Menu
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?
696
The vector<bool> class is a partial specialization of vector for elements of type bool . It has an allocator for the underlying type that's used by the specialization, which provides space optimization by storing one bool value per bit.
First, what's wrong with vector<bool> ? Because vector<bool> holds bits instead of bools, it can't return a bool& from its indexing operator or iterator dereference. This can play havoc on quite innocent looking generic code.
If vector1 and vector2 are two vectors of the same length, then vector1 == vector2 outputs a vector of that same length. Element i of the vector created by vector1 == vector2 equals TRUE if vector1[i] == vector2[i] and equals FALSE otherwise. A vector in which every element is TRUE or FALSE is called a Boolean vector.
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.
144
If you love us? You can donate to us via Paypal or buy me a coffee so we can maintain and grow! Thank you!
Donate Us With
