C++20: Concepts – Predefined Concepts

C++20 has many predefined concepts. Consequentially, before you define your concept, you should know the existing ones. This post gives you an overview of the predefined concepts. To get a general idea of concepts, read my previous posts.

Don’t reinvent the wheel. This golden rule also applies to concepts, by the way. The C++ Core Guidelines are very clear about this rule: T.11: Whenever possible use standard concepts.

My information is from the newest C++20 draft: N4842. To find all predefined concepts is quite a job. Most concepts are in Chapter 18 (concepts library) and Chapter 24 (ranges library). Additionally, a few concepts are in Chapter 17 (language support library), chapter 20 (general utility library), chapter 23 (iterators library), and Chapter 26 (numerics library). The document also shows how the concepts are implemented.

Honestly, I was astonished that I found no concept of concurrency such as Lockable. Initially, the concepts were written in CamelCase or Wiki syntax. Now, they are written with underscores. E.g., DerivedFrom became derived_from.

Here are the concepts according to their ordering in the C++20 draft. I ignore this presentation the unique or auxiliary concepts. I also ignore the concepts from the ranges library. To understand them, you first have to understand the new ideas of the ranges library. I write about them when I write about the ranges library. The ranges library will follow the concepts immediately.

I must warn you: if you don’t like technical posts, you probably don’t like this post.

Language support library

This section has one interesting concept three_way_comparable to support the three-way comparison operator.

Modernes C++ Mentoring

"Fundamentals for C++ Professionals" (open)

"Design Patterns and Architectural Patterns with C++" (open)

"C++20: Get the Details" (open)

"Concurrency with Modern C++" (open)

"Generic Programming (Templates) with C++": October 2024

"Embedded Programming with Modern C++": October 2024

"Clean Code: Best Practices for Modern C++": March 2025

Do you want to stay informed: Subscribe.

If you want it more formal. Let a and b values of type T. They are essential three_way_comparable only if:

(a <=> b == 0) == bool(a == b) is true,
(a <=> b != 0) == bool(a != b) is true,
((a <=> b) <=> 0) and (0 <=> (b <=> a)) are equal,
(a <=> b < 0) == bool(a < b) is true,
(a <=> b > 0) == bool(a > b) is true,
(a <=> b <= 0) == bool(a <= b) is true,
(a <=> b >= 0) == bool(a >= b) is true, and

Concepts Library

I assume these are the concepts we mainly use.

Language-related concepts

This section has around 15 concepts that should be self-explanatory. These concepts express relationships between types, type classifications, and fundamental type properties. Their implementation is often directly based on the corresponding function from the type-traits library. I rearranged them and added a few words if necessary.

same_as
derived_from
convertible_to
common_reference_with: common_reference_with<T, U> must be well-formed, and T and U must be convertible to a reference type C
common_with: similar to common_reference_with, but the common type C has not to be a reference type
assignable_from
swappable

Arithmetic

integral
signed_integral
unsigned_integral
floating_point

Their definition is straightforward:

template<class T>
concept integral = is_integral_v<T>;

template<class T>
concept signed_integral = integral<T> && is_signed_v<T>;

template<class T>
concept unsigned_integral = integral<T> && !signed_integral<T>;

template<class T>
concept floating_point = is_floating_point_v<T>;

Lifetime

destructible
constructible_from
default_constructible
move_constructible
copy_constructible

Comparison concepts

boolean: specifies if a type T is usable as a truth value; interestingly, pointers, smart pointers, and types with an explicit conversion operator to bool are not boolean types.
equality_comparable
totally_ordered

Maybe you know it from your math. For values a, b, and c of type T, T models totally_ordered only if

Exactly one of bool(a < b), bool(a > b), or bool(a == b) is true.
If bool(a < b) and bool(b < c), then bool(a < c).
bool(a > b) == bool(b < a).
bool(a <= b) == !bool(b < a).
bool(a >= b) == !bool(a < b).

