Defining namespaces
- C++ course
- Beginner
- Lesson 106/116
- 5 min
Learn to define a namespace with the namespace keyword.
Namespaces with namespace
In large projects, functions or variables with the same name can clash. C++ solves this with namespace; you group related declarations under a name.
namespace math {
int add(int a, int b) {
return a + b;
}
}Here the add function lives inside the math namespace. To call it from outside, you write math::add(2, 3). The :: is the scope resolution operator and lets you reach the items inside a namespace.
The scope resolution operator
To access an item inside a namespace from outside, you use the :: operator. The namespace name comes first, the item name second.
int n = math::add(2, 3); // n = 5Here math::add calls the add function inside the math namespace. The same operator is also used to reach class members and global variables. With nested namespaces you chain them: outer::inner::foo().
Practice math namespace and call
Define namespace math. Inside it write the function int add(int a, int b) { return a + b; }. In main, call it with int n = math::add(2, 3);, print with cout << n << endl;.
Starter code
#include <iostream>
using namespace std;
// Define the math namespace
int main() {
// call the function in the namespace and print
return 0;
}Sample solution
#include <iostream>
using namespace std;
namespace math {
int add(int a, int b) {
return a + b;
}
}
int main() {
int n = math::add(2, 3);
cout << n << endl;
return 0;
}Expected output
5
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Which keyword is used to define a namespace?
- package
- module
- namespace Correct answer
- scope
Explanation: In C++ a namespace is defined with namespace.
2 Fill in the blank: namespace call
int n = math____:add(2, 3);int n = math::add(2, 3);Explanation: The :: operator reaches the item in a namespace.
3 Match each expression with its purpose
| Term | Meaning |
|---|---|
namespace math |
defines a namespace |
math::add |
calls the inner function |
:: |
scope operator |