1

Someone once hinted that doing this in a header file is not advised:

using namespace std;

Why is it not advised?

Could it cause linker errors like this: (linewrapped for convenience)

error LNK2005: "public: __thiscall std::basic_string<char,struct 
std::char_traits<char>,class std::allocator<char> >::
~basic_string<char,struct std::char_traits<char>,class std::allocator<char> > 
(void)" (??1?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@std@@QAE@XZ) 
already defined in tools.lib(Exception.obj)
Mooing Duck
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Tony The Lion
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2 Answers2

10

Because it forces anyone who uses your header file to bring the std namespace into global scope. This could be a problem if they have a class that has the same name as one of the standard library classes.

Eric Petroelje
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1

If the file gets included elsewhere the compilation unit will implicitely get the using directive. This can lead to confusing errors when names overlap.

pmr
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  • could it cause linker errors like this: error LNK2005: "public: __thiscall std::basic_string,class std::allocator >::~basic_string,class std::allocator >(void)" (??1?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@std@@QAE@XZ) already defined in tools.lib(Exception.obj) ?? – Tony The Lion Jul 06 '10 at 12:49
  • @Tony: You should add this to the question. It would be easier to read with proper formatting. – pmr Jul 06 '10 at 12:58