what's wrong with vector::reserve? [duplicate] - c++

Question

This question already has answers here:
Choice between vector::resize() and vector::reserve()
(4 answers)
Why can't you access memory allocated by vector::reserve
(3 answers)
Closed 3 years ago.
I am just trying to learn STL for Competitive programming and stuck with this doubt!
1. When i use vector::reserve(n) my loops labeled as loop1 and loop2 don't print anything.
2. but when i use vector::assign(n,0) my loops labeled as loop 1 and loop 2 works fine.
why is it so?
#include <iostream>
#include <cstdio>
#include <cstdlib>
#include <vector>
#include <algorithm>
#include <iterator>
using namespace std;
int main()
{
int test;
scanf("%d", &test);
while (test > 0) {
int n;
scanf("%d", &n);
vector<int> arr;
arr.reserve(n);
//arr.assign(n,0);
for (int i = 0; i < n; i++) {
scanf("%d", &arr[i]);
}
sort(arr.begin(), arr.end());
vector<int>::iterator itr;
// loop1
for (int x : arr) {
printf("%d ", x);
}
//loop2
for (itr = arr.begin(); itr != arr.end(); itr++) {
printf("%d ", *itr);
}
test--;
}
return 0;
}

Answer 1

This is a common mistake. std::vector::reserve does not create elements or change the size of the container; you're actually causing undefined behavior. reserve changes just the capacity. You are looking for std::vector::resize to change the size. Here's an example for clarity:
#include <iostream>
#include <vector>
int main() {
std::vector<int> ivec;
std::cout << ivec.size() << " - " << ivec.capacity() << '\n'; // 0 - 0
ivec.reserve(100);
std::cout << ivec.size() << " - " << ivec.capacity() << '\n'; // 0 - 100
ivec.resize(30);
std::cout << ivec.size() << " - " << ivec.capacity() << '\n'; // 30 - 100
}

Answer 2

vector::reserve doesn't change the size of the vector. Instead, it just allocates additional memory, increasing the capacity of the vector for operations such as push_back.
For example:
std::vector<int> v;
// v.size() == 0, v.capacity() == 0
for(int i = 0; i < 100; i++) {
v.push_back(i); // This will resize the vector a few times
}
// v.size() == 100, v.capacity() >= 100
Versus
std::vector<int> v;
v.reserve(100);
// v.size() == 0, BUT v.capacity() >= 100
for(int i = 0; i < 100; i++) {
v.push_back(i); // This won't resize the vector now
}
If you want to change the size of the vector, use vector::resize.