C++ Namespaces

In this page:

What is a Namespace?
The Standard Namespace
Nested Namespaces
Inline Namespaces
Namespaces Best Practices

What is a Namespace?

A namespace is a feature in C++ used to group related code under a unique name. It helps prevent naming conflicts. This is especially useful when your project uses multiple external libraries with identical class or function names.

Example: What is a Namespace?

cpp

#include <iostream>

namespace MyLibrary {
	void greet() { std::cout << "Hello from MyLibrary" << std::endl; }
}

int main() {
	MyLibrary::greet();
	return 0;
}

The Standard Namespace

All standard library templates, classes, and objects are grouped inside the std namespace. You can use the scope resolution operator or write a using directive to import standard elements into your current scope.

Example: The Standard Namespace

cpp

#include <iostream>

int main() {
	std::cout << "Using the std namespace" << std::endl;
	return 0;
}

Nested Namespaces

You can declare a namespace inside another namespace. This establishes a clear hierarchy for your code modules, similar to nested folders on your computer.

Example: Nested Namespaces

cpp

#include <iostream>

namespace Company {
	namespace Project {
		void run() { std::cout << "Running" << std::endl; }
	}
}

int main() {
	Company::Project::run();
	return 0;
}

Inline Namespaces

An inline namespace allows you to define versioned APIs. Members of an inline namespace are automatically visible in the parent namespace. This helps you deploy code updates while preserving backward compatibility.

Example: Inline Namespaces

cpp

#include <iostream>

namespace Library {
	inline namespace V2 {
		void greet() { std::cout << "Version 2" << std::endl; }
	}
}

int main() {
	Library::greet();
	return 0;
}

Namespaces Best Practices

Avoid putting global using directives in header files. This can pollute the global scope of any file that includes your header, leading to hard-to-track naming conflicts.

Example: Namespaces Best Practices

cpp

#include <iostream>

// Avoid "using namespace std;" in header files -- it would pollute the
// global scope of every file that includes this header.

int main() {
	std::cout << "Prefer explicit std:: prefixes in headers" << std::endl;
	return 0;
}

🔒

Chapter Quiz — Complete all 17 topics to unlock

0/17 topics done

Complete these topics first:

← Previous Lesson C++ Input Validation Next Lesson → C++ Header Files