C/C++编程：输出流类模板std :: basic_ostream

女爷i 2022-05-18 04:12 220阅读 0赞

>     template<
>         class CharT,
>         class Traits = std::char_traits<CharT>
>     > class basic_ostream : virtual public std::basic_ios<CharT, Traits>
> 
> 作用：包装给定的抽象设备（[std::basic\_streambuf][std_basic_streambuf]）并提供高层输出接口
> 
> 头文件：[<ostream>][ostream]
> 
> ![20210304135036530.png][]
> 
> **模板实例化：**
> 
> 标准库提供二个对常见字符类型的特化：
> 
> <table style="width:500px;"> 
>  <tbody> 
>   <tr> 
>    <td>类型</td> 
>    <td>类型</td> 
>   </tr> 
>   <tr> 
>    <td>ostream</td> 
>    <td>basic_ostream&lt;char&gt;</td> 
>   </tr> 
>   <tr> 
>    <td>wostream</td> 
>    <td>basic_ostream&lt;wchar_t&gt;</td> 
>   </tr> 
>  </tbody> 
> </table>
> 
> 标准库提供六个全局 basic\_ostream 对象：
> 
> <table style="width:500px;"> 
>  <tbody> 
>   <tr> 
>    <td> <p><a href="https://zh.cppreference.com/w/cpp/io/cout" rel="nofollow">cout、wcout</a></p> </td> 
>    <td>写入到标准 C 输出流&nbsp;<a href="https://zh.cppreference.com/w/cpp/io/c" rel="nofollow">stdout</a></td> 
>   </tr> 
>   <tr> 
>    <td> <p><a href="https://zh.cppreference.com/w/cpp/io/cerr" rel="nofollow">cerr、wcerr</a></p> </td> 
>    <td>写入到标准 C 错误流&nbsp;<a href="https://zh.cppreference.com/w/cpp/io/c" rel="nofollow">stderr</a>， 无缓冲</td> 
>   </tr> 
>   <tr> 
>    <td> <p><a href="https://zh.cppreference.com/w/cpp/io/clog" rel="nofollow">clog、wclog</a></p> </td> 
>    <td>写入到标准 C 错误流&nbsp;<a href="https://zh.cppreference.com/w/cpp/io/c" rel="nofollow">stderr</a></td> 
>   </tr> 
>  </tbody> 
> </table>

# 成员函数 #

<table style="width:500px;"> 
 <tbody> 
  <tr> 
   <td> <p><a href="https://zh.cppreference.com/w/cpp/io/basic_ostream/basic_ostream" rel="nofollow">(构造函数)</a></p> </td> 
   <td>&nbsp;</td> 
  </tr> 
  <tr> 
   <td> <p><a href="https://zh.cppreference.com/w/cpp/io/basic_ostream/~basic_ostream" rel="nofollow">(析构函数)</a>&nbsp;[虚]</p> </td> 
   <td>析构对象</td> 
  </tr> 
  <tr> 
   <td> <p><a href="https://zh.cppreference.com/w/cpp/io/basic_ostream/operator%3D" rel="nofollow">operator=</a></p> <p>(C++11)</p> </td> 
   <td>从另一&nbsp;<code>basic_ostream</code>&nbsp;移动赋值</td> 
  </tr> 
 </tbody> 
</table>

## std::basic\_ostream::basic\_ostream ##

>     explicit basic_ostream( std::basic_streambuf<CharT, Traits>* sb );   (1)	
>     
>     protected:
>     basic_ostream( const basic_ostream& rhs ) = delete;     (2)	(C++11 起)
>     
>     protected:
>     basic_ostream( basic_ostream&& rhs );                   (3)	(C++11 起)
> 
> **构造对象**
> 
> 1) 构造 `basic_ostream` 对象，通过调用 basic\_ios::init(sb) 赋初值给基类。
> 
> 2) 复制构造函数被删除。输出流不可复制。
> 
> 3) 移动构造函数用 `basic_ios<CharT, Traits>::move(rhs)` 从 `rhs` 移动所有 basic\_ios ，除了 `rdbuf()` 到 `*this` 中。此移动构造函数受保护：它为可移动输出流类 [std::basic\_ofstream][std_basic_ofstream] 和 [std::basic\_ostringstream][std_basic_ostringstream] 的移动构造函数所调用，它们知道如何正确地移动关联的流缓冲。
> 
> **参数**
> 
> <table> 
>  <tbody> 
>   <tr> 
>    <td>sb</td> 
>    <td>-</td> 
>    <td>用作输出序列的流缓冲</td> 
>   </tr> 
>   <tr> 
>    <td>rhs</td> 
>    <td>-</td> 
>    <td>初始化来源的 basic_ostream</td> 
>   </tr> 
>  </tbody> 
> </table>

