blocks|key|896180|text|为了读取，你应该创建一个ofstream的实例，而不是ofstream的实例。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|896181|ifstream+iusrfile;|code-block|syntax|javascript|896182|您应该在读取模式下打开文件。|896183|iusrfile.open("usrfile.txt",+ifstream::in);|896184|同样，这句话是不正确的。|896185|cout<<iusrfile;|896186|如果您正在尝试打印从文件中读取的数据，则应执行以下操作：|896187|cout<<usr;|896188|您可以阅读更多关于ifstream及其API+here的内容。|offset|length|896189|entityMap|0|LINK|mutability|MUTABLE|url|http://www.cplusplus.com/reference/iostream/ifstream^0|0|0|0|0|0|0|0|0|N|4|0|0^^$0|@$1|2|3|4|5|6|7|14|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|15|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|16|8|@]|9|@]|A|$]]|$1|I|3|J|5|D|7|17|8|@]|9|@]|A|$E|F]]|$1|K|3|L|5|6|7|18|8|@]|9|@]|A|$]]|$1|M|3|N|5|D|7|19|8|@]|9|@]|A|$E|F]]|$1|O|3|P|5|6|7|1A|8|@]|9|@]|A|$]]|$1|Q|3|R|5|D|7|1B|8|@]|9|@]|A|$E|F]]|$1|S|3|T|5|6|7|1C|8|@]|9|@$U|1D|V|1E|1|1F]]|A|$]]|$1|W|3|-4|5|6|7|1G|8|@]|9|@]|A|$]]]|X|$Y|$5|Z|10|11|A|$12|13]]]]

To read you should create an instance of ifsteam and not ofstream.

<pre><code>ifstream iusrfile;
</code></pre>

You should open the file in read mode.

<pre><code>iusrfile.open("usrfile.txt", ifstream::in);
</code></pre>

Also this statement is not correct.

<pre><code>cout&lt;&lt;iusrfile;
</code></pre>

If you are trying to print the data you read from the file you should do:

<pre><code>cout&lt;&lt;usr;
</code></pre>

You can read more about ifstream and its API <a href="http://www.cplusplus.com/reference/iostream/ifstream" rel="nofollow">here</a>

blocks|key|679446|text|所需的头文件：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|679447|#include+<iostream>
#include+<fstream>|code-block|syntax|javascript|679448|声明输入文件流：|679449|ifstream+in("in.txt");|679450|声明输出文件流：|679451|ofstream+out("out.txt");|679452|如果您希望使用变量作为文件名，而不是对其进行硬编码，请使用以下命令：|679453|string+file_name+=+"my_file.txt";
ifstream+in2(file_name.c_str());|679454|从文件读取到变量(假设文件中有两个整型变量)：|679455|int+num1,num2;
in+>>+num1+>>+num2;|679456|或者，从文件中一次读取一行：|679457|string+line;
while(getline(in,line)){
//do+something+with+the+line
}|679458|将变量写回文件：|679459|out+<<+num1+<<+num2;|679460|关闭文件：|679461|in.close();
out.close();|679462|entityMap^0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|1A|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|1B|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|1C|8|@]|9|@]|A|$]]|$1|I|3|J|5|D|7|1D|8|@]|9|@]|A|$E|F]]|$1|K|3|L|5|6|7|1E|8|@]|9|@]|A|$]]|$1|M|3|N|5|D|7|1F|8|@]|9|@]|A|$E|F]]|$1|O|3|P|5|6|7|1G|8|@]|9|@]|A|$]]|$1|Q|3|R|5|D|7|1H|8|@]|9|@]|A|$E|F]]|$1|S|3|T|5|6|7|1I|8|@]|9|@]|A|$]]|$1|U|3|V|5|D|7|1J|8|@]|9|@]|A|$E|F]]|$1|W|3|X|5|6|7|1K|8|@]|9|@]|A|$]]|$1|Y|3|Z|5|D|7|1L|8|@]|9|@]|A|$E|F]]|$1|10|3|11|5|6|7|1M|8|@]|9|@]|A|$]]|$1|12|3|13|5|D|7|1N|8|@]|9|@]|A|$E|F]]|$1|14|3|15|5|6|7|1O|8|@]|9|@]|A|$]]|$1|16|3|17|5|D|7|1P|8|@]|9|@]|A|$E|F]]|$1|18|3|-4|5|6|7|1Q|8|@]|9|@]|A|$]]]|19|$]]

Header files needed:

<pre><code>#include &lt;iostream&gt;
#include &lt;fstream&gt;
</code></pre>

declare input file stream:

<pre><code>ifstream in("in.txt");
</code></pre>

declare output file stream:

<pre><code>ofstream out("out.txt");
</code></pre>

if you want to use variable for a file name, instead of hardcoding it, use this:

<pre><code>string file_name = "my_file.txt";
ifstream in2(file_name.c_str());
</code></pre>

reading from file into variables (assume file has 2 int variables in):

