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C++笔记2——标准输入输出库 | 雅乐网 </title>
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<ul id="content-index-contents"><li class="content-index-level-1"><a href="http://www.yalewoo.com/cpp_notes_2_input_and_output_library.html#一、iostream" title="一、iostream"><em>1</em><span>一、iostream</span></a><ul class="children"><li class="content-index-level-2"><a href="http://www.yalewoo.com/cpp_notes_2_input_and_output_library.html#向流写入数据（输出）" title="向流写入数据（输出）"><em>1.1</em><span>向流写入数据（输出）</span></a></li><li class="content-index-level-2"><a href="http://www.yalewoo.com/cpp_notes_2_input_and_output_library.html#命名空间" title="命名空间"><em>1.2</em><span>命名空间</span></a></li><li class="content-index-level-2"><a href="http://www.yalewoo.com/cpp_notes_2_input_and_output_library.html#从流读取数据（输入）" title="从流读取数据（输入）"><em>1.3</em><span>从流读取数据（输入）</span></a></li><li class="content-index-level-2"><a href="http://www.yalewoo.com/cpp_notes_2_input_and_output_library.html#c中的输入输出函数" title="c中的输入输出函数"><em>1.4</em><span>c中的输入输出函数</span></a></li></ul></li></ul></div>
<h3 id="一、iostream">一、iostream</h3>
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<p>iostream库又叫做标准输入输出库（Standard Input / Output Streams Library），它包含两个基础类型：istream和ostream，分别表示输入流和输出流。其中定义了4个IO对象：</p>
<p>cin是istream对象，称为标准输入（standard input）</p>
<p>cout是ostream对象，称为标准输出（standard output）</p>
<p>另外还有两个ostream对象：cerr（标准错误）和clog（用来输出运行时的日志信息）</p>
<p>一般系统把程序运行窗口和这些对象关联，读取cin时就会从窗口读取，向cout、cerr和clog写数据时，会写到窗口。</p>
<p>要使用iostream库，首先要包含该头文件</p>
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#include <iostream></textarea></div>
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<div class="crayon-nums-content" style="font-size: 20px !important; line-height: 30px !important;"><div class="crayon-num" data-line="crayon-5a77f1c62ea27685691437-1">1</div></div>
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<td class="crayon-code"><div class="crayon-pre" style="font-size: 20px !important; line-height: 30px !important; -moz-tab-size:4; -o-tab-size:4; -webkit-tab-size:4; tab-size:4;"><div class="crayon-line" id="crayon-5a77f1c62ea27685691437-1"><span class="crayon-p">#include <iostream></span></div></div></td>
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</div><p>但是，之后还不能直接使用cout，这是因为c++中引入了命名空间（namespace）的概念，它可以帮助避免一些命名冲突的问题。标准库中定义的名字都在命名空间std中，当使用时需要通过作用域运算符（::）来指出。这样使用cout就应该用std::cout。</p>
<h4 id="向流写入数据（输出）">向流写入数据（输出）</h4>
<p>一个输出hello world的例子如下：</p>
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#include <iostream>
int main()
{
std::cout << "hello world" << std::endl;
return 0;
}</textarea></div>
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<div class="crayon-nums-content" style="font-size: 20px !important; line-height: 30px !important;"><div class="crayon-num" data-line="crayon-5a77f1c62ea33727782734-1">1</div><div class="crayon-num" data-line="crayon-5a77f1c62ea33727782734-2">2</div><div class="crayon-num" data-line="crayon-5a77f1c62ea33727782734-3">3</div><div class="crayon-num" data-line="crayon-5a77f1c62ea33727782734-4">4</div><div class="crayon-num" data-line="crayon-5a77f1c62ea33727782734-5">5</div><div class="crayon-num" data-line="crayon-5a77f1c62ea33727782734-6">6</div><div class="crayon-num" data-line="crayon-5a77f1c62ea33727782734-7">7</div></div>
</td>
