Defining Concepts with Requires Expressions


In my last post, "Define Concepts", I defined the concepts Integral, SignedIntegral, and UnsigendIntegral using logical combinations of existing concepts and compile-time predicates. Today, I use Requires Expressions to define concepts.


Before I write about the use of requires expressions to define a concept, here is a short reminder:

The syntax to define a concept is straightforward:
template <template-parameter-list>
concept concept-name = constraint-expression;
A concept definition starts with the keyword template and has a template parameter list. It uses the keyword concept followed by the concept name and the constraint expression.
A constraint-expression is a compile-time predicate. A compile-time predicate is a function that runs at compile time and returns a boolean. This compile-time predicate can either be:

  • A logical combination of other concepts or compile-time predicates using conjunctions (&&), disjunctions (||), or negations (!). I wrote about syntactical form in my previous post t "Define Concepts".

  • A requires expression
    • Simple requirements
    • Type requirements
    • Compound requirements
    • Nested requirements

Now, let's write about requires expression.


Rainer D 6 P2 540x540Modernes C++ Mentoring

Be part of my mentoring programs:





Do you want to stay informed about my mentoring programs: Subscribe via E-Mail.

Requires Expression

Thanks to requires expressions, you can define powerful concepts. A requires expression has the following form:

requires (parameter-list(optional)) {requirement-seq}  

  • parameter-list: A comma-separated list of parameters, such as in a function declaration
  • requirement-seq: A sequence of requirements, consisting of simple, type, compound, or nested requirements
Requires expressions can also be used as a standalone feature when a compile-time predicate is required. I will write about this feature later.

Simple Requirements

The following concept Addable is a simple requirement:

template<typename T>
concept Addable = requires (T a, T b) {
    a + b;

The concept Addable requires that the addition a + b of two values of the same type T is possible.
Before I continue with type requirements, I want to add. This concept has only one purpose: exemplifying simple requirements. Writing a concept that checks a type for supporting the + operator is bad. A concept should model an idea, such as Arithmetic.

Type Requirements

In a type requirement, you have to use the keyword typename together with a type name.

template<typename T>
concept TypeRequirement = requires {
    typename T::value_type;
    typename Other<T>;    

The concept TypeRequirement requires that type T has a nested member value_type, and that the class template Other can be instantiated with T.

Let's try this out:

#include <iostream>
#include <vector>

template <typename>
struct Other;  

template <>
struct Other<std::vector<int>> {};

template<typename T>
concept TypeRequirement = requires {
    typename T::value_type;             // (2)
    typename Other<T>;                  // (3)

int main() {

    TypeRequirement auto myVec= std::vector<int>{1, 2, 3};  // (1)


The expression TypeRequirement auto myVec = std::vector<int>{1, 2, 3} (line 1) is valid. A std::vector has an inner member value_type (line 2) and the class template Other (line 2) can be instantiated with std::vector<int> (line 3).
The concept TypeRequirement , in combination with auto in line 1 is called a constrained placeholder.  In my previous post, "C++20: Concepts, the Placeholder Syntax", read more about constrained and unconstrained placeholders.

Compound Requirements

A compound requirement has the form

{expression} noexcept(optional) return-type-requirement(optional);    

In addition to a simple requirement, a compound requirement can have a noexcept specifier and a requirement on its return type. You essentially express with the noexcept specifier that this expression does not throw an exception, and if it throw, you do not care and let the program just crash. Read more about the noexcept specifier in my previous post: C++ Core Guidelines: The noexcept specifier and operator.

The concept Equal, demonstrated in the following example, uses compound requirements.

