C++ Namespaces

What is a Namespace?

A namespace groups related identifiers to prevent name collisions. This is how std::cout is organized — everything in the C++ standard library lives in the std namespace.

#include <iostream>
using namespace std;

namespace Math {
    const double PI = 3.14159265;
    double square(double x)  { return x * x; }
    double cube(double x)    { return x * x * x; }
}

namespace Geometry {
    double circleArea(double r) { return Math::PI * Math::square(r); }
}

int main() {
    cout << Math::PI              << endl;   // 3.14159265
    cout << Math::square(4)      << endl;   // 16
    cout << Geometry::circleArea(5) << endl; // 78.5398
    return 0;
}

Output:

3.14159265
16
78.5398

using Declarations

using Math::PI;       // import specific name
using namespace Math; // import all names (use sparingly)
cout << PI;           // no prefix needed

Nested Namespaces (C++17)

namespace App::Utils::String {
    bool isEmpty(const string& s) { return s.empty(); }
}
// Use: App::Utils::String::isEmpty("test")

#include <iostream>
using namespace std;

namespace Imperial {
    double toKph(double mph) { return mph * 1.60934; }
}
namespace Metric {
    double toMph(double kph) { return kph / 1.60934; }
}

int main() {
    cout << "60 mph = " << Imperial::toKph(60) << " kph" << endl;
    cout << "100 kph = " << Metric::toMph(100) << " mph" << endl;
    return 0;
}

Summary

• Namespaces group identifiers to avoid name conflicts.
• Access members with NamespaceName::identifier.
• Avoid using namespace in headers.