c++ - 当我包含向量时，为什么 std::begin 可以无条件地访问？

Question

我认为我总是需要 的std::资格std::something，除非我是using std::something或整体namespace std，但似乎这#include <vector>使得，例如，std::begin可用begin。

为什么会这样？

#include <vector>
int main(){
    int w[3];
    begin(w); // Ok, this doesn't compile, as I'm not `using std::begin` nor `using namespace std`.
    std::vector<int> v;
    v.begin(); // Ok, as this is the member function.
    begin(v); // Why does this compile? What is making this free-function available unqualifiedly?
}