<td class="crayon-code"><div class="crayon-pre" style="font-size: 20px !important; line-height: 30px !important; -moz-tab-size:4; -o-tab-size:4; -webkit-tab-size:4; tab-size:4;"><div class="crayon-line" id="crayon-5a77f1c62ea33727782734-1"><span class="crayon-p">#include <iostream></span></div><div class="crayon-line" id="crayon-5a77f1c62ea33727782734-2"> </div><div class="crayon-line" id="crayon-5a77f1c62ea33727782734-3"><span class="crayon-t">int</span><span class="crayon-h"> </span><span class="crayon-e">main</span><span class="crayon-sy">(</span><span class="crayon-sy">)</span></div><div class="crayon-line" id="crayon-5a77f1c62ea33727782734-4"><span class="crayon-sy">{</span></div><div class="crayon-line" id="crayon-5a77f1c62ea33727782734-5"><span class="crayon-h"> </span><span class="crayon-v">std</span><span class="crayon-o">::</span><span class="crayon-r">cout</span><span class="crayon-h"> </span><span class="crayon-o"><<</span><span class="crayon-h"> </span><span class="crayon-s">"hello world"</span><span class="crayon-h"> </span><span class="crayon-o"><<</span><span class="crayon-h"> </span><span class="crayon-v">std</span><span class="crayon-o">::</span><span class="crayon-v">endl</span><span class="crayon-sy">;</span></div><div class="crayon-line" id="crayon-5a77f1c62ea33727782734-6"><span class="crayon-h"> </span><span class="crayon-st">return</span><span class="crayon-h"> </span><span class="crayon-cn">0</span><span class="crayon-sy">;</span></div><div class="crayon-line" id="crayon-5a77f1c62ea33727782734-7"><span class="crayon-sy">}</span></div></div></td>
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</div><p>其中endl是一个特殊的操纵符，它的效果是结束当前行，并将缓冲区的内容写入到设备。</p>
<p>左移运算符<<在c++中又称为输出运算符，它左侧是ostream对象，右侧就是要输出的值，作用就是在输出对象上打印消息。由于它的运算结果正好是左侧的那个ostream对象，因此可以连用：</p>
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std::cout << "hello" << "world";</textarea></div>
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<div class="crayon-nums-content" style="font-size: 20px !important; line-height: 30px !important;"><div class="crayon-num" data-line="crayon-5a77f1c62ea3a306534074-1">1</div></div>
</td>
<td class="crayon-code"><div class="crayon-pre" style="font-size: 20px !important; line-height: 30px !important; -moz-tab-size:4; -o-tab-size:4; -webkit-tab-size:4; tab-size:4;"><div class="crayon-line" id="crayon-5a77f1c62ea3a306534074-1"><span class="crayon-v">std</span><span class="crayon-o">::</span><span class="crayon-v">cout</span><span class="crayon-h"> </span><span class="crayon-o"><<</span><span class="crayon-h"> </span><span class="crayon-s">"hello"</span><span class="crayon-h"> </span><span class="crayon-o"><<</span><span class="crayon-h"> </span><span class="crayon-s">"world"</span><span class="crayon-sy">;</span></div></div></td>
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</div><p>std::cout << “hello”的结果还是std::cout，上面这句话相当于两句：</p>
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std::cout << "hello";
std::cout << "world";</textarea></div>
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<div class="crayon-nums-content" style="font-size: 20px !important; line-height: 30px !important;"><div class="crayon-num" data-line="crayon-5a77f1c62ea3f510740751-1">1</div><div class="crayon-num" data-line="crayon-5a77f1c62ea3f510740751-2">2</div></div>
</td>
