A namespace is a named scope that groups related declarations and prevents name clashes between libraries.

Declaring and using

C++

#include <iostream>

namespace geometry {
    constexpr double pi = 3.14159;
    double circleArea(double r) { return pi * r * r; }

    namespace units {                      // nested namespace
        double toCm(double inches) { return inches * 2.54; }
    }
}

namespace app::config {                     // C++17 nested-namespace shorthand
    int retries = 3;
}

int main() {
    std::cout << geometry::circleArea(1.0) << '\n';
    std::cout << geometry::units::toCm(10) << '\n';
    namespace gu = geometry::units;         // namespace alias
    std::cout << gu::toCm(1) << ' ' << app::config::retries << '\n';
    using geometry::pi;                     // using-declaration: one name
    std::cout << pi << '\n';
}

Output

Anonymous namespaces & `::`

C++

#include <iostream>

namespace {                    // anonymous: visible only in this .cpp (internal linkage)
    int helperCalls = 0;
    void helper() { ++helperCalls; }
}

int value = 1;                 // global

int main() {
    int value = 2;             // local shadows the global
    helper();
    helper();
    std::cout << value << ' ' << ::value << ' ' << helperCalls << '\n';   // ::value = global scope
}

Output

2 1 2

Construct	Effect
`ns::name`	Qualified access
`using ns::name;`	Using-declaration: brings one name into scope
`using namespace ns;`	Using-directive: makes all names visible (risk of ambiguity)
`namespace a = long::name;`	Alias
`namespace { … }`	Anonymous — replaces `static` for file-local functions/variables
`inline namespace v2 { … }`	Members are also visible in the enclosing namespace (library versioning)

Argument-dependent lookup (ADL)

When you call an unqualified function, C++ also searches the namespaces of the argument types. That's why std::cout << x finds std::operator<< and swap(a, b) can find a custom swap next to your type.

C++

#include <iostream>
namespace shop {
    struct Item { int price; };
    void print(const Item& i) { std::cout << "Item $" << i.price << '\n'; }
}
int main() {
    shop::Item it{5};
    print(it);        // no `shop::` needed — found by ADL via the argument's type
}

Output

Item $5

What is the difference between an anonymous namespace and static at file scope?

Declaring and using

Anonymous namespaces & ::

Argument-dependent lookup (ADL)

Anonymous namespaces & `::`