When working with user-defined types in C++, it is often convenient to be able to create literal representations of those types. C++11 introduced user-defined literals, which allow programmers to define custom literals for their classes or types.

User-defined literals provide a way to extend the language’s built-in literals (e.g., integer literals, floating-point literals) to support custom types. This can be particularly useful when dealing with units of measurement, physical quantities, or any other domain-specific representation.

To create a custom class literal, follow these steps:

Step 1: Define a Literal Operator

The first step is to define a literal operator for your class. A literal operator is a function that is called when the compiler encounters a literal expression. It has the following syntax:

return_type operator"" suffix (parameter_type parameter) {
    // implementation
}

Where suffix is the suffix used in the literal expression, and parameter is the value passed to the literal operator.

Step 2: Implement the Literal Operator

The next step is to implement the literal operator. This is where you define what the literal expression should represent and how it should be constructed. You can use this opportunity to validate the input and perform any necessary conversions.

Here’s an example of a custom class Distance representing a distance in meters:

class Distance {
public:
    Distance(double meters) : meters_(meters) {}

    double meters() const { return meters_; }

private:
    double meters_;
};

Distance operator"" _m(long double value) {
    return Distance(static_cast<double>(value));
}

In this example, the literal suffix _m is used to represent meters. The operator"" _m function takes a long double parameter representing the value of the literal expression. It constructs a Distance object using that value and returns it.

Step 3: Using the Custom Class Literal

Once you have defined the literal operator, you can use the custom class literal in your code. Here’s an example of using the Distance class literal:

int main() {
    Distance distance = 10.0_m;
    std::cout << "Distance: " << distance.meters() << " meters" << std::endl;

    return 0;
}

In this example, 10.0_m creates a Distance object representing 10 meters. The value is then printed to the console.

Conclusion

Custom class literals in C++ provide a powerful way to extend the language’s built-in literals to support user-defined types. They allow for more readable and expressive code when working with domain-specific representations. By defining a literal operator and implementing it for your class, you can create meaningful literal expressions that enhance the clarity and usability of your code.

For more details about user-defined literals and their limitations, refer to the C++ documentation.

#tags: C++ #user-defined-literals