<td class="crayon-code"><div class="crayon-pre" style="font-size: 20px !important; line-height: 30px !important; -moz-tab-size:4; -o-tab-size:4; -webkit-tab-size:4; tab-size:4;"><div class="crayon-line" id="crayon-5a77f1c62ea3f510740751-1"><span class="crayon-v">std</span><span class="crayon-o">::</span><span class="crayon-v">cout</span><span class="crayon-h"> </span><span class="crayon-o"><<</span><span class="crayon-h"> </span><span class="crayon-s">"hello"</span><span class="crayon-sy">;</span></div><div class="crayon-line" id="crayon-5a77f1c62ea3f510740751-2"><span class="crayon-v">std</span><span class="crayon-o">::</span><span class="crayon-v">cout</span><span class="crayon-h"> </span><span class="crayon-o"><<</span><span class="crayon-h"> </span><span class="crayon-s">"world"</span><span class="crayon-sy">;</span></div></div></td>
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</div><p></p>
<h4 id="命名空间">命名空间</h4>
<p>除了每次使用std::使用标准库中的名字，还可以使用using声明来一劳永逸。它的格式如下：</p>
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using namespace::name;</textarea></div>
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<div class="crayon-nums-content" style="font-size: 20px !important; line-height: 30px !important;"><div class="crayon-num" data-line="crayon-5a77f1c62ea45177323062-1">1</div></div>
</td>
<td class="crayon-code"><div class="crayon-pre" style="font-size: 20px !important; line-height: 30px !important; -moz-tab-size:4; -o-tab-size:4; -webkit-tab-size:4; tab-size:4;"><div class="crayon-line" id="crayon-5a77f1c62ea45177323062-1"><span class="crayon-r">using</span><span class="crayon-h"> </span><span class="crayon-t">namespace</span><span class="crayon-o">::</span><span class="crayon-v">name</span><span class="crayon-sy">;</span></div></div></td>
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</div><p>使用之后可以直接访问这个name</p>
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#include <iostream>
using std::cout;
using std::endl;
int main()
{
cout << "hello world" << endl;
return 0;
}</textarea></div>
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<div class="crayon-nums-content" style="font-size: 20px !important; line-height: 30px !important;"><div class="crayon-num" data-line="crayon-5a77f1c62ea4a305764331-1">1</div><div class="crayon-num" data-line="crayon-5a77f1c62ea4a305764331-2">2</div><div class="crayon-num" data-line="crayon-5a77f1c62ea4a305764331-3">3</div><div class="crayon-num" data-line="crayon-5a77f1c62ea4a305764331-4">4</div><div class="crayon-num" data-line="crayon-5a77f1c62ea4a305764331-5">5</div><div class="crayon-num" data-line="crayon-5a77f1c62ea4a305764331-6">6</div><div class="crayon-num" data-line="crayon-5a77f1c62ea4a305764331-7">7</div><div class="crayon-num" data-line="crayon-5a77f1c62ea4a305764331-8">8</div></div>
</td>
<td class="crayon-code"><div class="crayon-pre" style="font-size: 20px !important; line-height: 30px !important; -moz-tab-size:4; -o-tab-size:4; -webkit-tab-size:4; tab-size:4;"><div class="crayon-line" id="crayon-5a77f1c62ea4a305764331-1"><span class="crayon-p">#include <iostream></span></div><div class="crayon-line" id="crayon-5a77f1c62ea4a305764331-2"><span class="crayon-r">using</span><span class="crayon-h"> </span><span class="crayon-v">std</span><span class="crayon-o">::</span><span class="crayon-r">cout</span><span class="crayon-sy">;</span></div><div class="crayon-line" id="crayon-5a77f1c62ea4a305764331-3"><span class="crayon-r">using</span><span class="crayon-h"> </span><span class="crayon-v">std</span><span class="crayon-o">::</span><span class="crayon-v">endl</span><span class="crayon-sy">;</span></div><div class="crayon-line" id="crayon-5a77f1c62ea4a305764331-4"><span