User-Defined Literals: Introduction

In this lesson, we will introduce user-defined literals.

We'll cover the following

User-Defined Literals

User-defined literals are a unique feature in all mainstream programming languages, since they enable us to combine values with units.

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Syntax

Literals are explicit values in a program, including a boolean like true, the number 3 or 4.15; the char a, or the C string "hello". The lambda function [](int a, int b){ return a+b; } is also a function literal.

With C++11, it is possible to generate user-defined literals by adding a suffix to a built-in literal for the int, float, char, and C strings.

User-defined literals must obey the following syntax:

<built_in-Literal>  _  <Suffix>

Usually, we use the suffix for a unit:

Examples

101000101_b
63_s
10345.5_dm
123.45_km
100_m
131094_cm
33_cent
"Hello"_i18n

What is the key benefit of user-defined literals? The C++ compiler maps the user-defined literals to the corresponding literal operator, and this literal operator must be implemented by the programmer.

The Magic

Let’s take a look at the user-defined literal 0101001000_b which represents a binary value. The compiler maps the user-defined literal 0101001000_b to the literal operator operator"" _b (long long int bin).

A few special rules are important to follow:

  • There must be a space between the quotation marks and the suffix.
  • The binary value (0101001000) is in the variable bin.
  • If the compiler doesn’t find the corresponding literal operator, the compilation will fail.

With C++14, we get an alternative syntax for user-defined types. They differ from the C++11 syntax because user-defined types in require no space. Therefore, it is possible to use reserved keywords like _C as a suffix and use a user-defined literal of the form. This can be seen with 11_C. The compiler will map 11_C to the literal operator"" _C (unsigned long long int). The simple rule states that we can use suffixes starting with an upper-case letter.

User-defined literals are a very helpful feature in modern C++ if we want to write safety-critical software. Why? Due to the automatic mapping of the user-defined literal to the literal operator, we can implement type-safe arithmetic.

The concept will be further explained with an example in the following lesson.