[C++]C++小笔记之Simple File read and write-- ofstream and ifstream

快来打我* 2022-09-19 15:28 147阅读 0赞

首先我们简单的复习一下cin.

int n;
    cin >> n;

if user input a word instead of a number, four things will happen:

1.  the value of n remain unchanged;
2.  mismatch input is left in input queue;
3.  An error flag is set to the cin object;
4.  the call to the cin method, if converted to type bool, returns false.  if ( cin >> n) returns false;

在cin 读取出错的情况下，因为被设了error flag, 所以在继续读取更多的input之前，需要使用 cin.clear()方法reset flag.  clear()方法不仅会reset error flag； 它还可以reset EOF condition.

### ofstream -- Simple File Write To a txt File. ###

与cout用法相同，当要写内容进入文件时, ofstream objects可以使用所有cout同名的函数。

*  must include a fstream header file;
 *  fstream header file defines an ofstream class for handling output.
 *  name one or more ofstream objects;
 *  using namespace std OR use std:: prefix;
 *  associate ofstream object with a specific file. One way to do so is to use open("test.txt") method; 
 *  use close() method to close the file when finished write the file.
 *  use ofstream object with << operator to outpu a variety of data types;

Create a file:

fout.open("test.txt"); // if no such file, it will be created; if have file, clear its contents.
    
    OR
    
    char filename[50];
    cin >> filename;  //enter the filename that wants to write
    fout.open(filename);
    double wt=125.8;
    fout<<wt;               //write a number to txt file
    char line[4] = "xyz";
    fout<< line <<endl;
    fout.close();

char automobile[50];
    int year;
    double a_price, d_price;
    
    ofstream outFile;
    outFile.open("carinfo.txt");
    
    cout << "Enter the make and model of the car: ";
    cin.getline(automobile, 50);
    cout << "Enter the model year: ";
    cin >> year;
    cout << "Enter the original asking price: ";
    cin >> a_price;
    d_price = 0.913 * a_price;
    
    cout << fixed;
    cout.precision(2);
    cout.setf(ios_base::showpoint);
    cout << "make and model: " << automobile << endl;
    cout << "year: " << year << endl;
    cout << "price: " << d_price << endl;
    
    outFile << fixed;
    outFile.precision(2);
    outFile.setf(ios_base::showpoint);
    outFile << "make and model: " << automobile << endl;
    outFile << "year: " << year << endl;
    outFile << "price: " << d_price << endl;
    outFile.close();

### ifstream -- Simple File Read From a txt file. ###

与cin用法相同，当要写内容进入文件时, ifstream objects可以使用所有cin同名的函数。

*  must include a fstream header file;
 *  fstream header file defines an ifstream class for handling input;
 *  name one or more ifstream objects;
 *  if stream use >> operator to read a variaty of data types;
 *  ifstream object use get() function to read a individual character and use getline() fucntion to read a line of characters.
 *  ifstream object use eof(), fail() functions to monitor for the success of an input attempt;
 *  ifstream object itself, when used as a test condition is converted to boolean value true if last read attemp scceded and to false if otherwise.

char filename[50];
    	ifstream readFile;
    	cout << "Enter the name of the data file: ";
    	cin.getline(filename, 50);
    	readFile.open(filename);
    	if (!readFile.is_open()) {
    		cout << "Could not open file " << filename << endl;
    		cout << "Program terminating.\n";
    		return false;
    	}
    	char content[100];
    	readFile >> content;
    	while (readFile.good()) {
    		cout << content;
    		readFile>>content;
    	}
    	if (readFile.eof()) {
    		cout << " Reached at the end of file." << endl;
    	} else if (readFile.fail()) {
    		cout<<"Input terminated by data mismatch" <<endl;
    	} else {
    		cout<<"Input terminated for unknown reason"<<endl;
    	}

is\_open("test.txt") function shows if a file can be opened succesfully.

fail() function returns true if read EOF or a type mis-match. e.g.

readFile >> content;

bad() function returns false when something unexpected may go wrong.