c++ ifstream, ofstream, fstream 之2一览

• 上面是这几个类的继承关系，都需要包含，ifstream输入，ofstream输出，fstream既输入又输出

ios::in 输入(读)
ios::out 输出(写)
ios::ate 初始位置：文件尾
ios::app 附加
ios::trunc 如果文件已存在则先删除该文件
ios::binary 二进制方式

ifstream 默认 ios::in
ofstream 默认 ios::out
fstream 默认 ios::in | ios::out

• 下面是一段小程序显示它们的用法：

---

    #include <fstream>
    #include <string>
    #include <iostream>

    using namespace std;

    int main(int argc, char* argv[])
    {
        // read file use fstream
        std::fstream iofile("./status.txt", ios::in | ios::out);
        if (iofile.fail())
        {
            fprintf(stderr, "cann't open status.txt \n");
            return -1;
        }

        string strLine;
        while (getline(iofile, strLine))
        {
            cout << strLine << endl;
        //    getchar();
        }

        iofile.close();


        // write file use fstream
        fstream outfile("./result.txt", ios::out | ios::app);
        if (outfile.fail())
        {
            fprintf(stderr, "cann't create result.txt \n");
            return -1;
        }

        int count = 0;
        while (++count < 100)
        {
            outfile << count << endl;
        }

        outfile.close();

        // read file use istream
        ifstream infile("./status.txt"/*, ios::in (as default)*/);
        if (infile.fail())
        {
            fprintf(stderr, "cann't open status.txt \n");
            return -1;
        }

        while (getline(infile, strLine))
        {
            cout << strLine << endl;
            //getchar();
        }

        infile.close();

        // write file use ostream
        ofstream ooutfile("./result2.txt"/*, ios::out | ios::trunc (as default) */);
        if (ooutfile.fail())
        {
            fprintf(stderr, "cann't open status.txt \n");
            return -1;
        }

        while (--count > 0)
        {
            ooutfile << count << endl;
        }

        ooutfile.close();

	    return 0;
    }