15周《C++语言基础》程序阅读——二进制文件及文件的随机读写（2）（3）

2、查看下面程序的输出，解释为什么会有这样的输出。

#include <iostream>
#include <fstream>
using namespace std;
int main( )
{
    unsigned char a[] = {0x32,0x30,0x31,0x35,0xA3,0xAC,0xCE,0xD2,0xC3,0xC7,0xB3,0xD4,
                           0xC1,0xCB,0xB5,0xDA,0xD2,0xBB,0xBF,0xDA,0xF3,0xA6,0xD0,0xB7};
    ofstream outfile("f3.dat",ios::out|ios::binary);
    outfile.write((char*)a, sizeof(a));
    outfile.close();
    return 0;
}

因为ox开头的代码代表汉字。

3、阅读下面的程序，指出其功能，体会seekg()、tellg()等函数的功能及其用法 
（1）

<code class="hljs cpp has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">#include<iostream></span>
<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">#include <fstream></span>
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">using</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">namespace</span> <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">std</span>;
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">const</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">char</span> * filename = <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"a.txt"</span>;
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> main ()
{
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">long</span> l,m;
    ifstream file (filename, ios::in|ios::binary);
    l = file.tellg();
    file.seekg (<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>, ios::end);
    m = file.tellg();
    file.close();
    <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">cout</span> << <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"size of "</span> << filename;
    <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">cout</span> << <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">" is "</span> << (m-l) << <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">" bytes.\n"</span>;
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>;
}</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li></ul>

（2）

<code class="hljs perl has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">#include <fstream></span>
using namespace std;
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> main (){
    long <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">pos</span>;
    ofstream outfile;
    outfile.<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">open</span> (<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"test.txt"</span>);
    outfile.<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">write</span> (<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"This is an apple"</span>,<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">16</span>);
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">pos</span>=outfile.tellp();
    outfile.seekp (<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">pos</span>-<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">7</span>);
    outfile.<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">write</span> (<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">" sam"</span>,<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">4</span>);
    outfile.<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">close</span>();
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>;
}</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li></ul>

（3）

<code class="hljs cpp has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">#include <iostream> </span>
<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">#include <fstream> </span>
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">using</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">namespace</span> <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">std</span>;
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> main() 
{     
    fstream outfile,infile;     
    outfile.open(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"data.txt"</span>,ios::out);     
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">for</span> (<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> i=<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>;i<<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">26</span>;i++)
       outfile<<(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">char</span>)(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'A'</span>+i);     
    outfile.close();     
    infile.open(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"data.txt"</span>,ios::in);     
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">char</span> ch;     
    infile.seekg(<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">6</span>,ios::beg);     
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span>(infile.get(ch))   
        <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">cout</span><<ch;     
    infile.seekg(<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">8</span>,ios::beg);     
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span>(infile.get(ch))         
        <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">cout</span><<ch;     
    infile.seekg(-<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">8</span>,ios::end);
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span>(infile.get(ch))         
        <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">cout</span><<ch;         
    <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">cout</span><<endl;     
    infile.close(); 
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>; 
}</code>