Constraints and Concepts in C++ 20

In this article, we will learn about the constraints and concepts with the help of examples in a simple way. So, it is a feature that was introduced in the latest version i.e. C++20 that specifies requirements on template arguments and enables more expressive and readable code.

What are Constraints in C++?

Constraints are the conditions or requirements that need to be satisfied by template arguments when using templates. It allows the user to specify what types or values can be used with a template.

Types of Constraints

1. Conjunctions

It combines multiple constraints using AND (&) operator. It specifies that all the constraints within the conjunction must be satisfied for the overall constraint to be satisfied.

template <typename T>
requires Constraint1 && Constraint2 && ... && ConstraintN
void functionName(T parameter) {
   // Function body
}

2. Disjunctions

It is used to combine multiple constraints with the help of the OR (||) operator. It specifies that at least one of the constraints within the disjunction must be satisfied for the overall constraint to be satisfied.

template <typename T>
requires Constraint1 || Constraint2 || ... || ConstraintN
void functionName(T parameter) {
   // Function body
}

3. Atomic constraints

They are individual constraints that define specific requirements on types, expressions, or template arguments.

template <typename T>
concept ConceptName = Constraint1 && Constraint2 && ... && ConstraintN;

template <typename T>
requires ConceptName<T>
void functionName(T parameter) {
   // Function body
}

Examples of Constraints

Example 1:

The below code demonstrates the usage of constraints in C++ to restrict the valid types that can be used with function templates.

// C++ program to illustrate the constrains
#include <iostream>
#include <type_traits>

using namespace std;

// Function template with constraint using requires clause
template <typename T>
requires is_integral_v<T> void print_integer(T value)
{
    cout << "The integer value is: " << value << endl;
}

// Concept definition
template <typename T> concept Printable = requires(T value)
{
    cout << value << endl;
};

// Function template with concept as a constraint
template <Printable T> void print(T value)
{
    cout << "The printable value is: " << value << endl;
}

int main()
{
    // Call the print_integer function with an integer
    // argument
    print_integer(42);

    // Call the print function with a string argument
    print("Hello, World!");
}

Output

The integer value is: 42
The printable value is: Hello, World!

Violations of constraints are typically detected during the template instantiation process in C++. The compiler generates an error message for the specific constraint that has been violated.

Example 2:

// C++ program to illustrate the voilation in specified
// constrains for template arguments
#include <concepts>
#include <iostream>

template <typename T> requires integral<T> void foo(T value)
{
    cout << "Value: " << value << endl;
}

int main()
{
    // Valid usage, T = int (integral type)
    foo(5);
    // Error: Violation of constraint, T = double
    // (not an integral type)
    foo(3.14);

    return 0;
}

Output

What are Concepts in C++?

Concepts are used to specify the requirements of the template arguments. Concepts allow you to define constraints to your template. It is a way using which we can specify some constraints for our template arguments in C++.

Syntax

template <typename T>
concept ConceptName = /* constraints or requirements */;

If we combine these two terms then we can say that constraints are the expression and concepts is a way to define these expressions.

Examples of Concepts in C++

Example 1:

The below code demonstrates the usage of concepts in C++ to define and apply requirements on template arguments.

// C++ Program to illustrate the use of concept to define
// constraints
#include <iostream>
#include <iterator>
#include <type_traits>
#include <vector>

using namespace std;

// Define the Container concept
template <typename T> concept Container = requires(T t)
{
    {
        std::size(t)
    }
    ->std::same_as<std::size_t>;
    {
        std::begin(t)
    }
    ->std::same_as<typename T::iterator>;
    {
        std::end(t)
    }
    ->std::same_as<typename T::iterator>;
};

// Define the ContainerWrapper class that works with
// containers satisfying the
// Container concept
template <Container C> class ContainerWrapper {
public:
    ContainerWrapper(C c)
        : container(c)
    {
    }

    // Print the elements of the container
    void print()
    {
        for (auto it = std::begin(container);
             it != std::end(container); ++it) {
            cout << *it << " ";
        }
        cout << endl;
    }

private:
    C container;
};

int main()
{
    // Create a vector container
    vector<int> v{ 1, 2, 3 };

    // Instantiate a ContainerWrapper object with the vector
    ContainerWrapper wrapper(v);

    // Print the elements of the container using the
    // ContainerWrapper
    wrapper.print();
}

Output

1 2 3

Note: Concepts in C++ can not refer to themselves in a recursive manner.

Example 2:

The below code demonstrates errors that occur when we define recursive concepts.

template <typename T>
// Attempting recursive reference
concept RecursiveConcept = RecursiveConcept<T*>;

template <typename T>
void foo(T value) requires RecursiveConcept<T>
{
    // Code
}

int main()
{
    // Will result in a compiler error
    foo(42);
    return 0;
}

The above code will result in an error as we have defined a concept RecursiveConcept in the code that refers to itself that is not allowed in C++.

Concepts can be named in an id-expression. When we use the concept in an id-expression, the value of the id-expression is determined by whether the constraint expression of the concept is satisfied by the template argument.

If the constraint expression is satisfied by the template argument, the value of the id-expression is true, else it is false.

Conclusion

Constraints and concepts in C++ provide a powerful mechanism for expressing requirements on template arguments. It helps us to avoid the unwanted argument values for our templates. One thing to keep in mind is that constraints are the expression and concepts is a way to define these expressions.