The #include<iostream> exists, but I get an error: identifier "cout" is undefined. Why?

Question

I learn C++ and COM through the books. In the IDE MS Visual Studio 2012 I have created new empty C++ project, and added some existing files to it. My CPP file contains #include<iostream> row, but in editor I got such messages:

Error: identifier "cout" is undefined

end

Error: identifier "endl" is undefined

Code:

#include<iostream>
#include"interfaces.h" // unknown.h, objbase.h, initguid.h

class CA {//: public IX, IY{
public:
    // Constructor
    CA();
    // Destructor
    ~CA();
    // IUnknown
    virtual HRESULT __stdcall QueryInterface(const IID& iid, void** ppv);
    virtual ULONG __stdcall AddRef();
    virtual ULONG __stdcall Release();
    // IX
    virtual void __stdcall Fx1();
    virtual void __stdcall Fx2();
    // IY
    virtual void __stdcall Fy1(){ cout << "Fy1" << endl; }  // errors here
    virtual void __stdcall Fy2(){ cout << "Fy2" << endl; }  // errors here also
private:
    long counter;
};

Why it happens?

Answer 1

You need to specify the std:: namespace:

std::cout << .... << std::endl;;

Alternatively, you can use a using directive:

using std::cout;
using std::endl;

cout << .... << endl;

I should add that you should avoid these using directives in headers, since code including these will also have the symbols brought into the global namespace. Restrict using directives to small scopes, for example

#include <iostream>

inline void foo()
{
  using std::cout;
  using std::endl;
  cout << "Hello world" << endl;
}

Here, the using directive only applies to the scope of foo() .

Answer 2

您可以在#include <iostream>之后的开头添加：

using namespace std;

Answer 3

cout is in std namespace, you shall use std::cout in your code. And you shall not add using namespace std; in your header file, it's bad to mix your code with std namespace, especially don't add it in header file.

Answer 4

The problem is the std namespace you are missing. cout is in the std namespace.
Add using namespace std; after the #include

Answer 5

If you have included #include iostream and using namespace std; it should work. If it still doesn't work, make sure to check that you haven't deleted anything in the iostream file. To get to you iostream file, just Ctrl +Click your #include iostream and it should take you to that file. You can paste the below original iostream file to your iostream file and it should work.

// Standard iostream objects -*- C++ -*-

// Copyright (C) 1997-2019 Free Software Foundation, Inc.
//
// This file is part of the GNU ISO C++ Library.  This library is free
// software; you can redistribute it and/or modify it under the
// terms of the GNU General Public License as published by the
// Free Software Foundation; either version 3, or (at your option)
// any later version.

// This library is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
// GNU General Public License for more details.

// Under Section 7 of GPL version 3, you are granted additional
// permissions described in the GCC Runtime Library Exception, version
// 3.1, as published by the Free Software Foundation.

// You should have received a copy of the GNU General Public License and
// a copy of the GCC Runtime Library Exception along with this program;
// see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
// <http://www.gnu.org/licenses/>.

/** @file include/iostream
 *  This is a Standard C++ Library header.
 */

//
// ISO C++ 14882: 27.3  Standard iostream objects
//

#ifndef _GLIBCXX_IOSTREAM
#define _GLIBCXX_IOSTREAM 1

#pragma GCC system_header

#include <bits/c++config.h>
#include <ostream>
#include <istream>

namespace std _GLIBCXX_VISIBILITY(default)
{
_GLIBCXX_BEGIN_NAMESPACE_VERSION

  /**
   *  @name Standard Stream Objects
   *
   *  The &lt;iostream&gt; header declares the eight <em>standard stream
   *  objects</em>.  For other declarations, see
   *  http://gcc.gnu.org/onlinedocs/libstdc++/manual/io.html
   *  and the @link iosfwd I/O forward declarations @endlink
   *
   *  They are required by default to cooperate with the global C
   *  library's @c FILE streams, and to be available during program
   *  startup and termination. For more information, see the section of the
   *  manual linked to above.
  */
  //@{
  extern istream cin;       /// Linked to standard input
  extern ostream cout;      /// Linked to standard output
  extern ostream cerr;      /// Linked to standard error (unbuffered)
  extern ostream clog;      /// Linked to standard error (buffered)

#ifdef _GLIBCXX_USE_WCHAR_T
  extern wistream wcin;     /// Linked to standard input
  extern wostream wcout;    /// Linked to standard output
  extern wostream wcerr;    /// Linked to standard error (unbuffered)
  extern wostream wclog;    /// Linked to standard error (buffered)
#endif
  //@}

  // For construction of filebuffers for cout, cin, cerr, clog et. al.
  static ios_base::Init __ioinit;

_GLIBCXX_END_NAMESPACE_VERSION
} // namespace

#endif /* _GLIBCXX_IOSTREAM */