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using

Learn to import a namespace with using.

Importing with using

Writing ns::item at every call is sometimes verbose. The using directive makes all names in a namespace available at once.

using namespace math;
int n = add(2, 3);  // no math:: needed

Here, after using namespace math, add can be called directly. This is convenient in small programs; but it is used carefully in large projects because it can cause name clashes.

Risks of using

Although using namespace is handy, it has risks. If two namespaces contain a function with the same name, the call becomes ambiguous; this leads to a compile error.

For this reason, large projects usually avoid using namespace; instead they prefer full names like std::cout. Writing using in header files is almost always a bad practice; because everyone who includes that file automatically imports the namespace.

Practice Call add with using

Define namespace math { int add(int a, int b) { return a + b; } }. In main, first write using namespace math;, then call int n = add(4, 5);. Print with cout << n << endl;.

Starter code

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

// Define the math namespace

int main() {
    // import with using, call add, print
    return 0;
}

Sample solution

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

namespace math {
    int add(int a, int b) {
        return a + b;
    }
}

int main() {
    using namespace math;
    int n = add(4, 5);
    cout << n << endl;
    return 0;
}

Expected output

9

Questions and answers

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

1 Fill in the blank: using directive

Question 
using ____ math;
int n = add(2, 3);
Answer 
using namespace math;
int n = add(2, 3);

Explanation: using namespace math imports the namespace.

2 How is add(2,3) called after using namespace?

  • math::add(2,3)
  • namespace::add(2,3)
  • math.add(2,3)
  • add(2,3) Correct answer

Explanation: After using, you simply write add(2,3).

3 Match each expression with its result

Term Meaning
using namespace math imports
math::add call with full name
using namespace std imports std

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