#include <iostream>
    
    int main()
    {
        // 错误：复制构造函数被删除
        // std::ostream myout(std::cout);
    
        // OK ：与 cout 共享缓冲
        std::ostream myout(std::cout.rdbuf());
    
        // 错误：移动构造函数受保护
      //  std::ostream s2(std::move(std::ostringstream() << 7.1));
    
    
    }

// ostream constructor
    #include <iostream>     // std::cout, std::ostream, std::ios
    #include <fstream>      // std::filebuf
    
    int main()
    {
        // 使用了 文件缓冲区 对象打开文件test.txt。然后将缓冲区作为参数传递给流构造函数，将其与流关联。
        std::filebuf  fd;
        fd.open("test.txt", std::ios::out);
    
        std::ostream  os(&fd);
        os << "test sentence\n";
        fd.close();
    }

## std::basic\_ostream::~basic\_ostream ##

>     virtual ~basic_ostream();
> 
> 销毁basic\_ostream对象。此析构函数不进行底层流缓冲（ `rdbuf()` ）上的任何操作

#include <sstream>
    #include <iostream>
    void add_words(std::streambuf* p)
    {
        std::ostream buf(p); // buf 与 s 共享缓冲
        buf << " is the answer";
    } // 调用 buf 的析构函数。 p 保持不受影响。
    int main()
    {
        std::ostringstream s;
        s << 42;
        add_words(s.rdbuf());
        s << ".";
        std::cout << s.str() << '\n';
    }

![20210304143012518.png][]

<table style="width:500px;"> 
 <tbody> 
  <tr> 
   <td colspan="2"> <p>&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;&nbsp; 有格式输出</p> </td> 
  </tr> 
  <tr> 
   <td> <p><a href="https://zh.cppreference.com/w/cpp/io/basic_ostream/operator_ltlt" rel="nofollow">operator&lt;&lt;</a></p> </td> 
   <td>插入带格式数据</td> 
  </tr> 
  <tr> 
   <td colspan="2">&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;无格式输出</td> 
  </tr> 
  <tr> 
   <td> <p><a href="https://zh.cppreference.com/w/cpp/io/basic_ostream/put" rel="nofollow">put</a></p> </td> 
   <td>插入字符</td> 
  </tr> 
  <tr> 
   <td> <p><a href="https://zh.cppreference.com/w/cpp/io/basic_ostream/write" rel="nofollow">write</a></p> </td> 
   <td>插入字符块</td> 
  </tr> 
 </tbody> 
</table>

## std::basic\_ostream::operator << ##

>     basic_ostream& operator<<( short value );
>     basic_ostream& operator<<( unsigned short value );          (1)	
>     
>     basic_ostream& operator<<( int value );
>     basic_ostream& operator<<( unsigned int value );            (2)	
>     
>     basic_ostream& operator<<( long value );
>     basic_ostream& operator<<( unsigned long value );           (3)	
>     
>     basic_ostream& operator<<( long long value );
>     basic_ostream& operator<<( unsigned long long value );      (4)	(C++11 起)
>     
>     basic_ostream& operator<<( float value );
>     basic_ostream& operator<<( double value );
>     basic_ostream& operator<<( long double value );             (5)	
>     
>     basic_ostream& operator<<( bool value );                    (6)	
>     
>     basic_ostream& operator<<( const void* value );             (7)	
>     
>     basic_ostream& operator<<( std::nullptr_t );                             (8)	(C++17 起)
>     
>     basic_ostream& operator<<( std::basic_streambuf<CharT, Traits>* sb);     (9)	
>     
>     basic_ostream& operator<<(
>         std::ios_base& (*func)(std::ios_base&) );                                  (10)                       	
>     basic_ostream& operator<<(
>         std::basic_ios<CharT,Traits>& (*func)(std::basic_ios<CharT,Traits>&) );    (11)	
>     
>     basic_ostream& operator<<(
>         std::basic_ostream<CharT,Traits>& (*func)(std::basic_ostream<CharT,Traits>&) );
> 
> 插入数据到流
> 
> **参数**
> 
> <table> 
>  <tbody> 
>   <tr> 
>    <td>value</td> 
>    <td>-</td> 
>    <td>要插入的整数、浮点、布尔或指针值</td> 
>   </tr> 
>   <tr> 
>    <td>func</td> 
>    <td>-</td> 
>    <td>要调用的函数</td> 
>   </tr> 
>   <tr> 
>    <td>sb</td> 
>    <td>-</td> 
>    <td>指向要自之读取数据的 streambuffer 的指针</td> 
>   </tr> 
>  </tbody> 
> </table>

