C++20 & C++23: The Power of `std::variant` and `std::optional`

Modern C++ continues to evolve in power, clarity, and expressiveness. Two key tools in this evolution are std::variant and std::optional — both part of the type-safe union and nullable value family, introduced in C++17 and significantly refined in C++20 and C++23.

This article presents a comprehensive guide to the improvements made to these utilities in C++20 and C++23, highlighting real-world use cases, best practices, and what every serious C++ developer should know.

`std::optional`: Enhancements in C++20 and C++23

Recap: What Is `std::optional`?


std::optional<int> maybe_value = get_value();
if (maybe_value) {
    std::cout << "Value: " << *maybe_value << '\n';
}

Represents an optional value, similar to Option in Rust or Maybe in Haskell.
Avoids the pitfalls of null pointers and exception-based returns.

C++20: Improvements to `std::optional`

constexpr Support You can now use std::optional in constexpr contexts, enabling powerful compile-time evaluations.


constexpr std::optional<int> square(int x) {
    return x > 0 ? std::optional{x * x} : std::nullopt;
}
static_assert(square(5).value() == 25);

Three-Way Comparison (<=>) std::optional now supports the spaceship operator for consistent and simplified comparison behavior.


std::optional<int> a = 10, b = 20;
if (a < b) { std::cout << "a < b"; }

C++23: Quality-of-Life Enhancements

Monadic Interfaces (proposed) Although not standardized yet, monadic functions like and_then, transform, and or_else are being explored to simplify functional-style chaining.
Potential future syntax:
```
maybe_value.and_then([](int v) { return do_something(v); });
```
Improved Debugging and Diagnostics C++23 improves error messages and constexpr feedback when using std::optional, particularly in template-heavy code.

`std::variant`: Powerful Type-Safe Union

Recap: What Is `std::variant`?


std::variant<int, std::string> data;
data = 42;
data = "text";

A type-safe alternative to traditional unions.
Enables runtime polymorphism without inheritance.
Excellent for APIs where a value may be one of multiple known types.

C++20: Major Enhancements to `std::variant`

constexpr Construction and Access Now fully usable in compile-time contexts.


constexpr std::variant<int, float> v = 42;
static_assert(std::holds_alternative<int>(v));

std::visit Becomes constexpr Visitors can now be evaluated at compile time.


constexpr int result = std::visit([](auto x) {
    return x + 1;
}, std::variant<int>{5});
static_assert(result == 6);

Improved Exception Guarantees More operations are marked noexcept, improving exception safety and performance.

C++23: Subtle but Impactful Improvements

Clearer Diagnostics Template instantiation errors involving variants are now easier to understand.
Better Overload Inference in std::visit Supports complex lambdas and deduced overloads more gracefully, reducing the need for boilerplate.
Library Conformance and Future Compatibility Implementation consistency ensures smoother upgrades and enables future enhancements like reflection and pattern matching.

Real-World Use Cases

Configuration Parsing Use std::variant to represent config values that could be int, bool, or string.
Command Results Use std::optional<T> to represent success/failure without exceptions or error codes.
Serialization Layers std::variant maps well to formats like JSON, YAML, or TOML where values can be heterogeneously typed.

Common Mistakes and Best Practices

Mistake	Better Approach
Using `.value()` unsafely	Check `.has_value()` or use `if (opt)`
Skipping `std::visit`	Always use `std::visit` to access variant values
Using heavy types in variants	Use smart pointers inside the variant

Looking Beyond C++23

The next wave of improvements may include:

Pattern Matching (expected in C++26)
Standardized Monadic Interfaces for optional and expected
Reflection to make generic programming safer and cleaner
Better Codegen as compilers optimize for variant layout and visitation

Conclusion

With std::optional and std::variant, Modern C++ provides elegant, type-safe alternatives to nullptr, unions, and verbose error handling. The enhancements in C++20 and C++23 bring compile-time power, safer APIs, and cleaner control flow to every C++ codebase.

If you're writing modern C++, these tools are not optional — they are essential. Understanding their capabilities, limitations, and best practices is a mark of true professionalism in contemporary C++ software development.

C++20 & C++23: The Power of std::variant and std::optional

std::optional: Enhancements in C++20 and C++23

Recap: What Is std::optional?

C++20: Improvements to std::optional