Object concepts

movable
copyable
semiregular
regular

Here is the definition of the four concepts:

template<class T>
concept movable = is_object_v<T> && move_constructible<T> && assignable_from<T&, T> && swappable<T>;

template<class T>
concept copyable = copy_constructible<T> && movable<T> && assignable_from<T&, const T&>;

template<class T>
concept semiregular = copyable<T> && default_constructible<T>;

template<class T>
concept regular = semiregular<T> && equality_comparable<T>;

I have to add a few words. The concept movable requires for T that is_object_v<T> holds. This means, under the hood, that T is either a scalar, an array, a union, or a class.

I will implement the concept of semiregular and regular in future posts. Informally, a semiregular type behaves similarly to an int, and a regular type behaves similarly to an int and is comparable with ==.

Callable concepts

invocable
regular_invocable: F models invocable and equality-preserving and does not modify the function arguments; equality-preserving means the F produces the same output when giving the same input
predicate: F models a predicate if F models invocable and returns a boolean

General Utility library

This chapter has only special memory concepts; therefore, I skip it.

Iterators library

The iterators library has many important concepts. Here are the iterator categories:

input_iterator
output_iterator
forward_iterator
bidirectional_iterator
random_access_iterator
contiguous_iterator

The six categories of iterators correspond to the iterator concepts. The image shows the three most prominent iterator categories and the containers of the Standard Template Library, respectively.

The following relation holds. A random-access iterator is a bidirectional iterator, and a bidirectional iterator is a forward iterator. A contiguous iterator is a random access iterator and requires that the elements of the container are stored contiguously in memory. This means std::array, std::vector, and std::string support contiguous iterators, but not std::deque.

Algorithm concepts

permutable: reordering of elements in place is possible
mergeable: merging sorted sequences into an output sequence is possible
sortable: permuting a sequence into an ordered sequence is possible

Numeric library

The numeric library has the concept of a uniform_random_bit_generator. A uniform_random_bit_generator g of type G has to return unsigned integers so that each value is equally probable. Additionally, the uniform random bit generator g of type G has to support the member functions G::min, and G::max.

What’s next?

With the next posts, it becomes more practical. I write about the definition of concepts such as integral, regular, and semiregular. Defining concepts is more than just putting constraints on type parameters.

Post Views: 6,938

Thanks a lot to my Patreon Supporters: Matt Braun, Roman Postanciuc, Tobias Zindl, G Prvulovic, Reinhold Dröge, Abernitzke, Frank Grimm, Sakib, Broeserl, António Pina, Sergey Agafyin, Андрей Бурмистров, Jake, GS, Lawton Shoemake, Jozo Leko, John Breland, Venkat Nandam, Jose Francisco, Douglas Tinkham, Kuchlong Kuchlong, Robert Blanch, Truels Wissneth, Mario Luoni, Friedrich Huber, lennonli, Pramod Tikare Muralidhara, Peter Ware, Daniel Hufschläger, Alessandro Pezzato, Bob Perry, Satish Vangipuram, Andi Ireland, Richard Ohnemus, Michael Dunsky, Leo Goodstadt, John Wiederhirn, Yacob Cohen-Arazi, Florian Tischler, Robin Furness, Michael Young, Holger Detering, Bernd Mühlhaus, Stephen Kelley, Kyle Dean, Tusar Palauri, Juan Dent, George Liao, Daniel Ceperley, Jon T Hess, Stephen Totten, Wolfgang Fütterer, Matthias Grün, Phillip Diekmann, Ben Atakora, Ann Shatoff, Rob North, Bhavith C Achar, Marco Parri Empoli, Philipp Lenk, Charles-Jianye Chen, Keith Jeffery, Matt Godbolt, and Honey Sukesan.

Thanks, in particular, to Jon Hess, Lakshman, Christian Wittenhorst, Sherhy Pyton, Dendi Suhubdy, Sudhakar Belagurusamy, Richard Sargeant, Rusty Fleming, John Nebel, Mipko, Alicja Kaminska, Slavko Radman, and David Poole.

My special thanks to Embarcadero
My special thanks to PVS-Studio
My special thanks to Tipi.build
My special thanks to Take Up Code
My special thanks to SHAVEDYAKS