Namespace

1 What is a Namespace?

In C++, namespaces are used to organize code and prevent name conflicts — especially in large projects or when using multiple libraries. A namespace groups related classes, functions, variables, etc., under a name.

Example:

#include <iostream>

namespace MathUtils {
    int add(int a, int b) {
        return a + b;
    }

    int square(int x) {
        return x * x;
    }
}

int main() {
    std::cout << MathUtils::add(3, 5) << std::endl;     
    std::cout << MathUtils::square(4) << std::endl;
    return 0;
}

Output:

2 Using using Declarations

If you use a namespace often, you can simplify access with using:

Option 1 — Use a single name:

#include <iostream>
using MathUtils::add;

int main() {
    std::cout << add(2, 3) << std::endl;  
    std::cout << square(4); 
}

Option 2 — Use the entire namespace:

#include <iostream>
using namespace MathUtils;

int main() {
    std::cout << add(2, 3) << std::endl;
    std::cout << square(4) << std::endl;
}

Be careful with using namespace in headers or large projects — it may cause name collisions.

3 Nested Namespaces

C++ allows nested namespaces for hierarchical organization.

#include <iostream>
namespace Company {
    namespace Project {
        void info() {
            std::cout << "Project info\n";
        }
    }
}

namespace Company::Project {
    void details() {
        std::cout << "Project details\n";
    }
}

int main() {
    Company::Project::info();
    Company::Project::details();
}

4 Anonymous Namespaces

Used to limit visibility of functions or variables to a single translation unit (file).

#include <iostream>
namespace {
    int secret = 42;

    void hiddenFunction() {
        std::cout << "Hidden value: " << secret << std::endl;
    }
}

int main() {
    hiddenFunction();  
}

Equivalent to using static in C for file-local scope.

5 Namespace Aliases

You can create shorter aliases for long namespace names.

#include <iostream>
namespace VeryLongNamespaceName {
    void greet() {
        std::cout << "Hello!\n";
    }
}

namespace VLN = VeryLongNamespaceName;

int main() {
    VLN::greet();  
}

6 Extending Namespaces

You can split namespace definitions across multiple files or locations.

namespace App {
    void start() { std::cout << "App started\n"; }
}

namespace App {
    void stop() { std::cout << "App stopped\n"; }
}

int main() {
    App::start();
    App::stop();
}

Summary

Concept	Syntax	Use Case
Basic namespace	namespace N { ... }	Organize code
Access member	N::member	Qualified access
Using declaration	using N::x;	Simplify usage
Using directive	using namespace N;	Temporarily bring all names into scope
Nested	namespace A::B {}	Hierarchical structure
Anonymous	namespace {}	File-local visibility
Alias	namespace short = long_name;	Shorter references