C++中處理文件類似于處理標準輸入和標準輸出。類ifstream、ofstream和fstream分別從類 istream、ostream和iostream派生而來。作為派生的類，它們繼承了插入和提取運算符（以及其他成員函數），還有與文件一起使用的成員和構造函數。可將文件<fstream> 包括進來以使用任何fstream。如果只執行輸入，使用ifstream類；如果只執行輸出，使用 ofstream類；如果要對流執行輸入和輸出，使用fstream類。可以將文件名稱用作構造函數參數。

ofstream: Stream class to write on files.

ifstream: Stream class to read from files.

fstream: Stream class to both read and write from/to files.

These classes are derived directly or indirectly from the classes istream and ostream.

對這些類的一個對象所做的第一個操作通常就是將它和一個真正的文件聯系起來，也就是說打開一個文件。被打開的文件在程序中由一個流對象(stream object)來表示 (這些類的一個實例) ，而對這個流對象所做的任何輸入輸出操作實際就是對該文件所做的操作。

要通過一個流對象打開一個文件，可以使用它的成員函數open()或直接通過構造函數。

void open (constchar * filename, openmode mode);

這里filename 是一個字符串，代表要打開的文件名，mode 是以下標志符的一個組合：

ios::in ?以輸入(讀)方式打開文件；

ios::out ?以輸出(寫)方式打開文件；

ios::ate ?初始位置：文件尾，文件打開后定位到文件尾；

ios::app ?以追加的方式打開文件，所有輸出附加在文件末尾；

ios::trunc ?如果文件已存在則先刪除該文件；

ios::binary ?二進制方式，以二進制方式打開文件；

這些標識符可以被組合使用，中間以”或”操作符(|)間隔。

這些類的成員函數open 都包含了一個默認打開文件的方式，只有當函數被調用時沒有聲明方式參數的情況下，默認值才會被采用。如果函數被調用時聲明了任何參數，默認值將被完全改寫，而不會與調用參數組合。ofstream類的默認打開方式是： ios::out | ios::trunc ;ifstream 類的默認打開方式是ios::in;fstream類的默認打開方式是： ios::in | ios::out.

? ? ? ??http://www.cplusplus.com/reference/fstream/fstream/中列出了fstream中可以使用的成員函數。

???????? C++ IO heads, templates and class (https://www.ntu.edu.sg/home/ehchua/programming/cpp/cp10_IO.html)：