#include <iostream>
    #include <iomanip>
    #include <sstream>
    int main()
    {
        std::istringstream input(" \"Some text.\" ");
        volatile int n = 42;
        double f = 3.14;
        bool b = true;
        std::cout << n   // int 重载
                  << ' ' // 非成员重载
                  << std::boolalpha << b // bool 重载
                  << " " // non-member overload
                  << std::fixed << f // double 重载
                  << input.rdbuf() // streambuf 重载
                  << &n // bool 重载： volatile int* 不转换成 const void*
                  << std::endl; // 函数重载
    }

![20210304145240194.png][]

// example on insertion
    #include <iostream>     // std::cout, std::right, std::endl
    #include <iomanip>      // std::setw
    
    int main () {
      int val = 65;
    
      std::cout << std::right;       // right-adjusted (manipulator)
      std::cout << std::setw(10);    // set width (extended manipulator)
    
      std::cout << val << std::endl; // multiple insertions
    
      return 0;
    }

![20210304145312895.png][]

## std::basic\_ostream::put ##

>     basic_ostream& put( char_type ch );
> 
> 表现为[无格式输出函数 (UnformattedOutputFunction) ][_UnformattedOutputFunction_]。构造并检查sentry对象后，写字符ch到输出流
> 
> **参数**
> 
> <table> 
>  <tbody> 
>   <tr> 
>    <td>ch</td> 
>    <td>要写入的字符</td> 
>   </tr> 
>  </tbody> 
> </table>
> 
> **返回值**
> 
> \*this

#include <fstream>
    #include <iostream>
     
    int main()
    {
        std::cout.put('a'); // 正常用法
        std::cout.put('\n');
     
        std::ofstream s("/does/not/exist/");
        s.clear(); // 假装流是好的
        std::cout << "Unformatted output: ";
        s.put('c'); // 这将设置 badbit ，但非 failbit
        std::cout << " fail=" << bool(s.rdstate() & s.failbit);
        std::cout << " bad=" << s.bad() << '\n';
        s.clear();
        std::cout << "Formatted output:   ";
        s << 'c'; // 这将设置 badbit 和 failbit
        std::cout << " fail=" << bool(s.rdstate() & s.failbit);
        std::cout << " bad=" << s.bad() << '\n';
    }

![20210304201015263.png][]

## std::basic\_ostream::write ##

>     basic_ostream& write( const char_type* s, std::streamsize count );
> 
> 表现为[无格式输出函数 (UnformattedOutputFunction) ][_UnformattedOutputFunction_]。构造并检查 sentry 对象后，输出来自首元素为 `s` 所指向的字符数组中的相继位置的字符。插入字符到输出序列，直至出现下列条件之一：
> 
>  *  准确插入了 `count` 个字符
>  *  插入到输出序列失败（该情况下调用 setstate(badbit) ）
> 
> ### 参数 ###
> 
> <table> 
>  <tbody> 
>   <tr> 
>    <td>s</td> 
>    <td>指向要写入的字符串的指针</td> 
>   </tr> 
>   <tr> 
>    <td>count</td> 
>    <td>要写入的字符数</td> 
>   </tr> 
>  </tbody> 
> </table>

#include <iostream>
    
    int main()
    {
        int n = 0x41424344;
        std::cout.write(reinterpret_cast<char*>(&n), sizeof n) << '\n';
    
        char c[]="This is sample text.";
        std::cout.write(c, 4).write("!\n", 2);
    
    }

![20210304201622567.png][]

<table style="width:500px;"> 
 <tbody> 
  <tr> 
   <td colspan="2">&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;寻位</td> 
  </tr> 
  <tr> 
   <td> <p><a href="https://zh.cppreference.com/w/cpp/io/basic_ostream/tellp" rel="nofollow">tellp</a></p> </td> 
   <td>返回输出位置指示器</td> 
  </tr> 
  <tr> 
   <td><a href="https://zh.cppreference.com/w/cpp/io/basic_ostream/seekp" rel="nofollow">seekp</a></td> 
   <td>设置输出位置指示器</td> 
  </tr> 
 </tbody> 
</table>

## std::basic\_ostream::tellp ##

>     pos_type tellp();
> 
> 作用：
> 
>  *  返回当前关联的streambuf对象的输出位置指示器
>  *  若 fail()==true ，则返回 pos\_type(-1) 。否则，返回 rdbuf()->pubseekoff(0, [std::ios\_base::cur][std_ios_base_cur], [std::ios\_base::out][std_ios_base_out]) 。

#include <iostream>
    #include <sstream>
    int main()
    {
        std::ostringstream s;
        std::cout << s.tellp() << '\n';
        s << 'h';
        std::cout << s.tellp() << '\n';
        s << "ello, world ";
        std::cout << s.tellp() << '\n';
        s << 3.14 << '\n';
        std::cout << s.tellp() << '\n' << s.str();
    }

![20210304202209440.png][]

