Forward declaration of std::wstring

Question

// 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::wstri         


        

Answer 1

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

namespace std {
  template
  struct char_traits;

  template
  struct allocator;

  template
  struct basic_string;

  typedef basic_string, allocator >
    wstring;
}

// simple test that it's compatible:
std::wstring *p;  // incomplete type at this point, but you can have a pointer
#include 
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
  struct char_traits;

  template
  struct allocator;

  template,
           class Allocator=allocator >
  struct basic_string;

  typedef basic_string wstring;
}

#include 

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