C++编程思想第2卷第4章输入输出流在输入输出流中定位

本文深入探讨了C++中文件流（iostreams）的操作，包括使用二进制流写入和读取文件，以及如何利用seekg()和seekp()函数进行文件指针的精确定位，展示了文件复制和修改的具体实现。

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每种类型的输入输出流都有下一个字符从哪里来如果是istream 或到哪里去
如果是ostream 的概念

用streampos进行绝对流位置定位，需要先调用一个“告知”函数，以便知道流指针
中的确切位置

第2种方法是相对定位，使用重载版本的seekp()和seekg()函数

文件进行定位

//: C04:Seeking.cpp
// From "Thinking in C++, Volume 2", by Bruce Eckel & Chuck Allison.
// (c) 1995-2004 MindView, Inc. All Rights Reserved.
// See source code use permissions stated in the file 'License.txt',
// distributed with the code package available at www.MindView.net.
// Seeking in iostreams.
#include <cassert>
#include <cstddef>
#include <cstring>
#include <fstream>
#include "../require.h"
using namespace std;

int main() {
  const int STR_NUM = 5, STR_LEN = 30;
  char origData[STR_NUM][STR_LEN] = {
    "Hickory dickory dus. . .",
    "Are you tired of C++?",
    "Well, if you have,",
    "That's just too bad,",
    "There's plenty more for us!"
  };
  char readData[STR_NUM][STR_LEN] = {{ 0 }};
  ofstream out("Poem.bin", ios::out | ios::binary);
  assure(out, "Poem.bin");
  for(int i = 0; i < STR_NUM; i++)
    out.write(origData[i], STR_LEN);
  out.close();
  ifstream in("Poem.bin", ios::in | ios::binary);
  assure(in, "Poem.bin");
  in.read(readData[0], STR_LEN);
  assert(strcmp(readData[0], "Hickory dickory dus. . .")
    == 0);
  // Seek -STR_LEN bytes from the end of file
  in.seekg(-STR_LEN, ios::end);
  in.read(readData[1], STR_LEN);
  assert(strcmp(readData[1], "There's plenty more for us!")
    == 0);
  // Absolute seek (like using operator[] with a file)
  in.seekg(3 * STR_LEN);
  in.read(readData[2], STR_LEN);
  assert(strcmp(readData[2], "That's just too bad,") == 0);
  // Seek backwards from current position
  in.seekg(-STR_LEN * 2, ios::cur);
  in.read(readData[3], STR_LEN);
  assert(strcmp(readData[3], "Well, if you have,") == 0);
  // Seek from the begining of the file
  in.seekg(1 * STR_LEN, ios::beg);
  in.read(readData[4], STR_LEN);
  assert(strcmp(readData[4], "Are you tired of C++?")
    == 0);
  getchar();
} ///:~

输出在工程目录下出现 Poem.bin
Hickory dickory dus. . . Are you tired of C++? Well, if you have, That's just too bad, There's plenty more for us!

这个程序使用二进制输出流写一首诗到文件中

重新打开ifstream文件后，使用seekg()获取指针位置

写文件的流对象不使用fstream对象

向两个对象之一写入数据 ifstream ostream

//: C04:Iofile.cpp
// From "Thinking in C++, Volume 2", by Bruce Eckel & Chuck Allison.
// (c) 1995-2004 MindView, Inc. All Rights Reserved.
// See source code use permissions stated in the file 'License.txt',
// distributed with the code package available at www.MindView.net.
// Reading & writing one file.
#include <fstream>
#include <iostream>
#include "../require.h"
using namespace std;

int main() {
  ifstream in("Iofile.cpp");
  assure(in, "Iofile.cpp");
  ofstream out("Iofile.out");
  assure(out, "Iofile.out");
  out << in.rdbuf(); // Copy file
  in.close();
  out.close();
  // Open for reading and writing:
  ifstream in2("Iofile.out", ios::in | ios::out);
  assure(in2, "Iofile.out");
  ostream out2(in2.rdbuf());
  cout << in2.rdbuf();  // Print whole file
  out2 << "Where does this end up?";
  out2.seekp(0, ios::beg);
  out2 << "And what about this?";
  in2.seekg(0, ios::beg);
  cout << in2.rdbuf();
  getchar();
} ///:~

输出工程目录放Iofile.cpp 不然运行闪退目录下输出 iofile.out
//: C04:Iofile.cpp
// From "Thinking in C++, Volume 2", by Bruce Eckel & Chuck Allison.
// (c) 1995-2004 MindView, Inc. All Rights Reserved.
// See source code use permissions stated in the file 'License.txt',
// distributed with the code package available at www.MindView.net.
// Reading & writing one file.
#include <fstream>
#include <iostream>
#include "../require.h"
using namespace std;

int main() {
ifstream in("Iofile.cpp");
assure(in, "Iofile.cpp");
ofstream out("Iofile.out");
assure(out, "Iofile.out");
out << in.rdbuf(); // Copy file
in.close();
out.close();
// Open for reading and writing:
ifstream in2("Iofile.out", ios::in | ios::out);
assure(in2, "Iofile.out");
ostream out2(in2.rdbuf());
cout << in2.rdbuf(); // Print whole file
out2 << "Where does this end up?";
out2.seekp(0, ios::beg);
out2 << "And what about this?";
in2.seekg(0, ios::beg);
cout << in2.rdbuf();
getchar();
} ///:~
And what about this?// From "Thinking in C++, Volume 2", by Bruce Eckel & Chuck Allison.
// (c) 1995-2004 MindView, Inc. All Rights Reserved.
// See source code use permissions stated in the file 'License.txt',
// distributed with the code package available at www.MindView.net.
// Reading & writing one file.
#include <fstream>
#include <iostream>
#include "../require.h"
using namespace std;

这是iofile.out
And what about this?// From "Thinking in C++, Volume 2", by Bruce Eckel & Chuck Allison.
// (c) 1995-2004 MindView, Inc. All Rights Reserved.
// See source code use permissions stated in the file 'License.txt',
// distributed with the code package available at www.MindView.net.
// Reading & writing one file.
#include <fstream>
#include <iostream>
#include "../require.h"
using namespace std;