C++ concept basics #01_concept_basic.cpp

 

**What it teaches:** C++ Concepts are like "type requirements"

```cpp

// Define a concept

template <typename T>

concept HasName = requires(T t) {

{ t.name } -> std::convertible_to<const char*>;

};

// Use in requires clause

template <typename T>

requires HasName<T> // ✅ Concept works here!

void printName(T animal) {

std::cout << animal.name;

}

```

**Key Point:** Concepts work in `requires` clauses

..

#include <iostream>

#include <concepts>

// ==========================================

// CONCEPT: Checks if a type has certain features

// ==========================================

// Simple concept: checks if type T has a "name" member

template <typename T>

concept HasName = requires(T t) {

{ t.name } -> std::convertible_to<const char*>;

};

// Another concept: checks if type T has a "bark()" method

template <typename T>

concept CanBark = requires(T t) {

{ t.bark() } -> std::same_as<void>;

};

// ==========================================

// Types that satisfy concepts

// ==========================================

struct Dog {

const char* name = "marearts.com Rex";

void bark() { std::cout << "Woof!\n"; }

};

struct Cat {

const char* name = "marearts.com Whiskers";

// No bark() method!

};

struct Robot {

// No name field!

void bark() { std::cout << "Beep boop!\n"; }

};

// ==========================================

// Functions that use concepts in "requires"

// ==========================================

// This function ONLY accepts types that have a name

template <typename T>

requires HasName<T> // ← Concept used in requires clause

void printName(T animal) {

std::cout << "Name: " << animal.name << "\n";

}

// This function ONLY accepts types that can bark

template <typename T>

requires CanBark<T> // ← Concept used in requires clause

void makeBark(T animal) {

animal.bark();

}

// ==========================================

// Main - Try it out!

// ==========================================

int main() {

Dog dog;

Cat cat;

Robot robot;

std::cout << "=== Using Concepts in 'requires' ===\n";

// ✅ Works - Dog has name

printName(dog);

// ✅ Works - Cat has name

printName(cat);

// ❌ Won't compile - Robot doesn't have name

// printName(robot); // Uncomment to see error!

std::cout << "\n";

// ✅ Works - Dog can bark

makeBark(dog);

// ❌ Won't compile - Cat can't bark

// makeBark(cat); // Uncomment to see error!

// ✅ Works - Robot can bark

makeBark(robot);

return 0;

}

..

How to build and run?

--------------------------------------------------------------------------------------------

>g++ -std=c++20 01_concept_basics.cpp -o 01_concept_basics

>./01_concept_basics

=== Using Concepts in 'requires' ===

Name: Rex

Name: Whiskers

Woof!

Beep boop!

--------------------------------------------------------------------------------------------

What is convertible_ to and same_as??

Used in your HasName concept:

01_concept_basics.cppLines 13-15

concept HasName = requires(T t) {

    { t.name } -> std::convertible_to<const char*>;

};

This checks that t.name returns something that can be converted to const char*. It's more flexible than requiring an exact type match - so it would accept const char*, char*, or any type convertible to const char*.

std::same_as

Used in your CanBark concept:

01_concept_basics.cppLines 19-21

concept CanBark = requires(T t) {

    { t.bark() } -> std::same_as<void>;

};

This checks that t.bark() returns exactly void - no conversions allowed. It's stricter than convertible_to.

And there are more predefined concepts?

These are C++20 Standard Concepts

Both are defined in the C++20 standard library <concepts> header. Other common standard concepts include:

• std::integral<T> - checks if T is an integral type

• std::floating_point<T> - checks if T is a floating point type

• std::derived_from<Derived, Base> - checks inheritance

• std::invocable<F, Args...> - checks if F can be called with Args

You can create your own concepts (like you did with HasName and CanBark), or use these predefined ones from the standard library! 🎯

Thank you!

MareArts