(N, false);

Every entry should be set to false. Isn't that what you need ?

Moreover, if you don't specify false, it'll take the default value, which is false for bool.