class="crayon-t">int</span><span class="crayon-h"> </span><span class="crayon-e">main</span><span class="crayon-sy">(</span><span class="crayon-sy">)</span></div><div class="crayon-line" id="crayon-5a77f1c62ea4a305764331-5"><span class="crayon-sy">{</span></div><div class="crayon-line" id="crayon-5a77f1c62ea4a305764331-6"><span class="crayon-h"> </span><span class="crayon-r">cout</span><span class="crayon-h"> </span><span class="crayon-o"><<</span><span class="crayon-h"> </span><span class="crayon-s">"hello world"</span><span class="crayon-h"> </span><span class="crayon-o"><<</span><span class="crayon-h"> </span><span class="crayon-v">endl</span><span class="crayon-sy">;</span></div><div class="crayon-line" id="crayon-5a77f1c62ea4a305764331-7"><span class="crayon-h"> </span><span class="crayon-st">return</span><span class="crayon-h"> </span><span class="crayon-cn">0</span><span class="crayon-sy">;</span></div><div class="crayon-line" id="crayon-5a77f1c62ea4a305764331-8"><span class="crayon-sy">}</span></div></div></td>
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</div><p>还可以直接使用using指示，引入一个命名空间的所有名字</p>
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#include <iostream>
using namespace std;
int main()
{
cout << "hello world" << endl;
return 0;
}</textarea></div>
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<div class="crayon-nums-content" style="font-size: 20px !important; line-height: 30px !important;"><div class="crayon-num" data-line="crayon-5a77f1c62ea4f332016925-1">1</div><div class="crayon-num" data-line="crayon-5a77f1c62ea4f332016925-2">2</div><div class="crayon-num" data-line="crayon-5a77f1c62ea4f332016925-3">3</div><div class="crayon-num" data-line="crayon-5a77f1c62ea4f332016925-4">4</div><div class="crayon-num" data-line="crayon-5a77f1c62ea4f332016925-5">5</div><div class="crayon-num" data-line="crayon-5a77f1c62ea4f332016925-6">6</div><div class="crayon-num" data-line="crayon-5a77f1c62ea4f332016925-7">7</div></div>
</td>
<td class="crayon-code"><div class="crayon-pre" style="font-size: 20px !important; line-height: 30px !important; -moz-tab-size:4; -o-tab-size:4; -webkit-tab-size:4; tab-size:4;"><div class="crayon-line" id="crayon-5a77f1c62ea4f332016925-1"><span class="crayon-p">#include <iostream></span></div><div class="crayon-line" id="crayon-5a77f1c62ea4f332016925-2"><span class="crayon-r">using</span><span class="crayon-h"> </span><span class="crayon-t">namespace</span><span class="crayon-h"> </span><span class="crayon-v">std</span><span class="crayon-sy">;</span></div><div class="crayon-line" id="crayon-5a77f1c62ea4f332016925-3"><span class="crayon-t">int</span><span class="crayon-h"> </span><span class="crayon-e">main</span><span class="crayon-sy">(</span><span class="crayon-sy">)</span></div><div class="crayon-line" id="crayon-5a77f1c62ea4f332016925-4"><span class="crayon-sy">{</span></div><div class="crayon-line" id="crayon-5a77f1c62ea4f332016925-5"><span class="crayon-h"> </span><span class="crayon-r">cout</span><span class="crayon-h"> </span><span class="crayon-o"><<</span><span class="crayon-h"> </span><span class="crayon-s">"hello world"</span><span class="crayon-h"> </span><span class="crayon-o"><<</span><span class="crayon-h"> </span><span class="crayon-v">endl</span><span class="crayon-sy">;</span></div><div class="crayon-line" id="crayon-5a77f1c62ea4f332016925-6"><span class="crayon-h"> </span><span class="crayon-st">return</span><span class="crayon-h"> </span><span class="crayon-cn">0</span><span class="crayon-sy">;</span></div><div class="crayon-line" id="crayon-5a77f1c62ea4f332016925-7"><span class="crayon-sy">}</span></div></div></td>