<pre><code>int num1,num2;
in &gt;&gt; num1 &gt;&gt; num2;
</code></pre>

or, reading a line a time from file:

<pre><code>string line;
while(getline(in,line)){
//do something with the line
}
</code></pre>

write variables back to the file:

<pre><code>out &lt;&lt; num1 &lt;&lt; num2;
</code></pre>

close the files:

<pre><code>in.close();
out.close();
</code></pre>

blocks|key|896301|text|文件IO的默认c%2B%2B机制称为流。流可以有三种风格:输入、输出和输入输出。输入流的作用类似于数据源。要从输入流中读取数据，可以使用>>运算符：|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|896302|istream+>>+my_variable;+//This+code+will+read+a+value+from+stream+into+your+variable.|code-block|syntax|javascript|896303|对于不同的类型，Operator+>>的作用也不同。如果在上面的例子中my_variable是一个整数，那么将从流中读取一个数字，如果my_variable是一个字符串，那么将读取一个单词，依此类推。您可以通过编写istream+>>+a+>>+b+>>+c;从流中读取多个值，其中a，b和c将是您的变量。|896304|输出流的作用类似于接收器，您可以将数据写入其中。要将数据写入流，请使用<<运算符。|896305|ostream+<<+my_variable;+//This+code+will+write+a+value+from+your+variable+into+stream.|896306|与输入流一样，您可以通过编写以下代码向流中写入几个值：|896307|ostream+<<+a+<<+b+<<+c;|896308|显然，输入输出流可以同时充当两者。|896309|在您的代码示例中，使用了cout和cin流对象。cout代表控制台输出，cin代表console-input。这些是用于与默认控制台交互的预定义流。|896310|要与文件交互，您需要使用ifstream和ofstream类型。与cin和cout类似，ifstream代表input-file-stream，ofstream代表output-file-stream。|896311|您的代码可能如下所示：|896312|#include+<iostream>
#include+<fstream>

using+namespace+std;

int+start()
{
++++cout+<<+"Welcome...";

++++//+do+fancy+stuff

++++return+0;
}

int+main+()
{
++++string+usreq,+usr,+yn,+usrenter;

++++cout+<<+"Is+this+your+first+time+using+TEST"+<<+endl;
++++cin+>>+yn;
++++if+(yn+==+"y")
++++{
++++++++ifstream+iusrfile;
++++++++ofstream+ousrfile;
++++++++iusrfile.open("usrfile.txt");
++++++++iusrfile+>>+usr;
++++++++cout+<<+iusrfile;+//+I'm+not+sure+what+are+you+trying+to+do+here,+perhaps+print+iusrfile+contents?
++++++++iusrfile.close();
++++++++cout+<<+"Please+type+your+Username.+\n";
++++++++cin+>>+usrenter;
++++++++if+(usrenter+==+usr)
++++++++{
++++++++++++start+();
++++++++}
++++}
++++else
++++{
++++++++cout+<<+"THAT+IS+NOT+A+REGISTERED+USERNAME.";
++++}

++++return+0;
}|896313|要进一步阅读，您可能需要查看c%2B%2B+I/O+reference|896314|entityMap|0|LINK|mutability|MUTABLE|url|http://www.cplusplus.com/reference/iostream/^0|1S|2|0|0|H|2|Z|B|1V|B|2Z|N|0|Z|2|0|0|0|0|0|C|4|H|3|O|4|15|D|0|C|8|L|8|X|3|11|4|18|8|1I|H|20|8|2A|I|0|0|0|E|H|0|0^^$0|@$1|2|3|4|5|6|7|1E|8|@$9|1F|A|1G|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|1H|8|@]|D|@]|E|$I|J]]|$1|K|3|L|5|6|7|1I|8|@$9|1J|A|1K|B|C]|$9|1L|A|1M|B|C]|$9|1N|A|1O|B|C]|$9|1P|A|1Q|B|C]]|D|@]|E|$]]|$1|M|3|N|5|6|7|1R|8|@$9|1S|A|1T|B|C]]|D|@]|E|$]]|$1|O|3|P|5|H|7|1U|8|@]|D|@]|E|$I|J]]|$1|Q|3|R|5|6|7|1V|8|@]|D|@]|E|$]]|$1|S|3|T|5|H|7|1W|8|@]|D|@]|E|$I|J]]|$1|U|3|V|5|6|7|1X|8|@]|D|@]|E|$]]|$1|W|3|X|5|6|7|1Y|8|@$9|1Z|A|20|B|C]|$9|21|A|22|B|C]|$9|23|A|24|B|C]|$9|25|A|26|B|C]]|D|@]|E|$]]|$1|Y|3|Z|5|6|7|27|8|@$9|28|A|29|B|C]|$9|2A|A|2B|B|C]|$9|2C|A|2D|B|C]|$9|2E|A|2F|B|C]|$9|2G|A|2H|B|C]|$9|2I|A|2J|B|C]|$9|2K|A|2L|B|C]|$9|2M|A|2N|B|C]]|D|@]|E|$]]|$1|10|3|11|5|6|7|2O|8|@]|D|@]|E|$]]|$1|12|3|13|5|H|7|2P|8|@]|D|@]|E|$I|J]]|$1|14|3|15|5|6|7|2Q|8|@]|D|@$9|2R|A|2S|1|2T]]|E|$]]|$1|16|3|-4|5|6|7|2U|8|@]|D|@]|E|$]]]|17|$18|$5|19|1A|1B|E|$1C|1D]]]]

