Nested namespaces

Learn nested namespaces and their usage.

Nested namespaces

Namespaces can be nested; this is the basic way to split large systems into subcategories. To reach an inner namespace, you write a scope chain.

namespace app {
    namespace util {
        int helper() { return 7; }
    }
}

Here the helper function is in the app::util namespace. To call it, you write app::util::helper(). Since C++17, the shorthand namespace app::util { ... } is also available.

Nested using usage

You can import an entire nested namespace with using namespace app::util;. After this line, helper can be called directly.

using namespace app::util;
int n = helper();  // no app::util:: needed

This is convenient when you work a lot with a specific sub-namespace. Still, the risk of name clashes in large projects should not be forgotten; in doubt, the full name app::util::helper() is preferred.

Exercise Define and call a nested namespace

Define namespace app { namespace util { int helper() { return 7; } } }. In main, call int n = app::util::helper();. Print with cout << n << endl;.

Show example solution

#include <iostream>
using namespace std;

namespace app {
    namespace util {
        int helper() { return 7; }
    }
}

int main() {
    int n = app::util::helper();
    cout << n << endl;
    return 0;
}

Test yourself

In namespace app { namespace util { ... } }, how is helper called?
- app.util.helper()
- app::util::helper()
- app-helper()
- util::helper()
Show answer

app::util::helper() — Nested namespaces use :: chaining.

Fill in the blank: nested call

int n = app::util____helper();

Show answer

int n = app::util::helper();

Match each expression with its purpose

namespace app::util

nested definition
app::util::helper

full call
using namespace app::util

imports

Next lesson: Namespace alias

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