</tr>
</table>
</div>
</div><p>使用之后，可以直接使用std下面的所有名字。一般不推荐使用，因为std中有很多名字，可能会和你自己的变量名字冲突。</p>
<h4 id="从流读取数据（输入）">从流读取数据（输入）</h4>
<p>右移运算符>> 也称为输入运算符，它左侧是一个istream对象，右侧是要写入内容的对象。它从istream中读取数据，并写入到右侧的对象中。和<<类似，它的结果也是左侧的istream对象，因此也可以连用</p>
<div id="crayon-5a77f1c62ea55657855097" class="crayon-syntax crayon-theme-classic crayon-font-consolas crayon-os-pc print-yes notranslate" data-settings=" minimize scroll-mouseover disable-anim wrap" style=" margin-top: 12px; margin-bottom: 12px; font-size: 20px !important; line-height: 30px !important;">
<div class="crayon-plain-wrap"><textarea class="crayon-plain print-no" data-settings="dblclick" readonly style="-moz-tab-size:4; -o-tab-size:4; -webkit-tab-size:4; tab-size:4; font-size: 20px !important; line-height: 30px !important;">
#include <iostream>
using namespace std;
int main()
{
int a, b;
cin >> a >> b;
cout << a << "+" << b << "=" << a+b << endl;
return 0;
}</textarea></div>
<div class="crayon-main" style="">
<table class="crayon-table">
<tr class="crayon-row">
<td class="crayon-nums " data-settings="show">
<div class="crayon-nums-content" style="font-size: 20px !important; line-height: 30px !important;"><div class="crayon-num" data-line="crayon-5a77f1c62ea55657855097-1">1</div><div class="crayon-num" data-line="crayon-5a77f1c62ea55657855097-2">2</div><div class="crayon-num" data-line="crayon-5a77f1c62ea55657855097-3">3</div><div class="crayon-num" data-line="crayon-5a77f1c62ea55657855097-4">4</div><div class="crayon-num" data-line="crayon-5a77f1c62ea55657855097-5">5</div><div class="crayon-num" data-line="crayon-5a77f1c62ea55657855097-6">6</div><div class="crayon-num" data-line="crayon-5a77f1c62ea55657855097-7">7</div><div class="crayon-num" data-line="crayon-5a77f1c62ea55657855097-8">8</div><div class="crayon-num" data-line="crayon-5a77f1c62ea55657855097-9">9</div></div>
</td>
<td class="crayon-code"><div class="crayon-pre" style="font-size: 20px !important; line-height: 30px !important; -moz-tab-size:4; -o-tab-size:4; -webkit-tab-size:4; tab-size:4;"><div class="crayon-line" id="crayon-5a77f1c62ea55657855097-1"><span class="crayon-p">#include <iostream></span></div><div class="crayon-line" id="crayon-5a77f1c62ea55657855097-2"><span class="crayon-r">using</span><span class="crayon-h"> </span><span class="crayon-t">namespace</span><span class="crayon-h"> </span><span class="crayon-v">std</span><span class="crayon-sy">;</span></div><div class="crayon-line" id="crayon-5a77f1c62ea55657855097-3"><span class="crayon-t">int</span><span class="crayon-h"> </span><span class="crayon-e">main</span><span class="crayon-sy">(</span><span class="crayon-sy">)</span></div><div class="crayon-line" id="crayon-5a77f1c62ea55657855097-4"><span class="crayon-sy">{</span></div><div class="crayon-line" id="crayon-5a77f1c62ea55657855097-5"><span class="crayon-h"> </span><span class="crayon-t">int</span><span class="crayon-h"> </span><span class="crayon-v">a</span><span class="crayon-sy">,</span><span class="crayon-h"> </span><span class="crayon-v">b</span><span class="crayon-sy">;</span></div><div class="crayon-line" id="crayon-5a77f1c62ea55657855097-6"><span class="crayon-h"> </span><span class="crayon-v">cin</span><span class="crayon-h"> </span><span