Namespaces in C++ prevent naming conflicts and enable controlled access between diffferent code sections. As projects grow larger, the likelihood of name collisions increases, especial when integrating third-party libraries that may define identical identifiers differently.

Defining Namespaces

namespace CustomSpace {}

Three Access Methods

1. Explicit Qualification

#include <iostream>
namespace Data {
    int value = 42;
}

int main() {
    std::cout << Data::value << std::endl;
    return 0;
}

2. Single Declaration

#include <iostream>
namespace Metrics {
    int width = 100;
    int height = 200;
}
using Metrics::width;

int main() {
    std::cout << width << std::endl;
    return 0;
}

3. Namespace Import

#include <iostream>
namespace Config {
    int timeout = 30;
    int retries = 3;
}
using namespace Config;

int main() {
    std::cout << timeout << std::endl;
    std::cout << retries << std::endl;
    return 0;
}

Special Namespace Cases

Global Namespace The default namespace contains all identifiers not explicitly placed in a named namespace. To access global identifiers from within a custom namespace:

int compute(int x);

namespace App {
    void process() {
        std::cout << ::compute(5);
    }
}

Nested Namespaces Namespaces can contain other namespaces:

namespace Outer {
    int count = 0;
    
    namespace Inner {
        void display() {
            std::cout << "Nested access" << std::endl;
        }
    }
}

int main() {
    Outer::Inner::display();
    return 0;
}

When name conflicts occur between nested scopes, C++ follows the nearest-scope resolution principle:

int x = 1;

namespace Layer1 {
    int x = 10;
    
    namespace Layer2 {
        int x = 20;
        void show() {
            std::cout << x; // Outputs 20
        }
    }
}