In C++, user-defined literals enable developers to define their own custom suffixes for literal constants. This enables the creation of convenient and expressive syntax for initializing values of user-defined types. User-defined literals can be defined for integer, floating-point, character, and string literals.

Syntax

To define a user-defined literal, the following syntax is used:

return_type operator"" suffix_name (parameter_type value);

The operator"" is a special operator that indicates a user-defined literal. suffix_name is the custom suffix that will be used, and parameter_type is the type of the literal passed to the user-defined literal. The return_type can be any valid C++ type.

Example

Suppose we have a class called Distance which represents a distance value and we want to define a user-defined literal to convert meters to Distance objects. The user-defined literal can be defined as follows:

class Distance {
    // class implementation
};

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

In this example, operator"" _m is the user-defined literal for meters. It takes a long double parameter, representing the value in meters, and returns a Distance object.

After defining this user-defined literal, we can create Distance objects using the custom suffix as follows:

auto d = 10.5_m; // creates a Distance object representing 10.5 meters

Restrictions

• User-defined literals must begin with an underscore _ followed by lowercase letters, e.g., _m, _km, etc.
• User-defined literals cannot be defined for built-in types like int or double.
• User-defined literals cannot have default arguments.

User-defined literals provide a powerful mechanism for creating more expressive and readable code. They enable the customization of literal constants for user-defined types, ultimately leading to more natural and intuitive syntax.

For more details on user-defined literals, please refer to the C++ Standard.