c++文件流基本用法(fstream, ifstream, ostream)

前言：
c++的文件流处理其实很简单，前提是你能够理解它。文件流本质是利用了一个buffer中间层。有点类似标准输出和标准输入一样。

c++ IO的设计保证IO效率，同时又兼顾封装性和易用性。本文将会讲述c++文件流的用法。

有错误和疏漏的地方，欢迎批评指证。

需要包含的头文件： <fstream> 

名字空间: std

也可以试用<fstream.h>

fstream提供了三个类，用来实现c++对文件的操作。（文件的创建，读写）。
ifstream -- 从已有的文件读

ofstream -- 向文件写内容

fstream - 打开文件供读写

支持的文件类型

实际上，文件类型可以分为两种： 文本文件和二进制文件.

文本文件保存的是可读的字符， 而二进制文件保存的只是二进制数据。利用二进制模式，你可以操作图像等文件。用文本模式，你只能读写文本文件。否则会报错。

 

例一: 写文件

声明一个ostream变量

1. 调用open方法,使其与一个文件关联
2. 写文件
3. 调用close方法.

<ol><li class="li1"><div class="de1"><span class="co2">#include <fstream.h></span></div></li><li class="li1"><div class="de1"> </div></li><li class="li1"><div class="de1"><span class="kw4">void</span> main</div></li><li class="li1"><div class="de1"><span class="br0">{</span></div></li><li class="li2"><div class="de2">ofstream <span class="kw4">file</span>;</div></li><li class="li1"><div class="de1"> </div></li><li class="li1"><div class="de1"><span class="kw4">file</span>.<span class="me1">open</span><span class="br0">(</span><span class="st0">"file.txt"</span><span class="br0">)</span>;</div></li><li class="li1"><div class="de1"> </div></li><li class="li1"><div class="de1">file<<<span class="st0">"Hello file<span class="es0">/n</span>"</span><<<span class="nu0">75</span>;</div></li><li class="li2"><div class="de2"> </div></li><li class="li1"><div class="de1"><span class="kw4">file</span>.<span class="me1">close</span><span class="br0">(</span><span class="br0">)</span>;</div></li><li class="li1"><div class="de1"><span class="br0">}</span></div></li></ol>

可以像试用cout一样试用操作符<<向文件写内容.
Usages:

<ol><li class="li1"><div class="de1"> </div></li><li class="li1"><div class="de1">file<<<span class="st0">"string<span class="es0">/n</span>"</span>;</div></li><li class="li1"><div class="de1"><span class="kw4">file</span>.<span class="me1">put</span><span class="br0">(</span><span class="st0">'c'</span><span class="br0">)</span>;</div></li></ol>

例二:  读文件

1. 声明一个ifstream变量.

2. 打开文件.

3. 从文件读数据

4. 关闭文件.

<ol><li class="li1"><div class="de1"><span class="co2">#include <fstream.h></span></div></li><li class="li1"><div class="de1"> </div></li><li class="li1"><div class="de1"><span class="kw4">void</span> main</div></li><li class="li1"><div class="de1"><span class="br0">{</span></div></li><li class="li2"><div class="de2">ifstream <span class="kw4">file</span>;</div></li><li class="li1"><div class="de1"><span class="kw4">char</span> output<span class="br0">[</span><span class="nu0">100</span><span class="br0">]</span>;</div></li><li class="li1"><div class="de1"><span class="kw4">int</span> x;</div></li><li class="li1"><div class="de1"> </div></li><li class="li1"><div class="de1"><span class="kw4">file</span>.<span class="me1">open</span><span class="br0">(</span><span class="st0">"file.txt"</span><span class="br0">)</span>;</div></li><li class="li2"><div class="de2"> </div></li><li class="li1"><div class="de1">file>>output;</div></li><li class="li1"><div class="de1">cout<<output;</div></li><li class="li1"><div class="de1">file>>x;</div></li><li class="li1"><div class="de1">cout<<x;</div></li><li class="li2"><div class="de2"> </div></li><li class="li1"><div class="de1"><span class="kw4">file</span>.<span class="me1">close</span><span class="br0">(</span><span class="br0">)</span>;</div></li><li class="li1"><div class="de1"><span class="br0">}</span></div><div class="de1"></div><div class="de1">同样的，你也可以像cin一样使用>>来操作文件。或者是调用成员函数</div><div class="de1">Usages:</div></li></ol>

<ol><li class="li1"><div class="de1"> </div></li><li class="li1"><div class="de1">file>>char *;</div></li><li class="li1"><div class="de1">file>>char;</div></li><li class="li2"><div class="de2"><span class="kw4">file</span>.<span class="me1">get</span><span class="br0">(</span><span class="kw4">char</span><span class="br0">)</span>;</div></li><li class="li1"><div class="de1"><span class="kw4">file</span>.<span class="me1">get</span><span class="br0">(</span><span class="kw4">char</span> *,<span class="kw4">int</span><span class="br0">)</span>;</div></li><li class="li1"><div class="de1"><span class="kw4">file</span>.<span class="me1">getline</span><span class="br0">(</span><span class="kw4">char</span> *,<span class="kw4">int</span> sz<span class="br0">)</span>;</div></li><li class="li1"><div class="de1"><span class="kw4">file</span>.<span class="me1">getline</span><span class="br0">(</span><span class="kw4">char</span> *,<span class="kw4">int</span> sz,<span class="kw4">char</span> eol<span class="br0">);</span></div></li></ol>

