Do not use namespace using directives?

Question

Namespace usings are for your convenience, not for you to inflict on others: Never write a using declaration or a using directive before an #include directive. Corollary: In header files, don't write namespace-level using directives or using declarations; instead, explicitly namespace-qualify all names. 21 сент. 2009 г.

What is using directive in namespace?

The using directive can appear: At the beginning of a source code file, before any namespace or type declarations. In any namespace, but before any namespaces or types declared in that namespace, unless the global modifier is used, in which case the directive must appear before all namespace and type declarations.

What if namespace is not used?

If this namespace is not used, then computer finds for the cout, cin and endl etc.. Computer cannot identify those and therefore it throws errors.

Why using namespace is a bad practice?

While this practice is okay for example code, pulling in the entire std namespace into the global namespace is not good as it defeats the purpose of namespaces and can lead to name collisions. This situation is called namespace pollution.

Why should you avoid using directives in a header?

In general, avoid putting using directives in header files (*. h) because any file that includes that header will bring everything in the namespace into scope, which can cause name hiding and name collision problems that are very difficult to debug. Always use fully qualified names in a header file.

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