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What is a namespace and why use one? What does 'using namespace std;' do and why is it discouraged in headers? How do you nest namespaces, and what is the C++17 shorthand? What is the global namespace and how do you refer to it? What is an anonymous (unnamed) namespace? What is a namespace alias? How do namespaces help organize a large codebase? What is argument-dependent lookup (ADL)? What is an inline namespace and what is it used for? How do namespaces interact with the one-definition rule and linkage?
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What is a using-declaration versus a using-directive?

Intermediate

Answer

A using-declaration (using std::vector;) brings a single name into scope. A using-directive (using namespace std;) brings all names from a namespace into scope. Using-declarations are more precise and safer; using-directives are broad and collision-prone. Prefer using-declarations, scoped as narrowly as possible.
using std::string;      // declaration: just string
using namespace std;    // directive: everything in std
Real-world example A function uses a targeted using-declaration for std::swap to enable ADL while keeping scope clean.

Common follow-ups: Which is more precise? | Why prefer using-declarations?

Namespaces Compilation & Linking Namespaces
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