C++20 Concepts: part 3

In this article we’ll dive deeper in the details of concepts and build upon the topics already explained in the first and second part.

1. Introduction

In the second part of the tutorial series, the idea behind the concepts imposing semantic restrictions on generic code was explained and the three ways of defining and using concepts were discussed. In this article, I’ll take a step further into writing the definitions of concepts.

2. Concepts with multiple semantic restrictions

Imagine a scenario where generic functions/methods have to be written whereby each function performs one mathematical operation (one of add, subtract, divide, multiply) on two input parameter and prints the result on the console. Lets call them sumAndPrint , subtractAndPrint , multiplyAndPrint anddivideAndPrint respecitvely.

The sumAndPrint generic function could look like below:

template <typename T>
void sumAndPrint(const T num1, const T num2)
{
    std::cout << (num1 + num2);
}

Similarly one could define the remaining three generic functions that can perform the remaining math operations viz - , *, \ and print the result on console. So how can one write a concept and ensure the template parameter follows the semantics of math operations and is as well streamable ?

One just needs to define a concept with multiple rules within the body:

template <typename T>
concept ConceptName = requires(T param1, T param2, ... )
{   
    {/* rule 1 */};
    {/* rule 2*/ };
          .
          .
          .
};

Coming back to our concept for the four generic methods, lets name it IsArithmeticOpAPrintable , can be defined as follows:

template <typename T>
concept IsArithmeticOpAPrintable = requires(T param1, T param2)
{
   {std::cout << (num1 + num2)};
   {std::cout << (num1 - num2)};
   {std::cout << (num1 * num2)};
   {std::cout << (num1 / num2)};
};

The body of the concept above imposes the semantic restrictions on the result of the mathematical operation performed with param1 and param2 , i.e param1 op param2 and says nothing about the parameters themselves!