Memory model: stack vs heap, new/delete, and why raw pointers are dangerous

Worked example

Read it before you run it

int on_stack{5};
// Lives on the stack; freed automatically at the closing brace.
int* on_heap = new int{5};
// Allocates an int on the heap and returns its address; you now own it.
std::cout << *on_heap << '\n';
// Read through the pointer.
delete on_heap;
// Release the memory; the pointer now dangles.
on_heap = nullptr;
// Clear the pointer so it cannot be used by mistake.
double* buffer = new double[100];
// Allocates an array of 100 doubles on the heap.
delete[] buffer;
// Arrays need delete[]; plain delete on an array is undefined behaviour.
// std::vector<double> better(100);
// The safe alternative: memory is freed automatically.

Practice code

Now type it yourself

Copy the starter into practice.cpp and replace every TODO with your own code. Build and run it: g++ -std=c++17 -Wall -Wextra -Wpedantic practice.cpp -o practice -pthread && ./practice

#include <iostream>

int main() {
  // TODO: allocate an array of 3 ints on the heap with new[], fill it with i * 10, add them up, then delete[] it
  std::cout << "sum " << sum << '\n';
  return 0;
}

sum 30

#include <iostream>

int main() {
  int* values = new int[3];
  int sum{0};
  for (int i = 0; i < 3; ++i) {
    values[i] = i * 10;
    sum += values[i];
  }
  delete[] values;
  std::cout << "sum " << sum << '\n';
  return 0;
}