In many programming languages, literals are used to represent values of built-in types like integers, floating-point numbers, or strings. However, there are cases when we need to represent values of custom types in a more concise and expressive way. This is where user-defined literals come into play.

What are user-defined literals?

User-defined literals allow users to define their own syntax for literal values of custom types. By overloading the appropriate operator, we can specify how a literal string should be converted into an instance of our custom type.

In C++, user-defined literals are denoted by a sequence of characters followed by an underscore and a suffix. For example, in the literal 42_km, km is the suffix indicating that we want to create a Distance object with a value of 42 kilometers.

How to define user-defined literals?

To define a user-defined literal in C++, we need to overload the appropriate operator for the desired type. The operators for user-defined literals are named operator"" followed by the suffix we want to use.

Here’s an example of how to define a user-defined literal for a Distance class:

class Distance {
    double value;

public:
    Distance(double v) : value(v) {}

    // Define the user-defined literal operator
    friend Distance operator"" _km(long double value) {
        return Distance(value);
    }
};

In this example, the operator""_km function should be defined as a friend function of the Distance class. It takes a long double parameter, which represents the value provided in the literal. Inside the function, we can create an instance of the Distance class using the provided value and return it.

Using user-defined literals

Once we have defined a user-defined literal, we can use it in our code just like any other literal. For example:

Distance d = 42_km;       // Creates a Distance object with a value of 42 kilometers
Distance d2 = 10.5_km;    // Creates a Distance object with a value of 10.5 kilometers

The 42_km and 10.5_km literals are automatically converted into Distance objects using the overloaded operator""_km function.

Benefits of user-defined literals

User-defined literals provide several benefits:

1. Improved readability: User-defined literals allow us to express values of custom types in a more natural and intuitive way, making our code easier to read and understand.

2. Type safety: With user-defined literals, we can ensure that the correct units or representations are used, reducing the risk of errors.

3. Code reusability: Once we define a user-defined literal, we can use it throughout our codebase, promoting code reusability and consistency.

Conclusion

User-defined literals provide a powerful way to define custom syntax for literal values of our own types. By overloading the appropriate operator, we can convert literal strings into instances of our custom types, improving readability, type safety, and code reusability.

References:

• C++ documentation: User-defined literals
• C++ documentation: Operator overloading