// conceptsDefinitionEqual.cpp

#include <concepts>
#include <iostream>

template<typename T>                                     // (1)
concept Equal = requires(T a, T b) {
    { a == b } -> std::convertible_to<bool>;
    { a != b } -> std::convertible_to<bool>;

bool areEqual(Equal auto a, Equal auto b){
  return a == b;

struct WithoutEqual{                                       // (2)
  bool operator==(const WithoutEqual& other) = delete;

struct WithoutUnequal{                                     // (3)
  bool operator!=(const WithoutUnequal& other) = delete;

int main() {
    std::cout << std::boolalpha << '\n';
    std::cout << "areEqual(1, 5): " << areEqual(1, 5) << '\n';
    bool res = areEqual(WithoutEqual(),  WithoutEqual());    // (4)
    bool res2 = areEqual(WithoutUnequal(),  WithoutUnequal());

    std::cout << '\n';

The concept Equal (line 1) requires that its type parameter T supports the equal and non-equal operators. Additionally, both operators have to return a convertible value to a boolean. Of course, int supports the concept Equal, but this does not hold for the types WithoutEqual (line 2) and WithoutUnequal (line 3). Consequently, when I use the type WithoutEqual (line 4), I get the following error message when using the GCC compiler.


Nested Requirements

A nested requirement has the form

requires constraint-expression;   

Nested requirements are used to specify requirements on type parameters.

In my previous post, "Define Concepts", I defined the concept UnsignedIntegral using logical combinations of existing concepts and compile-time predicates. Now, I define it using nested requirements:
// nestedRequirements.cpp

#include <type_traits>

template <typename T>
concept Integral = std::is_integral<T>::value;

template <typename T>
concept SignedIntegral = Integral<T> && std::is_signed<T>::value;

// template <typename T>                               // (2)
// concept UnsignedIntegral = Integral<T> && !SignedIntegral<T>;

template <typename T>                                  // (1)
concept UnsignedIntegral = Integral<T> &&
requires(T) {
    requires !SignedIntegral<T>;

int main() {

    UnsignedIntegral auto n = 5u;  // works
    // UnsignedIntegral auto m = 5;   // compile time error, 5 is a signed literal


Line (1) uses the concept SignedIntegral as a nested requirement to refine the concept Integral. The commented-out concept UnsignedIntegral in line (2) is more convenient to read.

What's next?

Typically, you use requires expressions to define a concept, but they can also be used as a standalone feature when a compile-time predicate is required. Therefore, use cases for requires expression can be a requires clause, static_assertor constexpr if. I will write in my next post about these unique use cases of requires expressions.

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, Animus24, Jozo Leko, John Breland, Venkat Nandam, Jose Francisco, Douglas Tinkham, Kuchlong Kuchlong, Robert Blanch, Truels Wissneth, Kris Kafka, 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, Matthieu Bolt, Stephen Kelley, Kyle Dean, Tusar Palauri, Dmitry Farberov, Juan Dent, George Liao, Daniel Ceperley, Jon T Hess, Stephen Totten, Wolfgang Fütterer, Matthias Grün, Phillip Diekmann, Ben Atakora, Ann Shatoff, and Rob North.


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



My special thanks to Embarcadero CBUIDER STUDIO FINAL ICONS 1024 Small


My special thanks to PVS-Studio PVC Logo


My special thanks to logo


My special thanks to Take Up Code TakeUpCode 450 60



I'm happy to give online seminars or face-to-face seminars worldwide. Please call me if you have any questions.

Bookable (Online)


Standard Seminars (English/German)

Here is a compilation of my standard seminars. These seminars are only meant to give you a first orientation.

  • C++ - The Core Language
  • C++ - The Standard Library
  • C++ - Compact
  • C++11 and C++14
  • Concurrency with Modern C++
  • Design Pattern and Architectural Pattern with C++
  • Embedded Programming with Modern C++
  • Generic Programming (Templates) with C++


  • Clean Code with Modern C++
  • C++20

Contact Me

Modernes C++,


Stay Informed about my Mentoring



English Books

Course: Modern C++ Concurrency in Practice

Course: C++ Standard Library including C++14 & C++17

Course: Embedded Programming with Modern C++

Course: Generic Programming (Templates)

Course: C++ Fundamentals for Professionals

Course: The All-in-One Guide to C++20

Course: Master Software Design Patterns and Architecture in C++

Subscribe to the newsletter (+ pdf bundle)

All tags

Blog archive

Source Code


Today 937

Yesterday 6503

Week 27194

Month 7440

All 12085649

Currently are 199 guests and no members online

Kubik-Rubik Joomla! Extensions

Latest comments