C++ Namespaces
What is a Namespace?
A namespace is a declarative region that provides a scope for identifiers (names of types, functions, variables). Namespaces prevent name collisions — two libraries can both define a function called print() without conflict if they're in different namespaces.
The entire C++ standard library lives in the std namespace — that's why you write std::cout or use using namespace std;.
#include <iostream>
using namespace std;
namespace Math {
const double PI = 3.14159265358979;
double circleArea(double r) { return PI * r * r; }
double square(double x) { return x * x; }
}
namespace Graphics {
void draw() { cout << "Drawing shape..." << endl; }
double square(double x) { return x * x; } // same name, different namespace
}
int main() {
// Access with scope resolution operator ::
cout << Math::PI << endl;
cout << Math::circleArea(5) << endl;
cout << Math::square(4) << endl; // 16
cout << Graphics::square(4) << endl; // 16 — no conflict!
Graphics::draw();
// using declaration — bring one name into scope
using Math::PI;
cout << PI << endl; // no Math:: needed
// using directive — bring entire namespace into scope
using namespace Math;
cout << circleArea(3) << endl;
return 0;
}Nested Namespaces and Aliases
#include <iostream>
using namespace std;
// Nested namespace
namespace App {
namespace Utils {
void log(const string &msg) {
cout << "[LOG] " << msg << endl;
}
}
namespace DB {
void connect() { cout << "DB connected" << endl; }
}
}
// C++17 nested namespace shorthand
namespace App::Config {
const string VERSION = "1.0.0";
}
// Namespace alias — shorter name for long namespace
namespace AU = App::Utils;
// Anonymous namespace — internal linkage (like static in C)
namespace {
int internalCounter = 0; // only visible in this file
void increment() { internalCounter++; }
}
int main() {
App::Utils::log("Starting app");
App::DB::connect();
cout << App::Config::VERSION << endl;
AU::log("Using alias"); // shorter
increment();
increment();
cout << "Counter: " << internalCounter << endl; // 2
return 0;
}