👀 Views: 498 💬 Answers: 1 📅 Created: 2025-06-10

c++ std-variant std-visit custom-types cpp

Could someone explain I'm learning this framework and I'm stuck on something that should probably be simple... I've searched everywhere and can't find a clear answer. I'm encountering unexpected behavior when using `std::variant` with `std::visit` in C++17..... My goal is to use `std::variant` to hold different types, including my custom type, and I want to perform operations based on the active type. However, I am running into issues when trying to access the custom type through `std::visit`. Here’s a simplified version of my code: ```cpp #include <iostream> #include <variant> #include <string> struct MyType { std::string name; MyType(const std::string& n) : name(n) {} }; using MyVariant = std::variant<int, MyType>; void processVariant(const MyVariant& var) { std::visit([](auto&& arg) { std::cout << "Value: " << arg << '\n'; }, var); } int main() { MyVariant v1 = 42; MyVariant v2 = MyType("Example"); processVariant(v1); processVariant(v2); return 0; } ``` When I run this code, the output for the `MyType` variant is incorrect. I get: `Value: <unresolved overload>`. I have verified that the `std::variant` is correctly initialized with `MyType`, but the lambda inside the `std::visit` does not seem to handle my custom type properly. I have also tried defining a specific overload for `MyType`, but the issue persists. I suspect this might be related to how I'm trying to output the `MyType` object. The default output stream operator for `MyType` is not defined, so I tried to add: ```cpp std::ostream& operator<<(std::ostream& os, const MyType& myType) { return os << myType.name; } ``` However, even after including this operator, I still see the unresolved overload issue. What am I missing here? Is there a step in the process that I might have overlooked when dealing with custom types in `std::variant`? I’m using GCC 9.3.0 and compiling with `-std=c++17`. Any advice on how to resolve this would be greatly appreciated. I'm working on a CLI tool that needs to handle this. I'd really appreciate any guidance on this. My development environment is Windows. Am I missing something obvious? This is part of a larger desktop app I'm building. What am I doing wrong? This is happening in both development and production on Debian. I'd love to hear your thoughts on this. For reference, this is a production web app. Any examples would be super helpful. Any pointers in the right direction?