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Showing content from https://en.cppreference.com/w/cpp/language/../memory/shared_ptr/../../container/vector/resize.html below:

std::vector<T,Allocator>::resize - cppreference.com

void resize( size_type count );

void resize( size_type count, const value_type& value );

Resizes the container to contain count elements:

• If count is equal to the current size, does nothing.
• If the current size is greater than count, the container is reduced to its first count elements.
• If the current size is less than count, then:

Additional

are appended.

2) Additional copies of value are appended.

Linear in the difference between the current size and count. Additional complexity possible due to reallocation if capacity is less than count.

If an exception is thrown for any reason, these functions have no effect (strong exception safety guarantee). Although not explicitly specified, std::length_error is thrown if the capacity required by the new vector would exceed max_size().

In overload (1), if T's move constructor is not noexcept and T is not CopyInsertable into *this, vector will use the throwing move constructor. If it throws, the guarantee is waived and the effects are unspecified.

If value-initialization in overload (1) is undesirable, for example, if the elements are of non-class type and zeroing out is not needed, it can be avoided by providing a custom Allocator::construct.
Vector capacity is never reduced when resizing to smaller size because that would invalidate all iterators, while the specification only invalidates the iterators to/after the erased elements.

#include <vector>
#include <iostream>
 
void print(auto rem, const std::vector<int>& c)
{
    for (std::cout << rem; const int el : c)
        std::cout << el << ' ';
    std::cout << '\n';
}
 
int main()
{
    std::vector<int> c = {1, 2, 3};
    print("The vector holds: ", c);
 
    c.resize(5);
    print("After resize up to 5: ", c);
 
    c.resize(2);
    print("After resize down to 2: ", c);
 
    c.resize(6, 4);
    print("After resize up to 6 (initializer = 4): ", c);
}

Output:

The vector holds: 1 2 3
After resize up to 5: 1 2 3 0 0
After resize down to 2: 1 2
After resize up to 6 (initializer = 4): 1 2 4 4 4 4

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

1. ↑ erase() can remove elements in the middle of a vector, so the value type is required to be MoveAssignable so that the elements following the removed section can be moved forward to fill the gap. However, resize() can only remove elements at the end of the vector, making the MoveAssignable unnecessary.
2. ↑ Removing elements by using pop_back() implies that the elements are required to be removed from back to front.

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