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  • 749
    Don't forget you can do: "using std::cout;" which means you don't have to type std::cout, but don't bring in the entire std namespace at the same time.
    – Bill
    Commented Sep 21, 2009 at 15:29
  • 127
    It is particularly bad to use 'using namespace std' at file scope in header files. Using it in source files (*.cpp) at file scope after all includes is not quite as bad, as its effect is limited to a single translation unit. Even less problematic is using it inside functions or classes, because its effect is limited to the function or class scope.
    – sh-
    Commented Jun 25, 2017 at 14:37
  • 17
    I would discourage to use using directive but for specific namespaces like std::literals::chrono_literals, Poco::Data:Keywords,Poco::Units and stuff that will deal with literals or readability tricks. Whenever it is in header or implementation files. It might be OK in a function scope I guess, but apart from literals and stuff, it is not useful.
    – Ludovic Zenohate Lagouardette
    Commented Jul 19, 2017 at 9:33
  • 25
    @Jon: It's got nothing to do with namespace std in particular. My emphasis was meant to be on "at file scope in header files". To put it as an advice: Do not use "using namespace" (std or other) at file scope in header files. It is OK to use it in implementation files. Sorry for the ambiguity.
    – sh-
    Commented Apr 9, 2018 at 14:10
  • 5
    I have no answer to add to the others, but I use: using [namespace]::[identifier]; in the local scope of relatively simple source files. For example, if I have a header frequently using the fully qualified: std::size_t type, I will typically prepend: using std::size_t; to the implementation. This simplifies reading the code, and only effects the local scope - that is: using std::[identifier]; employs the principle of least surprise. There are too many identifiers in the std namespace to keep track of. Later C++ revisions may add identifiers that clash with your own!
    – Brett Hale
    Commented Aug 7, 2021 at 13:34