CodeBlanks Logo Icon CodeBlanks
Login Register

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.

Practice 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;.

Starter code

main.cpp
#include <iostream>
using namespace std;

// Define app and util as nested namespaces

int main() {
    // call the inner helper with its full name and print
    return 0;
}

Sample solution

main.cpp
#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;
}

Expected output

7

Questions and answers

Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.

1 In namespace app { namespace util { ... } }, how is helper called?

  • app.util.helper()
  • app::util::helper() Correct answer
  • app-helper()
  • util::helper()

Explanation: Nested namespaces use :: chaining.

2 Fill in the blank: nested call

Question 
int n = app::util____helper();
Answer 
int n = app::util::helper();

Explanation: The :: operator reaches the inner namespace.

3 Match each expression with its purpose

Term Meaning
namespace app::util nested definition
app::util::helper full call
using namespace app::util imports

All lessons in Beginner

1 C++ basics

  1. What is C++?
  2. Your first program
  3. std::cout
  4. std::cin
  5. Comments
  6. Summary - C++ basics

2 Variables

  1. Declaring variables
  2. auto
  3. const
  4. constexpr
  5. Type conversion
  6. Summary - Variables

3 Data types

  1. Integer types
  2. Floating-point types
  3. bool and char
  4. std::string
  5. sizeof
  6. Summary - Data types

4 Project: Calculator

  1. Project design
  2. Add and subtract
  3. Multiply and divide
  4. Error handling
  5. Putting it together

5 Operators

  1. Arithmetic operators
  2. Assignment operators
  3. Comparison operators
  4. Logical operators
  5. Bitwise operators
  6. Summary - Operators

6 Strings

  1. std::string basics
  2. String concatenation
  3. String length
  4. Substring
  5. String search
  6. Summary - Strings

7 Conditionals

  1. The if statement
  2. else and else if
  3. The switch statement
  4. The ternary operator
  5. Nested conditionals
  6. Summary - Conditionals

8 Loops

  1. The for loop
  2. The while loop
  3. The do-while loop
  4. Range-based for
  5. break and continue
  6. Summary - Loops

9 Project: Shopping cart

  1. Cart design
  2. Adding products
  3. Calculating totals
  4. Listing the cart
  5. Putting it together

10 Arrays

  1. C-style arrays
  2. Array access
  3. std::array
  4. Multidimensional arrays
  5. Arrays and loops
  6. Summary - Arrays

11 std::vector

  1. Declaring vectors
  2. push_back
  3. size and at
  4. Vector access
  5. Vector loop
  6. Summary - std::vector

12 Functions

  1. Defining functions
  2. Return values
  3. Parameters
  4. Function overloading
  5. Default parameters
  6. Summary - Functions

13 References

  1. What is a reference?
  2. Declaring references
  3. Reference parameters
  4. const references
  5. Dangling references
  6. Summary - References

14 Pointer basics

  1. What is a pointer?
  2. The & address operator
  3. Pointer dereference
  4. nullptr
  5. Pointer arithmetic
  6. Summary - Pointer basics

15 Project: To-do CLI

  1. Task model
  2. Adding tasks
  3. Listing tasks
  4. Completing tasks
  5. Putting it together

16 Advanced string operations

  1. String streams
  2. stoi and stod
  3. to_string
  4. String comparison
  5. String modification
  6. Summary - Advanced strings

17 Structs

  1. Defining structs
  2. Using structs
  3. Struct and function
  4. Array of structs
  5. Nested structs
  6. Summary - Structs

18 Exception basics

  1. try/catch
  2. throw
  3. std::exception
  4. Custom exceptions
  5. catch all
  6. Summary - Exceptions

19 Namespaces

  1. Defining namespaces
  2. using
  3. std namespace
  4. Nested namespaces
  5. Namespace alias
  6. Summary - Namespaces

20 Beginner final

  1. Project design
  2. Data model
  3. Business logic
  4. Error handling
  5. Putting it together