class="crayon-o">>></span><span class="crayon-h"> </span><span class="crayon-v">a</span><span class="crayon-h"> </span><span class="crayon-o">>></span><span class="crayon-h"> </span><span class="crayon-v">b</span><span class="crayon-sy">;</span></div><div class="crayon-line" id="crayon-5a77f1c62ea55657855097-7"><span class="crayon-h"> </span><span class="crayon-r">cout</span><span class="crayon-h"> </span><span class="crayon-o"><<</span><span class="crayon-h"> </span><span class="crayon-v">a</span><span class="crayon-h"> </span><span class="crayon-o"><<</span><span class="crayon-h"> </span><span class="crayon-s">"+"</span><span class="crayon-h"> </span><span class="crayon-o"><<</span><span class="crayon-h"> </span><span class="crayon-v">b</span><span class="crayon-h"> </span><span class="crayon-o"><<</span><span class="crayon-h"> </span><span class="crayon-s">"="</span><span class="crayon-h"> </span><span class="crayon-o"><<</span><span class="crayon-h"> </span><span class="crayon-v">a</span><span class="crayon-o">+</span><span class="crayon-v">b</span><span class="crayon-h"> </span><span class="crayon-o"><<</span><span class="crayon-h"> </span><span class="crayon-v">endl</span><span class="crayon-sy">;</span></div><div class="crayon-line" id="crayon-5a77f1c62ea55657855097-8"><span class="crayon-h"> </span><span class="crayon-st">return</span><span class="crayon-h"> </span><span class="crayon-cn">0</span><span class="crayon-sy">;</span></div><div class="crayon-line" id="crayon-5a77f1c62ea55657855097-9"><span class="crayon-sy">}</span></div></div></td>
</tr>
</table>
</div>
</div><p>当需要读取的数据个数未知时，可以检测cin对象的值。下面是一个计算求和的例子</p>
<div id="crayon-5a77f1c62ea5b524248822" class="crayon-syntax crayon-theme-classic crayon-font-consolas crayon-os-pc print-yes notranslate" data-settings=" minimize scroll-mouseover disable-anim wrap" style=" margin-top: 12px; margin-bottom: 12px; font-size: 20px !important; line-height: 30px !important;">
<div class="crayon-plain-wrap"><textarea class="crayon-plain print-no" data-settings="dblclick" readonly style="-moz-tab-size:4; -o-tab-size:4; -webkit-tab-size:4; tab-size:4; font-size: 20px !important; line-height: 30px !important;">
#include <iostream>
using namespace std;
int main()
{
int sum = 0, value;
while (cin >> value)
{
sum += value;
}
cout << "sum = " << sum;
return 0;
}</textarea></div>
<div class="crayon-main" style="">
<table class="crayon-table">
<tr class="crayon-row">
<td class="crayon-nums " data-settings="show">
<div class="crayon-nums-content" style="font-size: 20px !important; line-height: 30px !important;"><div class="crayon-num" data-line="crayon-5a77f1c62ea5b524248822-1">1</div><div class="crayon-num" data-line="crayon-5a77f1c62ea5b524248822-2">2</div><div class="crayon-num" data-line="crayon-5a77f1c62ea5b524248822-3">3</div><div class="crayon-num" data-line="crayon-5a77f1c62ea5b524248822-4">4</div><div class="crayon-num" data-line="crayon-5a77f1c62ea5b524248822-5">5</div><div class="crayon-num" data-line="crayon-5a77f1c62ea5b524248822-6">6</div><div class="crayon-num" data-line="crayon-5a77f1c62ea5b524248822-7">7</div><div class="crayon-num" data-line="crayon-5a77f1c62ea5b524248822-8">8</div><div class="crayon-num" data-line="crayon-5a77f1c62ea5b524248822-9">9</div><div class="crayon-num" data-line="crayon-5a77f1c62ea5b524248822-10">10</div><div class="crayon-num" data-line="crayon-5a77f1c62ea5b524248822-11">11</div><div class="crayon-num" data-line="crayon-5a77f1c62ea5b524248822-12">12</div></div>
