How do I handle errors when using std::variant?

When using std::variant, there are a few key ways to handle errors and exceptional situations:

std::bad_variant_access

This exception is thrown if you call std::get with an index or type that doesn't match the currently active variant member. To avoid this, you can:Here's an example of using std::get_if:

• Use std::holds_alternative to check if the variant holds a specific type before trying to access it.
• Use std::get_if, which returns a pointer to the held value if it's of the specified type, or nullptr otherwise.
• Use std::visit with a visitor that handles all possible types.

1std::variant<int, std::string> v = 42;
2if (auto pint = std::get_if<int>(&v)) {
3    std::cout << "int: " << *pint << '\n';
4} else if (auto pstr = std::get_if<std::string>(&v)) {
5    std::cout << "string: " << *pstr << '\n';
6}

Valueless by exception

A variant can become "valueless by exception" if an exception is thrown during assignment or emplacement. You can check for this state with the valueless_by_exception() member function. A variant in this state can still be assigned to, but any other operation will throw std::bad_variant_access.

1try {
2  v.emplace<std::string>(invalid_string);
3} catch (...) {
4  if (v.valueless_by_exception()) {
5    std::cout << "variant became valueless\n";
6  }
7}

Visitor exceptions

If an exception is thrown from a visitor function during a call to std::visit(), the exception will propagate out of the call to visit. Ensure your visitors handle exceptions appropriately.

1try {
2  std::visit(my_visitor, v);
3} catch (const my_exception& e) {
4  std::cerr << "Caught exception from visitor: "
5    << e.what() << '\n';
6}

Some general best practices:

• Use std::variant only when the set of types is known at compile-time and is relatively small.
• Prefer std::visit over directly accessing variant members. It's more type-safe and cleaner.
• Handle std::bad_variant_access exceptions, either by preventing them with checks like std::holds_alternative, or catching them if they're expected.
• Be aware of the "valueless by exception" state and handle it appropriately.
• Ensure your visitor functions are exception-safe.

By following these practices, you can use std::variant safely and effectively in your C++ code.