头文件和命名空间是否必须同时引用?

c++中开始的两行一般为

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#include <iostream>
using namespace std;

但也可以使用std::cin方式显示指定namespace。那么不使用”using namespace std;”是否可以直接采用std::cin调用呢?试验后却发现会报错。

头文件iostream中定义了std namespace,如果不用using namespace std,则cin cout不可见?目前未搞懂,先记录

iostream源码如下:

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// iostream standard header for Microsoft
#pragma once
#ifndef _IOSTREAM_
#define _IOSTREAM_
#ifndef RC_INVOKED
#include <istream>

#pragma pack(push,_CRT_PACKING)
#pragma warning(push,3)
#pragma push_macro("new")
#undef new
_STD_BEGIN
#ifdef _M_CEE_PURE
__PURE_APPDOMAIN_GLOBAL extern istream cin, *_Ptr_cin;
__PURE_APPDOMAIN_GLOBAL extern ostream cout, *_Ptr_cout;
__PURE_APPDOMAIN_GLOBAL extern ostream cerr, *_Ptr_cerr;
__PURE_APPDOMAIN_GLOBAL extern ostream clog, *_Ptr_clog;

__PURE_APPDOMAIN_GLOBAL extern wistream wcin, *_Ptr_wcin;
__PURE_APPDOMAIN_GLOBAL extern wostream wcout, *_Ptr_wcout;
__PURE_APPDOMAIN_GLOBAL extern wostream wcerr, *_Ptr_wcerr;
__PURE_APPDOMAIN_GLOBAL extern wostream wclog, *_Ptr_wclog;

#else /* _M_CEE_PURE */
// OBJECTS
__PURE_APPDOMAIN_GLOBAL extern _CRTDATA2 istream cin, *_Ptr_cin;
__PURE_APPDOMAIN_GLOBAL extern _CRTDATA2 ostream cout, *_Ptr_cout;
__PURE_APPDOMAIN_GLOBAL extern _CRTDATA2 ostream cerr, *_Ptr_cerr;
__PURE_APPDOMAIN_GLOBAL extern _CRTDATA2 ostream clog, *_Ptr_clog;

__PURE_APPDOMAIN_GLOBAL extern _CRTDATA2 wistream wcin, *_Ptr_wcin;
__PURE_APPDOMAIN_GLOBAL extern _CRTDATA2 wostream wcout, *_Ptr_wcout;
__PURE_APPDOMAIN_GLOBAL extern _CRTDATA2 wostream wcerr, *_Ptr_wcerr;
__PURE_APPDOMAIN_GLOBAL extern _CRTDATA2 wostream wclog, *_Ptr_wclog;

// CLASS _Winit
class _CRTIMP2_PURE _Winit {
public:
__thiscall _Winit();
__thiscall ~_Winit() _NOEXCEPT;
private:
__PURE_APPDOMAIN_GLOBAL static int _Init_cnt;
};
#endif /* _M_CEE_PURE */
_STD_END
#pragma pop_macro("new")
#pragma warning(pop)
#pragma pack(pop)
#endif /* RC_INVOKED */
#endif /* _IOSTREAM_ */

/*
* Copyright (c) by P.J. Plauger. All rights reserved.
* Consult your license regarding permissions and restrictions.
V6.50:0009 */

参考

[C++]头文件(Header Files)和命名空间(Namespace)

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http://2tu.github.io/
本文出自Tu’s blog