</td>
<td class="crayon-code"><div class="crayon-pre" style="font-size: 20px !important; line-height: 30px !important; -moz-tab-size:4; -o-tab-size:4; -webkit-tab-size:4; tab-size:4;"><div class="crayon-line" id="crayon-5a77f1c62ea5b524248822-1"><span class="crayon-p">#include <iostream></span></div><div class="crayon-line" id="crayon-5a77f1c62ea5b524248822-2"><span class="crayon-r">using</span><span class="crayon-h"> </span><span class="crayon-t">namespace</span><span class="crayon-h"> </span><span class="crayon-v">std</span><span class="crayon-sy">;</span></div><div class="crayon-line" id="crayon-5a77f1c62ea5b524248822-3"><span class="crayon-t">int</span><span class="crayon-h"> </span><span class="crayon-e">main</span><span class="crayon-sy">(</span><span class="crayon-sy">)</span></div><div class="crayon-line" id="crayon-5a77f1c62ea5b524248822-4"><span class="crayon-sy">{</span></div><div class="crayon-line" id="crayon-5a77f1c62ea5b524248822-5"><span class="crayon-h"> </span><span class="crayon-t">int</span><span class="crayon-h"> </span><span class="crayon-v">sum</span><span class="crayon-h"> </span><span class="crayon-o">=</span><span class="crayon-h"> </span><span class="crayon-cn">0</span><span class="crayon-sy">,</span><span class="crayon-h"> </span><span class="crayon-t">value</span><span class="crayon-sy">;</span></div><div class="crayon-line" id="crayon-5a77f1c62ea5b524248822-6"><span class="crayon-h"> </span><span class="crayon-st">while</span><span class="crayon-h"> </span><span class="crayon-sy">(</span><span class="crayon-v">cin</span><span class="crayon-h"> </span><span class="crayon-o">>></span><span class="crayon-h"> </span><span class="crayon-t">value</span><span class="crayon-sy">)</span></div><div class="crayon-line" id="crayon-5a77f1c62ea5b524248822-7"><span class="crayon-h"> </span><span class="crayon-sy">{</span></div><div class="crayon-line" id="crayon-5a77f1c62ea5b524248822-8"><span class="crayon-h"> </span><span class="crayon-v">sum</span><span class="crayon-h"> </span><span class="crayon-o">+=</span><span class="crayon-h"> </span><span class="crayon-t">value</span><span class="crayon-sy">;</span></div><div class="crayon-line" id="crayon-5a77f1c62ea5b524248822-9"><span class="crayon-h"> </span><span class="crayon-sy">}</span></div><div class="crayon-line" id="crayon-5a77f1c62ea5b524248822-10"><span class="crayon-h"> </span><span class="crayon-r">cout</span><span class="crayon-h"> </span><span class="crayon-o"><<</span><span class="crayon-h"> </span><span class="crayon-s">"sum = "</span><span class="crayon-h"> </span><span class="crayon-o"><<</span><span class="crayon-h"> </span><span class="crayon-v">sum</span><span class="crayon-sy">;</span></div><div class="crayon-line" id="crayon-5a77f1c62ea5b524248822-11"><span class="crayon-h"> </span><span class="crayon-st">return</span><span class="crayon-h"> </span><span class="crayon-cn">0</span><span class="crayon-sy">;</span></div><div class="crayon-line" id="crayon-5a77f1c62ea5b524248822-12"><span class="crayon-sy">}</span></div></div></td>
</tr>
</table>
</div>
</div><p>当遇到文件结束符（end-of-file，windows命令行中可以通过Ctrl+z模拟）或者遇到无效输入（上例中输入不是数字）时，istream对象的状态会变为无效，从而跳出while循环。</p>
<h4 id="c中的输入输出函数">c中的输入输出函数</h4>
<p>c++中也可以使用printf和scanf这些c中的输入输出函数，头文件为cstdio。</p>
<p>c中的大部分头文件都有对应的c++版本，头文件名字为c中的名字去掉.h并在最前面加c。</p>
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