C++: containers and standard algorithms

Prefer a standard container and an algorithm that expresses the operation clearly. This example uses C++17.

#include <iostream>
#include <numeric>
#include <vector>

int main() {
    const std::vector<int> values{1, 2, 3, 4, 5};
    const int total = std::accumulate(values.begin(), values.end(), 0);
    std::cout << total << '\n';  // 15
}

Compile and run:

c++ -std=c++17 -Wall -Wextra -pedantic sum.cpp -o sum
./sum

std::accumulate visits each element once: \(O(n)\) time and \(O(1)\) auxiliary space. The initial value determines the accumulator type; use 0LL when a wider integer sum is needed.