Understading ostream and how to declare in a class definition

Question

Hi guys recently i'm starting to understand better c++ and i found different problems and most of them are starting to be clear. One thing that i've not understood is a error that the compiler found when i try to declare a ostream or the every stream in a class declaration. For example

class Test{
  stringbuff buff;
  ostream out (&buff)

; }

The compiler returns this error message:

expected identifier before ‘&’ token

Oneother is when i try with:

stringstream stream(std::in|std::out);

compiler returns

error: ‘std::ios<char, std::char_traits<char> >::in’ is not a type
   stringstream out(ios::in|ios::out);

The question is why can't i call these 'functions' in class declaration and what king of methods are. For example to be more clear how can declare a same methoud to use in this in the same way of ostream o (method);

Thanks to all and sorry for my english.

Answer 1

Your problem is that statement ostream out (&buff) ; is treated by compiler as an attempt to declare a function member, not a data member ; that is a generalized case of the Most vexing parse.

"Using the new uniform initialization syntax introduced in C++11 solves this issue" for in-class initialization also : ostream out{ &buff };.

To be more specific, c++11 just allows you to use direct-initialization with {} or copy-initialization with =, and not () in any of its "direct-init" usages for data members in-class initialization.

Another option is to initialize your data member within constructor's init list.

class Test
{
  std::stringbuf buff ;
  std::ostream out ;
  public :
    Test () : out( & buff ) { }
} ;