Forward declaration of std::wstring

// This is a header file.

class MyClass; // It can be forward declared because the function uses reference.
// However, how can I do forward declaraion about std::wstring?开发者_如何学Python
// class std::wstring; doesn't work.
VOID Boo(const MyClass& c);
VOID Foo(const std::wstring& s);

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

The reason is that wstring is defined in namespace std and is typedef'd to std::basic_string<wchar_t>. More elaborately, std::wstring is std::basic_string<wchar_t, std::char_traits<wchar_t> >. This means that in order to forward-declare std::wstring you'd have to forward-declare std::char_traits<> and std::basic_string<> inside namespace std. Because (apart from a few exceptions) the standard forbids adding definitions or declarations to namespace std (17.4.3.1/1) ultimately you can't forward-declare any standard template or type in a standard-conforming way. Specifically, this means you can't forward-declare std::wstring.

And yes, we all agree it would be convenient to have a <stringfwd> header, like <iosfwd> for <iostream>. But there isn't. <string> is also not nearly as hardcore to compile as <iostream>, but nevertheless. You have two choices: #include<string> or use an opaque pointer.

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std::wstring is a template instantiation, so you can't just forward declare it. You'll have to use the header file.

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I don't think avoiding #include <string> gains you any real benefit, but this is how you could do it:

namespace std {
  template<class Char>
  struct char_traits;

  template<class T>
  struct allocator;

  template<class Char, class Traits, class Allocator>
  struct basic_string;

  typedef basic_string<wchar_t, char_traits<wchar_t>, allocator<wchar_t> >
    wstring;
}

// simple test that it's compatible:
std::wstring *p;  // incomplete type at this point, but you can have a pointer
#include <string>
int main() {
  std::wstring s = L"Hello, world!";
  p = &s;
  return 0;
}

You have to be careful of default parameters; in particular, this would not work:

namespace std {
  template<class Char>
  struct char_traits;

  template<class T>
  struct allocator;

  template<class Char, class Traits=char_traits<Char>,
           class Allocator=allocator<Char> >
  struct basic_string;

  typedef basic_string<wchar_t> wstring;
}

#include <string>

Compiled with the include shows the incompatibility:

In file included from /usr/include/c++/4.4/string:41,
                 from example.cpp:15:
/usr/include/c++/4.4/bits/stringfwd.h:52: error: redefinition of default argument for ‘class _Traits’
example.cpp:8: note: original definition appeared here

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You can't forward declare std::wstring in a conforming implementation, not because it is a typedef for a template specialization or that there is any possibility that it has an unknown number of template arguments (it doesn't; these are strictly specified) but because there is a constraint on conforming programs that prohibits them from adding any declarations or definitions to the std namespace other than explicit specializations of standard templates which are specialized on a user-defined type.

This constraint is stated in 17.4.3.1 [lib.reserved.names] / 1. There is no exception for forward declarations of std::wstring, you must #include <string> to make a declaration std::wstring available in a conforming way.

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class std::wstring; doesn't compile. But this does:

namespace std{
class wstring;
}

However, this declaration is incompatible with the <string> header file, which you should see if you #include <string> after it. So it's really not safe.