c++ - Forward declaration of std::wstring -

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// header file.  class myclass; // can forward declared because function uses reference. // however, how can forward declaraion std::wstring? // class std::wstring; doesn't work. void boo(const myclass& c); void foo(const std::wstring& s);

you can't. #include <string>, have (almost) no choice.

the reason wstring defined in namespace std , typedef'd std::basic_string<wchar_t>. more elaborately, std::wstring std::basic_string<wchar_t, std::char_traits<wchar_t> >. means in order forward-declare std::wstring you'd have forward-declare std::char_traits<> , std::basic_string<> inside namespace std. because (apart few exceptions) standard forbids adding definitions or declarations namespace std (17.4.3.1/1) can't forward-declare standard template or type in standard-conforming way. specifically, means can't forward-declare std::wstring.

and yes, agree convenient have <stringfwd> header, <iosfwd> <iostream>. there isn't. <string> not hardcore compile <iostream>, nevertheless. have 2 choices: #include<string> or use opaque pointer.


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