C++17 std::variant not handling custom types correctly when combining with std::visit
I just started working with I'm prototyping a solution and I'm reviewing some code and I'm prototyping a solution and I've searched everywhere and can't find a clear answer. I've been banging my head against this for hours. I'm working with an scenario with `std::variant` in C++17 when trying to use it with `std::visit`. I have defined a variant that includes a couple of custom types and I'm trying to implement a visitor function that should process these types. However, I'm running into a question where the visitor is not being invoked for certain types, leading to unexpected behavior. Here’s the code snippet: ```cpp #include <iostream> #include <variant> #include <string> struct TypeA { int value; }; struct TypeB { std::string name; }; using MyVariant = std::variant<TypeA, TypeB>; struct MyVisitor { void operator()(TypeA& a) { std::cout << "Processing TypeA with value: " << a.value << '\n'; } void operator()(TypeB& b) { std::cout << "Processing TypeB with name: " << b.name << '\n'; } }; int main() { MyVariant v1 = TypeA{10}; MyVariant v2 = TypeB{"Sample"}; std::visit(MyVisitor{}, v1); std::visit(MyVisitor{}, v2); std::visit(MyVisitor{}, std::monostate{}); // This line throws compilation behavior } ``` When I run this code, the visitor works fine for `v1` and `v2`, but when I try to invoke it with `std::monostate`, it fails to compile with an behavior: `no matching function for call to 'MyVisitor::operator()(std::monostate&)`. I didn't expect this since I thought `std::variant` should handle this without issues. My understanding was that I could include `std::monostate` to create a default state, but it seems like I'm missing something. I've tried modifying the visitor struct to include a handler for `std::monostate`, but that hasn’t resolved the scenario. Any insights on how I can manage this situation or correct my approach? Additionally, is there a best practice for using `std::variant` with custom types in terms of design patterns or performance considerations? My development environment is Ubuntu. How would you solve this? I'm working on a service that needs to handle this. This issue appeared after updating to C++ latest. Thanks in advance! I'm on Windows 10 using the latest version of C++. Thanks, I really appreciate it! I'm on CentOS using the latest version of C++. Thanks for any help you can provide! For context: I'm using C++ on macOS. Any feedback is welcome!