What is std::monostate in C++?

The introduction of std::monostate as part of the C++17 Standard Library improvements has provided C++ programmers with a new type that is lightweight and default constructible. std::monostate is essentially a type that does nothing and serves as a placeholder. Unlike other types, it does not hold any data, and its primary purpose is to assist in certain scenarios where a variant type is necessary but does not need to contain a meaningful value.

Why use `std::monostate`?

The primary use of std::monostate is with the std::variant type, a type-safe union introduced in C++17. A std::variant can hold one of several types, but what if we want a variant that can be empty or hold no meaningful value? Here, std::monostate comes into play. By including std::monostate as one of the types in a variant, we allow the variant to be default-constructed in a valid state without holding any other types.

Why having a variant with an “empty” state is useful?

Having an “empty” state, like that facilitated by std::monostate in variant types, is useful in practice for several reasons:

Default construction: It allows for the default construction of variant types even when other alternatives within the variant cannot be default constructed. This ensures that variants can always be initialized, enhancing flexibility and usability.
State representation: An “empty” state can represent scenarios where a value is intentionally missing or undefined, such as in optional fields or state machines where some states might not carry specific data.
Error handling: It can indicate error conditions or exceptional cases without resorting to exceptions or null pointers, making error handling more explicit and less error-prone.
Logical flow: In programming constructs where logic branches might not always produce a value, an “empty” state represents “no result” or “no action” scenarios, facilitating clearer logic and decision-making.
Simplification: It simplifies code by avoiding the need for additional structures or types to represent the absence of data, making codebases more maintainable and easier to understand.

How does `std::monostate` work?

std::monostate is essentially an empty class. It contains no member variables or methods except default constructors, assignment operators, and destructors, making it trivially constructive and destructible.

Example scenario

Imagine we’re dealing with a situation where we need a std::variant to encapsulate different types of states—perhaps it could be an integer, a string, or there might be cases where it doesn’t need to hold any value at all, essentially an “empty” state. Such scenarios are common in applications involving state machines, optional values, or when navigating through complex conditional logic where not every path results in a data-bearing state. In these instances, std::monostate is the perfect candidate to represent this “empty” state within a std::variant.

To put this into a concrete example, consider a std::variant<int, std::string, std::monostate> type. This versatile construct allows the variant to hold an integer or a string or adopt a state of std::monostate, indicating that it currently doesn’t encapsulate an integer or a string. Essentially, it enables the std::variant to be instantiated in a valid state without necessarily committing to holding a specific value like an integer or string right from the start.

Coding example

Let’s look at a practical example to understand how std::monostate is used in C++ programming.

#include <iostream>
#include <variant>
#include <string>
int main() {
    // Define a variant to hold an int, a string, or represent an 'empty' state using std::monostate
    std::variant<std::monostate, int, std::string> myVariant;
    // Check if myVariant currently holds the 'empty' state (std::monostate)
    if (std::holds_alternative<std::monostate>(myVariant)) {
        std::cout << "Variant is currently empty." << std::endl;
    }
    // Assign an integer value to myVariant
    myVariant = 42;
    // Check and output if myVariant holds an integer
    if (std::holds_alternative<int>(myVariant)) {
        std::cout << "Variant now holds an integer: " << std::get<int>(myVariant) << std::endl;
    }
    // Assign a string value to myVariant
    myVariant = "Hello, World!";
    // Check and output if myVariant holds a string
    if (std::holds_alternative<std::string>(myVariant)) {
        std::cout << "Variant now holds a string: " << std::get<std::string>(myVariant) << std::endl;
    }
    return 0;
}

The coding example for std::monostate

In the above example, we create a std::variant that can hold an integer, a string, or a std::monostate. Initially, the variant holds a std::monostate, signifying an “empty” state. We then assign it an integer and a string, demonstrating how the held value can change.

In the example mentioned above, we create a std::variant that has the capacity to hold an integer, a string, or a std::monostate. Initially, the variant is set to hold a std::monostate, indicating an “empty” state. Following that, we assign it an integer and a string, which shows how the held value can be changed.

Conclusion

In summary, std::monostate is a simple but useful tool in C++ programming, especially when working with std::variant. It allows us to define variants that can be in a valid yet empty state. Understanding how to use std::monostate it can help us write more flexible and robust C++ code, especially in applications that require type-safe unions with a potentially empty state.

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