JuniorTheoryCommon

What do the vectors `a` and `b` hold after `reserve(5)` and `resize(5)`?

C++

#include <vector>

int main() {
    std::vector<int> a;
    a.reserve(5);
    std::vector<int> b;
    b.resize(5);
}

a stays empty but has room for at least 5 elements; b holds five zeros

Expecting reserve to create elements: it only allocates memory, so a.size() stays 0, and even a[0] is out of range.
Expecting resize only to allocate memory: it sets b.size() to 5 and adds elements with the value 0, the same value int x{} gets.

Why call reserve before a loop that adds a known number of elements with push_back?
What does a later b.resize(2) do to b?

JuniorTheoryCommon

C++

#include <vector>

int main() {
    std::vector<int> a;
    a.reserve(5);
    std::vector<int> b;
    b.resize(5);
}

a stays empty but has room for at least 5 elements; b holds five zeros

Expecting reserve to create elements: it only allocates memory, so a.size() stays 0, and even a[0] is out of range.
Expecting resize only to allocate memory: it sets b.size() to 5 and adds elements with the value 0, the same value int x{} gets.

Why call reserve before a loop that adds a known number of elements with push_back?
What does a later b.resize(2) do to b?

What do the vectors a and b hold after reserve(5) and resize(5)?