1.同样的，你也可以使用构造函数开打开一个文件、你只要把文件名作为构造函数的

第一个参数就可以了。

<ol><li class="li1"><div class="de1">ofstream <span class="kw4">file</span><span class="br0">(</span><span class="st0">"fl.txt"</span><span class="br0">)</span>;</div></li><li class="li1"><div class="de1">ifstream <span class="kw4">file</span><span class="br0">(</span><span class="st0">"fl.txt"</span><span class="br0">)</span>;</div></li></ol>

上面所讲的ofstream和ifstream只能进行读或是写，而fstream则同时提供读写的功能。
void main()

<ol><li class="li1"><div class="de1"><span class="br0">{</span></div></li><li class="li1"><div class="de1">fstream <span class="kw4">file</span>;</div></li><li class="li1"><div class="de1"> </div></li><li class="li2"><div class="de2"><span class="kw4">file</span>.<span class="me1">open</span><span class="br0">(</span><span class="st0">"file.ext"</span>,iso::<span class="me2">in</span>|ios::<span class="me2">out</span><span class="br0">)</span></div></li><li class="li1"><div class="de1"> </div></li><li class="li1"><div class="de1"><span class="co1">//do an input or output here</span></div></li><li class="li1"><div class="de1"> </div></li><li class="li1"><div class="de1"><span class="kw4">file</span>.<span class="me1">close</span><span class="br0">(</span><span class="br0">)</span>;</div></li><li class="li2"><div class="de2"><span class="br0">}</span></div></li></ol>

open函数的参数定义了文件的打开模式。总共有如下模式

<ol><li class="li1"><div class="de1">属性列表</div></li><li class="li1"><div class="de1"> </div></li><li class="li1"><div class="de1">ios::<span class="me2">in</span> 读</div></li><li class="li1"><div class="de1">ios::<span class="me2">out</span>写</div></li><li class="li2"><div class="de2">ios::<span class="me2">app</span>从文件末尾开始写</div></li><li class="li1"><div class="de1"><span class="me1">ios</span>::<span class="me2">binary       二进制模式</span></div></li><li class="li1"><div class="de1">ios::<span class="me2">nocreate</span>打开一个文件时，如果文件不存在，不创建文件。</div></li><li class="li1"><div class="de1">ios::<span class="me2">noreplace</span>打开一个文件时，如果文件不存在，创建该文件</div></li><li class="li1"><div class="de1">ios::<span class="me2">trunc</span>打开一个文件，然后清空内容</div></li><li class="li2"><div class="de2">ios::<span class="me2">ate</span>打开一个文件时，将位置移动到文件尾</div></li></ol>

Notes

• 默认模式是文本
• 默认如果文件不存在，那么创建一个新的
• 多种模式可以混合，用|(按位或)
• 文件的byte索引从0开始。（就像数组一样）

我们也可以调用read函数和write函数来读写文件。

 

文件指针位置在c++中的用法：

<ol><li class="li1"><div class="de1">ios::<span class="me2">beg</span>文件头</div></li><li class="li2"><div class="de2">ios::<span class="me2">end</span>文件尾</div></li><li class="li1"><div class="de1">ios::<span class="me2">cur</span>当前位置</div><div class="de1">例子：</div></li></ol>

<ol><li class="li1"><div class="de1"><span class="kw4">file</span>.<span class="me1">seekg</span><span class="br0">(</span><span class="nu0">0</span>,ios::<span class="me2">end</span><span class="br0">)</span>;</div></li><li class="li1"><div class="de1"> </div></li><li class="li1"><div class="de1"><span class="kw4">        int</span> fl_sz = <span class="kw4">file</span>.<span class="me1">tellg</span><span class="br0">(</span><span class="br0">)</span>;</div></li><li class="li1"><div class="de1"></div></li><li class="li1"><div class="de1"><span class="kw4">file</span>.<span class="me1">seekg</span><span class="br0">(</span><span class="nu0">0</span>,ios::<span class="me2">beg</span><span class="br0">)</span>;</div></li></ol>

常用的错误判断方法:

<ol><li class="li1"><div class="de1">good<span class="br0">(</span><span class="br0">)</span>如果文件打开成功</div></li><li class="li1"><div class="de1">bad<span class="br0">(</span><span class="br0">)</span>打开文件时发生错误</div></li><li class="li2"><div class="de2"><span class="kw2">eof</span><span class="br0">(</span><span class="br0">)</span>到达文件尾</div><div class="de2">例子：</div></li></ol>

<ol><li class="li1"><div class="de1"><span class="kw4">char</span> ch;</div></li><li class="li1"><div class="de1">ifstream <span class="kw4">file</span><span class="br0">(</span><span class="st0">"kool.cpp"</span>,ios::<span class="me2">in</span>|ios::<span class="me2">out</span><span class="br0">)</span>;</div></li><li class="li1"><div class="de1"> </div></li><li class="li1"><div class="de1"><span class="kw1">if</span><span class="br0">(</span><span class="kw4">file</span>.<span class="me1">good</span><span class="br0">(</span><span class="br0">)</span><span class="br0">)</span> cout<<<span class="st0">"The file has been opened without problems;</span></div></li><li class="li2"><div class="de2"><span class="st0">else cout<<"</span>An Error has happend on opening the <span class="kw4">file</span>;</div></li><li class="li1"><div class="de1"> </div></li><li class="li1"><div class="de1"><span class="kw1">while</span><span class="br0">(</span>!<span class="kw4">file</span>.<span class="kw2">eof</span><span class="br0">(</span><span class="br0">)</span><span class="br0">)</span></div></li><li class="li1"><div class="de1"><span class="br0">{</span></div></li><li class="li1"><div class="de1">file>>ch;</div></li><li class="li2"><div class="de2">cout<<ch;</div></li><li class="li1"><div class="de1"><span class="br0">}</span></div></li></ol>