Openai/6957acb5-7d70-8002-a8e3-6e77a3f0799e

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gpt-5-2-thinking

User: what will this program print?

what will this program print? I'm building with g++.

1. include <vector>
2. include <iostream>

struct A{

   int i{};

   A(int i_)
   :i(i_)
   {}
   ~A() {
       std::cout << "Destruct " << i << std::endl;
   }

};

int main() {

   std::vector<A> vec;
   vec.reserve(5);

   vec.emplace_back(1);
   vec.emplace_back(2);

}

Assistant: With vec.reserve5 there’s no reallocation, so no extra moves/copies/destructions happen during the emplacebacks.

With vec.reserve(5) there’s no reallocation, so no extra moves/copies/destructions happen during the emplace_backs. The only destructors run are when vec is destroyed at the end of main.

With g++ (libstdc++), the vector destructor destroys elements from begin() to end(), so you’ll get:

Destruct 1
Destruct 2

The C++ standard doesn’t require a specific destruction order for std::vector’s elements, so a different standard library could legally print the opposite order. en.cppreference.com