## std::basic\_ostream::seekp ##

>     basic_ostream& seekp( pos_type pos );                                     (1)	
>     basic_ostream& seekp( off_type off, std::ios_base::seekdir dir );         (2)
> 
> 设置当前关联streambuf对象的输出位置指示器
> 
>  *  1) 通过调用 rdbuf()->pubseekpos(pos, [std::ios\_base::out][std_ios_base_out]) ，设置输出位置指示器为绝对（相对于文件起始）值 `pos` 。若调用返回 (pos\_type)-1 ，则执行 setstate(failbit) 。
>  *  2) 通过调用 rdbuf()->pubseekoff(off, dir, [std::ios\_base::out][std_ios_base_out]) 设置输出位置指示器为相对于 `dir` 的偏移 `off` 。若调用返回 (pos\_type)-1 ，则执行 setstate(failbit) 。

#include <sstream>
    #include <iostream>
     
    int main()
    {
        std::ostringstream os("hello, world");
        os.seekp(7);
        os << 'W';
        os.seekp(0, std::ios_base::end);
        os << '!';
        os.seekp(0);
        os << 'H';
        std::cout << os.str() << '\n';
    }

![20210304202620572.png][]

<table style="width:500px;"> 
 <tbody> 
  <tr> 
   <td colspan="2">&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;其他</td> 
  </tr> 
  <tr> 
   <td> <p><a href="https://zh.cppreference.com/w/cpp/io/basic_ostream/flush" rel="nofollow">flush</a></p> </td> 
   <td>与底层存储设备同步</td> 
  </tr> 
  <tr> 
   <td> <p><a href="https://zh.cppreference.com/w/cpp/io/basic_ostream/swap" rel="nofollow">swap</a></p> <p>(C++11)</p> </td> 
   <td>交换流对象，除了关联缓冲区</td> 
  </tr> 
 </tbody> 
</table>

## std::basic\_ostream::flush ##

>     basic_ostream& flush();
> 
> 写入未提交更改到底层输出序列。
> 
> 若 rdbuf() 为空指针，则不做任何事。
> 
> 否则，表现为[无格式输出函数 (UnformattedOutputFunction) ][_UnformattedOutputFunction_](C++11 起)。构造并检查 sentry 对象后，调用 rdbuf()->pubsync() 。若该调用返回 -1 ，则调用 setstate(badbit)

#include <thread>
    #include <iostream>
    #include <chrono>
    
    void f(){
        std::cout << "Output from thread ...";
        std::this_thread::sleep_for(std::chrono::seconds(2));
        std::cout << ".... thread calls flush\n";
        std::cout.flush();
    
    }
    
    int main()
    {
        std::thread t1(f);
        std::this_thread::sleep_for(std::chrono::seconds(1));
        std::clog << "Output from main\n";
        t1.join();
    }

![20210304203058696.png][]

##  ##

参考：

*  [http://www.cplusplus.com/reference/ostream/basic\_ostream/?kw=basic\_ostream][http_www.cplusplus.com_reference_ostream_basic_ostream_kw_basic_ostream]

[std_basic_streambuf]: https://zh.cppreference.com/w/cpp/io/basic_streambuf
[ostream]: https://zh.cppreference.com/w/cpp/header/ostream
[20210304135036530.png]: /images/20220518/592b063e8c094997bd33106fc7b47a58.png
[std_basic_ofstream]: https://zh.cppreference.com/w/cpp/io/basic_ofstream
[std_basic_ostringstream]: https://zh.cppreference.com/w/cpp/io/basic_ostringstream
[20210304143012518.png]: /images/20220518/02f5afab0e8c4abfa6aa15d8e2b0e112.png
[20210304145240194.png]: /images/20220518/c77e249b0b1248dca1ad304c7109d266.png
[20210304145312895.png]: /images/20220518/4c8f04d582d14cdabff66f8874b762ca.png
[_UnformattedOutputFunction_]: https://zh.cppreference.com/w/cpp/named_req/UnformattedOutputFunction
[20210304201015263.png]: /images/20220518/cee423a6f85d48c0bb6fb4227938af21.png
[20210304201622567.png]: /images/20220518/b2ee1aa97f5a48c3bdf71a90a2ca54b5.png
[std_ios_base_cur]: http://zh.cppreference.com/w/cpp/io/ios_base/seekdir
[std_ios_base_out]: http://zh.cppreference.com/w/cpp/io/ios_base/openmode
[20210304202209440.png]: /images/20220518/7a4f7b86051d47d6a8ee832938bc74c8.png
[20210304202620572.png]: /images/20220518/77b800336ee74b749285dfaf330580f5.png
[20210304203058696.png]: /images/20220518/a9931a4f6a6b4eeeb65d0be7bf3ef141.png
[http_www.cplusplus.com_reference_ostream_basic_ostream_kw_basic_ostream]: http://www.cplusplus.com/reference/ostream/basic_ostream/?kw=basic_ostream