Turning GetLastError() into an exception

Question

I have a Visual Studio 2008 C++ project that uses a Win32Exception class in cases where there is an exceptional error. The Win32Exception class looks like this:

/// defines an exception based on Win32 error codes. The what() function will
/// return a formatted string returned from FormatMessage()
class Win32Exception : public std::runtime_error
{
public:
    Win32Exception() : std::runtime_error( ErrorMessage( &error_code_ ) )
    {
    };

    virtual ~Win32Exception() { };

    /// return the actual error code
    DWORD ErrorCode() const throw() { return error_code_; };

private:

    static std::string ErrorMessage( DWORD* error_code )
    {
        *error_code = ::GetLastError();

        std::string error_messageA;
        wchar_t* error_messageW = NULL;
        DWORD len = ::FormatMessageW( FORMAT_MESSAGE_FROM_SYSTEM | 
                                      FORMAT_MESSAGE_ALLOCATE_BUFFER |
                                      FORMAT_MESSAGE_IGNORE_INSERTS,
                                      NULL,
                                      *error_code,
                                      MAKELANGID( LANG_NEUTRAL, SUBLANG_DEFAULT ),
                                      reinterpret_cast< LPWSTR >( &error_messageW ),
                                      0,
                                      NULL );
        if( NULL != error_messageW )
        {
            // this may generate a C4244 warning. It is safe to ignore.
            std::copy( error_messageW, 
                       error_messageW + len, 
                       std::back_inserter( error_messageA ) );
            ::LocalFree( error_messageW );
        }
        return error_messageA;
    };

    /// error code returned by GetLastError()
    DWORD error_code_;

}; // class Win32Exception

The class works well in the situations it has been used in. What I would like to know is if there are any obvious cases where this will fail that I should be aware of. Any other gotchas, caveats, or general suggestions on improvements are welcome.

Please note that the boost library is not an option for this code.

Answer 1

• This has already done by several people, including yours truly
  • https://github.com/BillyONeal/Instalog/blob/master/LogCommon/Win32Exception.hpp
  • https://github.com/BillyONeal/Instalog/blob/master/LogCommon/Win32Exception.cpp

• Ironically, your code is not exception safe.

  if( NULL != error_messageW )
  {
      // this may generate a C4244 warning. It is safe to ignore.
      std::copy( error_messageW, 
                 error_messageW + len, 
                 std::back_inserter( error_messageA ) );
      ::LocalFree( error_messageW );
  }
  

Note that if the back_inserter causes std::bad_alloc to be thrown, the memory allocated inside FormatMessage is leaked.