fostream创建文件

Flag     Function
ios::in  Opens an input file. Use this as an open mode for an ofstreamto prevent truncating an existing file.
ios::out  Opens an output file. When used for an ofstreamwithout ios::app, ios::ateor ios::in, ios::truncis implied.
ios::app  Opens an output file for appending.
ios::ate  Opens an existing file (either input or output) and seeks the end.
ios::nocreate  Opens a file only if it already exists. (Otherwise it fails.)
ios::noreplace  Opens a file only if it does not exist. (Otherwise it fails.)
ios::trunc  Opens a file and deletes the old file, if it already exists.
ios::binary  Opens a file in binary mode. Default is text mode.

#include "require.h"
#include <iostream>
#include <fstream>
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());//输出流
    cout << in2.rdbuf();  // Print whole file
    out2 << "Where does this end up?";//这句话跟在结尾
    out2.seekp(0, ios::beg);//设置了out2的输出起始位置
    out2 << "And what about this?";//这句话在开始写入，覆盖了原始数据
    in2.seekg(0, ios::beg);//从起始位置开始读入数据
    cout << in2.rdbuf();//写到标准输出
} ///:~

这段程序首先打开一个已有的文件，并将所有的数据copy到一个输出文件中去。

然后打开一个文件IOfile.out并打开一个输出流ostream out2设置为in2.rdbuf(),然后首次输出了in2的内容。这里out2可以对in2.rdbuf()中的数据进行写入。

然后分别在输出流中的内容中前边和后边分别插入了一段字符。输出了in2.rdbuf();