👀 Views: 1492 💬 Answers: 1 📅 Created: 2025-06-12

c++17 optional move-semantics best-practices cpp

I'm learning this framework and I'm experiencing unexpected behavior when using `std::optional` in C++17 with my custom type that implements move semantics. I have a simple class, `Resource`, that manages a resource and has a move constructor and a move assignment operator. However, when I store an instance of `Resource` in an `std::optional`, I get inconsistent results after moving the `optional` object. Here's a minimal example: ```cpp #include <iostream> #include <optional> class Resource { public: Resource() { std::cout << "Resource created\n"; } Resource(const Resource&) = delete; Resource(Resource&&) noexcept { std::cout << "Resource moved\n"; } Resource& operator=(const Resource&) = delete; Resource& operator=(Resource&&) noexcept { std::cout << "Resource move assigned\n"; return *this; } ~Resource() { std::cout << "Resource destroyed\n"; } }; int main() { std::optional<Resource> opt1 = Resource(); std::optional<Resource> opt2 = std::move(opt1); if (opt1) { std::cout << "opt1 is still valid\n"; } else { std::cout << "opt1 is empty!\n"; } return 0; } ``` When I run this code, I expect `opt1` to be empty after moving it to `opt2`, but it prints "opt1 is still valid!". Furthermore, it outputs "Resource moved" for the move constructor, which suggests that `std::optional` is not behaving as expected. I have confirmed that `Resource` is indeed movable. I've tried different combinations of constructors and destructors, but the behavior remains the same. Is there something about how `std::optional` interacts with move semantics that I'm missing? Are there any best practices for using `std::optional` with custom types? Any insights would be appreciated! I'd really appreciate any guidance on this.