C++20: Concepts, the Details


In my last post C++20: Two Extremes and the Rescue with Concepts, I gave the first motivation for concepts. Concepts put semantic constraints on template parameters. Today, I present different use-cases for concepts in a compact form.


The Details

Just do keep it in mind: What are the advantages of concepts?

  • Requirements for templates are part of the interface.
  • The overloading of functions or specialisation of class templates can be based on concepts.
  • We get improved error message because the compiler compares the requirements of the template parameter with the actual template arguments
  • You can use predefined concepts or define your own.
  • The usage of auto and concepts is unified. Instead of auto, you can use a concept.
  • If a function declaration uses a concept, it automatically becomes a function template. Writing function templates is, therefore, as easy as writing a function.

This post is about the first three points. Let me show many different usages of concepts:

Three Ways

There are three ways to use the concept Sortable. For simplicity reasons, I only show the declaration of the function template.

Requires Clause

template<typename Cont>
    requires Sortable<Cont>
void sort(Cont& container);

Trailing Requires Clause

template<typename Cont>
void sort(Cont& container) requires Sortable<Cont>;

Constrained Template Parameters

template<Sortable Cont>
void sort(Cont& container)


The algorithm sort requires in this case that the container is sortable. Sortable has to be a constant expression and a predicate.


You can define a class template which only accepts objects.

template<Object T>
class MyVector{};

MyVector<int> v1;   // OK
MyVector<int&> v2;  // ERROR: int& does not satisfy the constraint Object


The compiler complains that and reference is not an object. Maybe you wonder, what an object is.? A possible implementation der type-traits function std::is_object gives the answer:

template< class T>
struct is_object : std::integral_constant<bool,
                     std::is_scalar<T>::value ||
                     std::is_array<T>::value  ||
                     std::is_union<T>::value  ||
                     std::is_class<T>::value> {};


An object is either a scalar, or an array, or a union, or a class.

Member Functions

template<Object T>
class MyVector{
    void push_back(const T& e) requires Copyable<T>{}


In this case, the member function requires that the template parameter T must be copyable.

Variadic Templates

 // allAnyNone.cpp

#include <iostream>
#include <type_traits> template<typename T> concept Arithmetic = std::is_arithmetic<T>::value; template<Arithmetic... Args> bool all(Args... args) { return (... && args); } template<Arithmetic... Args> bool any(Args... args) { return (... || args); } template<Arithmetic... Args> bool none(Args... args) { return !(... || args); } int main(){ std::cout << std::boolalpha << std::endl; std::cout << "all(5, true, 5.5, false): " << all(5, true, 5.5, false) << std::endl; std::cout << "any(5, true, 5.5, false): " << any(5, true, 5.5, false) << std::endl; std::cout << "none(5, true, 5.5, false): " << none(5, true, 5.5, false) << std::endl; }


You can use concepts in variadic templates.  The definition of the function templates are based on fold expressions. all, any, and none requires from it type parameter T that is has to support the concept Arithmetic. Arithmetic essential means that T is either integral or floating-point.

The brand-new Microsoft compiler 19.23 supports partially as the only one the proposed concepts syntax.


More Requirements

Of course, you can use more than one requirement for the template parameters.

template <SequenceContainer S,   
          EqualityComparable<value_type<S>> T>
Iterator_type<S> find(S&& seq, const T& val){

The function template find requires that the container S is a SequenceContainer and that its elements are EqualityComparable.


std::advance(iter, n) puts its iterator iter n position further. Depending on the iterator, the implementation can use pointer arithmetic or just go n times further. In the first case, the execution time is constant; in the second case, the execution time depends on the stepsize n. Thanks to concepts, you can overload std::advance on the iterator category.

template<InputIterator I>
void advance(I& iter, int n){...}

template<BidirectionalIterator I>
void advance(I& iter, int n){...}

template<RandomAccessIterator I>
void advance(I& iter, int n){...}

// usage

std::vector<int> vec{1, 2, 3, 4, 5, 6, 7, 8, 9};
auto vecIt = vec.begin();
std::advance(vecIt, 5);       //  RandomAccessIterator

std::list<int> lst{1, 2, 3, 4, 5, 6, 7, 8, 9};
auto lstIt = lst.begin();
std::advance(lstIt, 5);       //  BidirectionalIterator

std::forward_list<int> forw{1, 2, 3, 4, 5, 6, 7, 8, 9};
auto forwIt = forw.begin();
std::advance(forwIt, 5);      //  InputIterator

Based on the iterator category, the containers std::vector, std::list, and std::forward_list support, the best fitting std::advance implementation is used.


Concepts also support template specialisations.

template<typename T>
class MyVector{};

template<Object T>
class MyVector{};

MyVector<int> v1;     // Object T
MyVector<int&> v2;    // typename T

  • MyVector<int&> goes to the unconstrained template parameter.

  • MyVector<int> goes to the constrained template parameter.

What's next?

My next post is about the syntactical unification in C++20. With C++20, you can use a constrained placeholder (concept) in each place you could use an unconstrained placeholder (auto) in C++11. But this is not the end of the unification. Defining a template becomes with C++20 a piece of cake. Just use a constrained or an unconstrained placeholder in the declaration of a function.

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+2 #1 Sebastian Büttner 2019-12-07 18:43
The article doesn't mention a fourth option for declaring a constrained function template. P1141 (Yet another approach for constrained declarations) was voted in the working draft in the 2018 San Diego meeting.
Using the terse syntax one could declare the sort function template like this:

void sort(Sortable auto container);

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