Default c++ mechanism for file IO is called streams.
Streams can be of three flavors: input, output and inputoutput.
Input streams act like sources of data. To read data from an input stream you use <code>&gt;&gt;</code> operator:

<pre><code>istream &gt;&gt; my_variable; //This code will read a value from stream into your variable.
</code></pre>

Operator <code>&gt;&gt;</code> acts different for different types. If in the example above <code>my_variable</code> was an int, then a number will be read from the strem, if <code>my_variable</code> was a string, then a word would be read, etc.
You can read more then one value from the stream by writing <code>istream &gt;&gt; a &gt;&gt; b &gt;&gt; c;</code> where a, b and c would be your variables.

Output streams act like sink to which you can write your data. To write your data to a stream, use <code>&lt;&lt;</code> operator.

<pre><code>ostream &lt;&lt; my_variable; //This code will write a value from your variable into stream.
</code></pre>

As with input streams, you can write several values to the stream by writing something like this:

<pre><code>ostream &lt;&lt; a &lt;&lt; b &lt;&lt; c;
</code></pre>

Obviously inputoutput streams can act as both.

In your code sample you use <code>cout</code> and <code>cin</code> stream objects.
<code>cout</code> stands for console-output and cin for <code>console-input</code>. Those are predefined streams for interacting with default console.

To interact with files, you need to use <code>ifstream</code> and <code>ofstream</code> types.
Similar to <code>cin</code> and <code>cout</code>, <code>ifstream</code> stands for <code>input-file-stream</code> and <code>ofstream</code> stands for <code>output-file-stream</code>.

Your code might look like this:

<pre><code>#include &lt;iostream&gt;
#include &lt;fstream&gt;

using namespace std;

int start()
{
 cout &lt;&lt; "Welcome...";

 // do fancy stuff

 return 0;
}

int main ()
{
 string usreq, usr, yn, usrenter;

 cout &lt;&lt; "Is this your first time using TEST" &lt;&lt; endl;
 cin &gt;&gt; yn;
 if (yn == "y")
 {
 ifstream iusrfile;
 ofstream ousrfile;
 iusrfile.open("usrfile.txt");
 iusrfile &gt;&gt; usr;
 cout &lt;&lt; iusrfile; // I'm not sure what are you trying to do here, perhaps print iusrfile contents?
 iusrfile.close();
 cout &lt;&lt; "Please type your Username. \n";
 cin &gt;&gt; usrenter;
 if (usrenter == usr)
 {
 start ();
 }
 }
 else
 {
 cout &lt;&lt; "THAT IS NOT A REGISTERED USERNAME.";
 }

 return 0;
}
</code></pre>

For further reading you might want to look at <a href="http://www.cplusplus.com/reference/iostream/" rel="nofollow noreferrer">c++ I/O reference</a>

Hey everyone, I have just started to learn C++ and I wanted to know how to read and write to a text file. I have seen many examples but they have all been hard to understand/follow and they have all varied. I was hoping that someone here could help. I am a total beginner so I need clear instructions. Here is an example of what i'm trying to do: 

<pre><code>#include &lt;iostream&gt;
#include &lt;fstream&gt;
using namespace std;
string usreq, usr, yn, usrenter;
int start ()
{
 cout &lt;&lt; "Welcome..."
int main ()
{
 cout &lt;&lt; "Is this your first time using TEST" &lt;&lt; endl;
 cin &gt;&gt; yn;
 if (yn == "y")
 {
 ofstream iusrfile;
 ofstream ousrfile;
 iusrfile.open("usrfile.txt", "w");
 iusrfile &gt;&gt; usr;
 cout &lt;&lt; iusrfile;
 iusrfile.close();
 cout &lt;&lt; "Please type your Username. \n";
 cin &gt;&gt; usrenter;
 if (usrenter == usr)
 {
 start ();
 }
 }
 else
 {
 cout &lt;&lt; "THAT IS NOT A REGISTERED USERNAME.";
 }

 return 0;

}
</code></pre>

How to read and write to a text file in C++?

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大家好，我刚刚开始学习C++，我想知道如何读写文本文件。我见过很多例子，但它们都很难理解/遵循，而且它们都是多种多样的。我希望这里有人能帮上忙。我是一个完全的初学者，所以我需要明确的指示。下面是我想要做的一个例子：#include <iostream>#include <fstream>using namespace ...

问如何在C++中读写文本文件？
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