Exception safety and committing changes

The idea

Stack unwinding destroys fully constructed automatic objects as an exception leaves their scopes, making RAII useful for cleanup. Cleanup alone does not undo mutations already made to an existing object. An algorithm that promises to leave its input unchanged on failure must arrange its operations accordingly rather than assuming exceptions provide transactions.

One common strategy is to build a replacement in local owned storage, perform all potentially failing validation and construction there, and commit with an operation whose failure properties are understood. Copying a vector before appending protects the original if construction throws. Catch exceptions where recovery or useful context is possible; catching everything and returning a success-looking value can hide both invalid input and implementation defects.

#include <iostream>
#include <vector>
#include <stdexcept>
void appendPositive(std::vector<int>& values, int n) {
  if (n <= 0) throw std::invalid_argument("positive required");
  auto next = values;
  next.push_back(n);
  values.swap(next);
}
int main() {
  std::vector<int> v{2};
  try { appendPositive(v, -1); } catch (const std::invalid_argument&) {}
  std::cout << v.size() << ' ' << v[0];
}

1 2

Validation and replacement construction occur before the swap commits the new vector.