C++ - reserve(), resize() - what why

What is reserve()

• reserve(10) reserves memory so you can e.g. push_back(10) elements without having a reallocation.

void reserve (size_type n);

• Request a change in capacity

• Requests that the vector capacity be at least enough to contain n elements.

• If n is greater than the current vector capacity, the function causes the container to reallocate its storage increasing its capacity to n (or greater).

• In all other cases, the function call does not cause a reallocation and the vector capacity is not affected.

• This function has no effect on the vector size and cannot alter its elements.

resize()

• The resize() method (and passing argument to constructor is equivalent to that) will insert or delete appropriate number of elements to the vector to make it given size (it has optional second argument to specify their value). It will affect the size(), iteration will go over all those elements, push_back will insert after them and you can directly access them using the operator[].

• The reserve() method only allocates memory, but leaves it uninitialized. It only affects capacity(), but size() will be unchanged. There is no value for the objects, because nothing is added to the vector. If you then insert the elements, no reallocation will happen, because it was done in advance, but that’s the only effect.

reserve() and push_back() is faster than resize()

std::vector reserve() and push_back() is faster than resize() and array index, why?

Example

// Example program
#include <iostream>
#include <vector>
#include <string>

int main()
{
    std::vector<int> v1;
    v1.resize(10); //allocation + instance creation
    std::cout << "v1.size(): " <<v1.size()<< std::endl;
    std::cout <<(v1.size() == 10)<< std::endl;   //prints 1
    std::cout <<(v1.capacity()==10)<< std::endl; //prints 1
    for(auto x : v1) {
        std::cout << x << std::endl;
    }
    
    std::vector<int> v2;
    v2.reserve(10); //only allocation    
    std::cout << "v2.size(): " <<v2.size()<< std::endl;
    std::cout <<(v2.size() == 10)<< std::endl;   //prints 0
    std::cout <<(v2.capacity()==10)<< std::endl; //prints 1
    for(auto x : v2) {
        std::cout << x << std::endl;
    }
    for(int i = 0; i < 10; i++) {
        v2.push_back(i);
    }
    std::cout << "v2.size(): " <<v2.size()<< std::endl;
    std::cout <<(v2.size() == 10)<< std::endl;   //prints 1
    std::cout <<(v2.capacity()==10)<< std::endl; //prints 1
    for(auto x : v2) {
        std::cout << x << std::endl;
    }
}

output

v1.size(): 10
1
1
0
0
0
0
0
0
0
0
0
0
v2.size(): 0
0
1
v2.size(): 10
1
1
0
1
2
3
4
5
6
7
8
9