👀 Views: 69 💬 Answers: 1 📅 Created: 2025-06-11

c++20 stdoptional emplace cpp

I'm building a feature where I'm deploying to production and I keep running into I'm sure I'm missing something obvious here, but I've been banging my head against this for hours. I'm encountering unexpected behavior when using `std::optional` in C++20. I tried to create an optional object using both `emplace` and direct initialization, but I'm seeing different outcomes that I didn't anticipate. Here's a simplified version of my code: ```cpp #include <iostream> #include <optional> #include <string> struct MyStruct { std::string name; int value; }; int main() { std::optional<MyStruct> opt1; std::optional<MyStruct> opt2; // Using emplace opt1.emplace("Test", 42); // Using direct initialization opt2 = MyStruct{"Test", 42}; if (opt1.has_value()) { std::cout << "opt1: " << opt1->name << " " << opt1->value << std::endl; } if (opt2.has_value()) { std::cout << "opt2: " << opt2->name << " " << opt2->value << std::endl; } // Resetting opt1 opt1.reset(); if (!opt1.has_value()) { std::cout << "opt1 has been reset." << std::endl; } // Trying to access opt1 after reset try { std::cout << opt1->name << std::endl; // Should throw an exception } catch (const std::bad_optional_access& e) { std::cout << "Caught exception: " << e.what() << std::endl; } return 0; } ``` My expectation was that both `opt1` and `opt2` would behave similarly, but after resetting `opt1`, I checked its value and I received a `std::bad_optional_access` exception, as expected. However, when I try to access `opt2`, it doesn't throw an exception, instead it leads to undefined behavior, as I tried to access it after the scope where it was initialized. I also noticed that when using `emplace`, it seems to handle the state of the object differently than direct assignment. Is there a reason for this discrepancy? Am I missing something fundamental about `std::optional`? Any insights would be greatly appreciated! I'm working on a web app that needs to handle this. I'd really appreciate any guidance on this. Any ideas how to fix this? My team is using Cpp for this service. I'm coming from a different tech stack and learning Cpp. What's the best practice here?