以下是測試代碼：

#include <iostream>
#include <fstream>
#include <string>
#include <cstdlib>
#include <vector>#include "fstream.hpp"/* reference: http://www.tutorialspoint.com/cplusplus/cpp_files_streams.htmhttps://www.ntu.edu.sg/home/ehchua/programming/cpp/cp10_IO.htmlhttp://www.bogotobogo.com/cplusplus/fstream_input_output.php
*/int test_file_size()
{std::ifstream in("E:/GitCode/Messy_Test/testdata/fstream_data.bin", std::ios::binary);if (!in.is_open()) {std::cout << "fail to open file\n";return -1;}std::streampos begin, end;begin = in.tellg();in.seekg(0, std::ios::end);end = in.tellg();in.close();std::cout << "this file's size is: " << (end - begin) << " bytes.\n";return 0;
}int test_fstream1()
{char data[100];// open a file in write mode.std::ofstream outfile;outfile.open("E:/GitCode/Messy_Test/testdata/fstream.dat");if (!outfile.is_open()) {std::cout << "fail to open file to write\n";return -1;}std::cout << "Writing to the file" << std::endl;std::cout << "Enter your name: ";std::cin.getline(data, 100);// write inputted data into the file.outfile << data << std::endl;std::cout << "Enter your age: ";std::cin >> data;std::cin.ignore();// again write inputted data into the file.outfile << data << std::endl;// close the opened file.outfile.close();// open a file in read mode.std::ifstream infile;infile.open("E:/GitCode/Messy_Test/testdata/fstream.dat");if (!infile.is_open()) {std::cout << "fail to open file to read\n";return -1;}std::cout << "Reading from the file" << std::endl;infile >> data;// write the data at the screen.std::cout << data << std::endl;// again read the data from the file and display it.infile >> data;std::cout << data << std::endl;// close the opened file.infile.close();return 0;
}int test_fstream2()
{/* Testing Simple File IO (TestSimpleFileIO.cpp) */std::string filename = "E:/GitCode/Messy_Test/testdata/test.txt";// Write to Filestd::ofstream fout(filename.c_str());  // default mode is ios::out | ios::truncif (!fout) {std::cerr << "error: open file for output failed!" << std::endl;abort();  // in <cstdlib> header}fout << "apple" << std::endl;fout << "orange" << std::endl;fout << "banana" << std::endl;fout.close();// Read from filestd::ifstream fin(filename.c_str());  // default mode ios::inif (!fin) {std::cerr << "error: open file for input failed!" << std::endl;abort();}char ch;while (fin.get(ch)) {  // till end-of-filestd::cout << ch;}fin.close();return 0;
}int test_fstream3()
{/* Testing Binary File IO (TestBinaryFileIO.cpp) */std::string filename = "E:/GitCode/Messy_Test/testdata/test.bin";// Write to Filestd::ofstream fout(filename.c_str(), std::ios::out | std::ios::binary);if (!fout.is_open()) {std::cerr << "error: open file for output failed!" << std::endl;abort();}int i = 1234;double d = 12.34;fout.write((char *)&i, sizeof(int));fout.write((char *)&d, sizeof(double));fout.close();// Read from filestd::ifstream fin(filename.c_str(), std::ios::in | std::ios::binary);if (!fin.is_open()) {std::cerr << "error: open file for input failed!" << std::endl;abort();}int i_in;double d_in;fin.read((char *)&i_in, sizeof(int));std::cout << i_in << std::endl;fin.read((char *)&d_in, sizeof(double));std::cout << d_in << std::endl;fin.close();return 0;
}int test_fstream4()
{std::string theNames = "Edsger Dijkstra: Made advances in algorithms, the semaphore (programming).\n";theNames.append("Donald Knuth: Wrote The Art of Computer Programming and created TeX.\n");theNames.append("Leslie Lamport: Formulated algorithms in distributed systems (e.g. the bakery algorithm).\n");theNames.append("Stephen Cook: Formalized the notion of NP-completeness.\n");std::ofstream ofs("E:/GitCode/Messy_Test/testdata/theNames.txt");if (!ofs)	{std::cout << "Error opening file for output" << std::endl;return -1;}ofs << theNames << std::endl;ofs.close();char letter;int i;std::string line;std::ifstream reader("E:/GitCode/Messy_Test/testdata/theNames.txt");if (!reader) {std::cout << "Error opening input file" << std::endl;return -1;}//for (i = 0; !reader.eof(); i++) {while (!reader.eof()) {reader.get(letter);std::cout << letter;//getline( reader , line ) ;//std::cout << line << std::endl;}reader.close();return 0;
}//
std::ofstream _file;int test_init_database()
{_file.open("E:/GitCode/Messy_Test/testdata/data.bin");if (!_file.is_open()) {fprintf(stderr, "open file fail\n");return -1;}return 0;
}int test_store_database()
{for (int i = 0; i < 10; ++i) {_file.write((char*)&i, sizeof(i));}return 0;
}int test_close_database()
{_file.close();return 0;
}int test_fstream5()
{test_init_database();for (int i = 0; i < 5; ++i) {test_store_database();}test_close_database();std::ifstream file("E:/GitCode/Messy_Test/testdata/data.bin");if (!file.is_open()) {fprintf(stderr, "open file fail\n");return -1;}int a[100];for (int i = 0; i < 50; ++i) {file.read((char*)&a[i], sizeof(int));}file.close();return 0;
}//
static void parse_string(char* line, std::string& image_name, std::vector<int>& rect)
{std::string str(line);rect.resize(0);int pos = str.find_first_of(" ");image_name = str.substr(0, pos);std::string str1 = str.substr(pos + 1, str.length());for (int i = 0; i < 4; ++i) {pos = str1.find_first_of(" ");std::string x = str1.substr(0, pos);str1 = str1.erase(0, pos+1);rect.push_back(std::stoi(x));}
}int test_fstream6()
{std::string name{ "E:/GitCode/Messy_Test/testdata/list.txt" };std::ifstream in(name.c_str(), std::ios::in);if (!in.is_open()) {fprintf(stderr, "open file fail: %s\n", name.c_str());return -1;}int count{ 0 };char line[256];in.getline(line, 256);count = atoi(line);std::cout << count << std::endl;//while (!in.eof()) {for (int i = 0; i < count; ++i) {in.getline(line, 256);std::cout << "line: "<< line << std::endl;std::string image_name{};std::vector<int> rect{};parse_string(line, image_name, rect);std::cout << "image name: " << image_name << std::endl;for (auto x : rect)std::cout << "  " << x << "  ";std::cout << std::endl;}in.close();return 0;
}//
int test_fstream7()
{std::string name{ "E:/GitCode/Messy_Test/testdata/list.txt" };std::ifstream in(name.c_str(), std::ios::in);if (!in.is_open()) {fprintf(stderr, "open file fail: %s\n", name.c_str());return -1;}int count{ 0 };std::string image_name{};int left{ 0 }, top{ 0 }, right{ 0 }, bottom{ 0 };in >> count;std::cout << "count: " << count << std::endl;for (int i = 0; i < count; ++i) {in >> image_name >> left >> top >> right >> bottom;fprintf(stdout, "image_name: %s, rect: %d, %d, %d, %d\n", image_name.c_str(), left, top, right, bottom);}in.close();return 0;
}

GitHub：https://github.com/fengbingchun/Messy_Test