How do you use a namespace in C++?

Either qualify each name with namespace:: (e.g. std::cout), or bring names into scope with using.

// Option 1: qualify every name
std::cout << "Hello" << std::endl;
std::vector<int> v;

// Option 2: using-declaration (one specific name)
using std::cout;
using std::endl;
cout << "Hello" << endl;

// Option 3: using-directive (the whole namespace)
using namespace std;
cout << "Hello" << endl;
vector<int> v;

Why namespaces exist: they prevent name collisions — two libraries can each have a sort or string without clashing, because each lives in its own namespace.

Best practices:

• Never put using namespace in a header file (it forces it on everyone who includes the header)
• In .cpp files either approach is fine
• In large projects, explicit std:: is clearer about where a name comes from

Go deeper:

• Namespaces — cppreference — declaration, using, and name-lookup rules.
• Namespace — Wikipedia — the general concept beyond C++.