标签:info c++ ret func ace ini exe cout feed
在C++中标准库提供三个类用于文件操作,统称为文件流类:
- ifstream:专用于从文件中读取数据;
- ofstream:专用于向文件中写入数据;
- fstream:既可用于从文件中读取数据,又可用于向文件中写入数据。
这三个文件流类都位于 头文件中,因此在使用它们之前,需要先引入此头文件。
有两种方式可以打开文件:
1、在构造函数中传入文件名
ifstream (const char* filename, ios_base::openmode mode = ios_base::in);
ifstream fin(filename, openmode) by default openmode = ios::in
ifstream fin(“filename”);
2、使用open
方法
Calling of default constructor
ifstream fin;
fin.open(filename, openmode)
fin.open(“filename”);
打开方式:
模式标记 |
适用对象 |
作用 |
ios::in |
ifstream、fstream |
打开文件用于读取数据。如果文件不存在,则打开出错。 |
ios::out |
ofstream、fstream |
打开文件用于写入数据。如果文件不存在,则新建该文件;如果文件原来就存在,则打开时清除原来的内容。 |
ios::app |
ofstream、fstream |
打开文件,用于在其尾部添加数据。如果文件不存在,则新建该文件。 |
ios::ate |
ifstream |
打开一个已有的文件,并将文件读指针指向文件末尾(读写指 的概念后面解释)。如果文件不存在,则打开出错。 |
ios:: trunc |
ofstream |
打开文件时会清空内部存储的所有数据,单独使用时与 ios::out 相同。 |
ios::binary |
ifstream、ofstream、fstream |
以二进制方式打开文件。若不指定此模式,则以文本模式打开。 |
ios::in | ios::out |
fstream |
打开已存在的文件,既可读取其内容,也可向其写入数据。文件刚打开时,原有内容保持不变。如果文件不存在,则打开出错。 |
ios::in | ios::out |
ofstream |
打开已存在的文件,可以向其写入数据。文件刚打开时,原有内容保持不变。如果文件不存在,则打开出错。 |
ios::in | ios::out | ios::trunc |
fstream |
打开文件,既可读取其内容,也可向其写入数据。如果文件本来就存在,则打开时清除原来的内容;如果文件不存在,则新建该文件。 |
使用 ifsream & ofstream classes实现文件读写的Demo:
#include
/* fstream header file for ifstream, ofstream,
fstream classes */
#include
using namespace std;
// Driver Code
int main()
{
// Creation of ofstream class object
ofstream fout;
string line;
// by default ios::out mode, automatically deletes
// the content of file. To append the content, open in ios:app
// fout.open("sample.txt", ios::app)
fout.open("sample.txt");
// Execute a loop If file successfully opened
while (fout) {
// Read a Line from standard input
getline(cin, line);
// Press -1 to exit
if (line == "-1")
break;
// Write line in file
fout
使用fstream class实现文件读写的demo:
#include
/* fstream header file for ifstream, ofstream,
fstream classes */
#include
using namespace std;
// Driver Code
int main()
{
// Creation of fstream class object
fstream fio;
string line;
// by default openmode = ios::in|ios::out mode
// Automatically overwrites the content of file, To append
// the content, open in ios:app
// fio.open("sample.txt", ios::in|ios::out|ios::app)
// ios::trunc mode delete all conetent before open
fio.open("sample.txt", ios::trunc | ios::out | ios::in);
// Execute a loop If file successfully Opened
while (fio) {
// Read a Line from standard input
getline(cin, line);
// Press -1 to exit
if (line == "-1")
break;
// Write line in file
fio
使用read、write方法读写文件,其语法如下:
To write object‘s data members in a file :
// Here file_obj is an object of ofstream
file_obj.write((char *) & class_obj, sizeof(class_obj));
To read file‘s data members into an object :
// Here file_obj is an object of ifstream
file_obj.read((char *) & class_obj, sizeof(class_obj));
Example:
Input :
Input.txt :
Michael 19 1806
Kemp 24 2114
Terry 21 2400
Operation : Print the name of the highest
rated programmer.
Output :
Terry
demo:
#include
#include
using namespace std;
// Class to define the properties
class Contestant {
public:
// Instance variables
string Name;
int Age, Ratings;
// Function declaration of input() to input info
int input();
// Function declaration of output_highest_rated() to
// extract info from file Data Base
int output_highest_rated();
};
// Function definition of input() to input info
int Contestant::input()
{
// Object to write in file
ofstream file_obj;
// Opening file in append mode
file_obj.open("Input.txt", ios::app);
// Object of class contestant to input data in file
Contestant obj;
// Feeding appropriate data in variables
string str = "Michael";
int age = 18, ratings = 2500;
// Assigning data into object
obj.Name = str;
obj.Age = age;
obj.Ratings = ratings;
// Writing the object‘s data in file
file_obj.write((char*)&obj, sizeof(obj));
// Feeding appropriate data in variables
str = "Terry";
age = 21;
ratings = 3200;
// Assigning data into object
obj.Name = str;
obj.Age = age;
obj.Ratings = ratings;
// Writing the object‘s data in file
file_obj.write((char*)&obj, sizeof(obj));
return 0;
}
// Function definition of output_highest_rated() to
// extract info from file Data Base
int Contestant::output_highest_rated()
{
// Object to read from file
ifstream file_obj;
// Opening file in input mode
file_obj.open("Input.txt", ios::in);
// Object of class contestant to input data in file
Contestant obj;
// Reading from file into object "obj"
file_obj.read((char*)&obj, sizeof(obj));
// max to store maximum ratings
int max = 0;
// Highest_rated stores the name of highest rated contestant
string Highest_rated;
// Checking till we have the feed
while (!file_obj.eof()) {
// Assigning max ratings
if (obj.Ratings > max) {
max = obj.Ratings;
Highest_rated = obj.Name;
}
// Checking further
file_obj.read((char*)&obj, sizeof(obj));
}
// Output is the highest rated contestant
cout
C++ 文件IO
标签:info c++ ret func ace ini exe cout feed
原文地址:https://www.cnblogs.com/Yuanb/p/14805819.html