What does libstdc++'s std::vector::data do?

Question

According to the standard, std::vector<bool> has no member function data(). However, the following snippet compiles fine with the latest GCC with libstdc++:

#include <vector>

int main () {
    std::vector<bool> v;
    v.data();
}

If we try to use the result, it turns out the return type is void.

Is this some gcc extension or a bug?
If the former is true, what does it do?

I disagree with the dupe. I am aware that the standard does not define the function in question, I'm asking about the behavior of a specific implementation. — Baum mit Augen, Oct 24 '16 at 21:03
@LightnessRacesinOrbit I admit I did not. Looks like that would have helped. :) — Baum mit Augen, Oct 24 '16 at 21:08
It looks like it also has `emplace()` and `emplace_back()`. They're probably no-ops, or something. — Justin Time - Reinstate Monica, Oct 24 '16 at 21:09

score 10 · Accepted Answer · answered Oct 24 '16 at 21:06

10

My /usr/include/c++/4.8/bits/stl_bvector.h has:

// _GLIBCXX_RESOLVE_LIB_DEFECTS
// DR 464. Suggestion for new member functions in standard containers.
// N.B. DR 464 says nothing about vector<bool> but we need something
// here due to the way we are implementing DR 464 in the debug-mode
// vector class.
void
data() _GLIBCXX_NOEXCEPT { }

In /usr/include/c++/4.8/debug/vector I see the declaration:

using _Base::data;

So that seems to be the reason: the debug version of std::vector<bool> wouldn't compile unless std::vector<bool>::data existed.

answered Oct 24 '16 at 21:06

Brian Bi

111,498
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[Link to the mentioned DR](https://gcc.gnu.org/onlinedocs/libstdc++/ext/lwg-defects.html#464) for reference. – Baum mit Augen Oct 24 '16 at 21:20
3

Sad. It would be useful if it returned a pointer to the packed buffer. Why not make it `private`? – Yakk - Adam Nevraumont Oct 24 '16 at 22:51

What does libstdc++'s std::vector::data do?

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