Class/Template Descriptions
basic_ifstream<> class template derived from basic_istream<>. Defines file streams used for input.
ifstream basic_ifstream class for char type
wifstream basic_ifstream class for wchar type
basic_ofstream<> class template derived from basic_ostream<>. Defines file streams used for output.
ofstream basic_ofstream class for char type
wofstream basic_ofstream class for wchar type
basic_fstream<> class template derived from basic_iostream<>. Defines file streams used for both input and output.
fstream basic_fstream class for char type
wfstream basic_fstream class for wchar type
basic_ifstream<> members
basic_ifstream();
explicit basic_ifstream(const char* filename, ios_base::openmode mode = ios_base::in);
void close();
bool is_open();
void open(const char* filename, ios_base::openmode mode = ios_base::in);
basic_ofstream<> members
basic_ofstream();
explicit basic_ofstream(const char* filename, ios_base::openmode mode = ios_base::out);
void close();
bool is_open();
void open(const char* filename, ios_base::openmode mode = ios_base::out);
basic_fstream<> members
basic_fstream();
explicit
basic_fstream(const char* filename, ios_base::openmode mode = ios_base::in | ios_base::out);
void close();
bool is_open();
void open(const char* filename, ios_base::openmode mode = ios_base::in | ios_base::out);
Example 8-9 - Simple File I/O
1 // File: ex8-9.cpp
2
3 #include <iostream>
4 #include <fstream>
5 #include <cstdlib>
6 using namespace std;
7
8 int main(void)
9 {
10 ifstream f1("ex8-9.cpp");
11 ifstream f2("nofile");
12
13 ofstream f3("file.one");
14 fstream f4("iofile");
15
16 char buff[80];
17
18 if (!f1) {
19 cout << "Hey, I can't find the \"ex8-9.cpp\" file\n";
20 exit(1);
21 }
22
23 cout << boolalpha; // turn on "true"/"false" for cout's bools
24
25 cout << "f2.rdstate()=" << f2.rdstate() << endl;
26 cout << "f2.fail()=" << f2.fail() << endl;
27 cout << "f2.bad()=" << f2.bad() << endl;
28 if (f2.fail())
29 cout << "Hey, I can't find \"nofile\", but who cares\n";
30 if (f3)
31 cout << "Hey, I've decided to create a \"file.one\" file\n";
32
33 f1 >> buff;
34 cout << buff << endl;
35 f3 << "Have a nice day\n" << endl;
36
37 // did f4 get opened?
38 cout << "f4.is_open()=" << f4.is_open()<<endl;
39 cout << "f4.good()=" << f4.good()<<endl;
40 cout << "f4.bad()=" << f4.bad()<<endl;
41 cout << "f4.fail()=" << f4.fail()<<endl;
42
43 // try to write using the f4 stream
44 f4 << buff << endl;
45
46 // recheck f4's status
47 cout << "f4.is_open()=" << f4.is_open()<<endl;
48 cout << "f4.good()=" << f4.good()<<endl;
49 cout << "f4.bad()=" << f4.bad()<<endl;
50 cout << "f4.fail()=" << f4.fail()<<endl;
51 return 0;
52 }
****** Output - MS Visual C++ 2008 ******
f2.rdstate()=2
f2.fail()=true
f2.bad()=false
Hey, I can't find "nofile", but who cares
Hey, I've decided to create a "file.one" file
//
f4.is_open()=false
f4.good()=false
f4.bad()=false
f4.fail()=true
f4.is_open()=false
f4.good()=false
f4.bad()=true
f4.fail()=true
****** Output - gnu version 4.32b ******
f2.rdstate()=4
f2.fail()=true
f2.bad()=false
Hey, I can't find "nofile", but who cares
Hey, I've decided to create a "file.one" file
//
f4.is_open()=false
f4.good()=false
f4.bad()=false
f4.fail()=true
f4.is_open()=false
f4.good()=false
f4.bad()=false
f4.fail()=true
More I/O Members and Types
ios_base class
typedefs
typedef T3 openmode;
constants
Open mode constants
These constants are used to assign a value to an openmode value. They represent the mode for opening a stream.
app position to the end of the stream before each write operation.
ate position to the end of the stream when the stream is opened.
binary open the stream in binary mode (newlines are 1 byte).
in open for input.
out open for output
trunc delete an existing file when opening.
Positioning constants
These constants are used to assign a value to a seekdir value. They used for relative positioning in a file stream with the seekg() and seekp() functions.
beg position is relative to the beginning of a file stream.
cur position is relative to the current position in a file stream.
end position is relative to the end of a file stream.
More basic_istream members
istream& seekg(ios_base::pos_type pos); positions to the location indicated by pos in a file stream. pos_type is the type returned by the tellg() function.
istream& seekg(ios_base::pos_type pos, ios_base::seekdir dir);
seeks to the position pos characters from dir in a file stream.
pos_type tellg(); returns the character position in the file stream. If the stream state is non-zero, then the function returns pos_type (-1).
More basic_ostream members
ostream& seekp(ios_base::pos_type pos); positions to the location indicated by pos in a file stream. pos_type is the type returned by the tellp() function.
ostream& seekp(ios_base::pos_type pos, ios_base::seekdir dir);
seeks to the position pos characters from dir in a file stream.
pos_type tellp(); returns the character position in the file stream. If the stream state is non-zero, then the function returns pos_type (-1).
Example 8-10 – File I/O – positioning in a file
1 // File ex8-10.cpp - file I/O
2
3 #include <fstream>
4 #include <iostream>
5 #include <cstdlib>
6 #include <cstring>
7 using namespace std;
8
9 int main()
10 {
11 int ch, i;
12
13 // Open a file for output
14 ofstream fout("da_file");
15
16 // Check file open
17 if (!fout) {
18 cerr << "I can't open \"da_file\"\n";
19 exit(EXIT_FAILURE); // EXIT_FAILURE signifies failure
20 }
21
22 // Write 4 lines into the file
23 fout << "Have a nice day\n";
24 fout << 7 << endl;
25 fout << 3.14159 << endl;
26 fout << hex << 123 << ' ' << oct << 123 << endl;
27
28 // Close the file
29 fout.close();
30
31 // Open the file for input
32 ifstream fin("da_file");
33
34 // Check input file open
35 if (!fin) {
36 cerr << "I can't open \"da_file\"\n";
37 exit(EXIT_FAILURE);
38 }
39
40 char buff[80];
41
42 // Read each line from the file
43 while (!fin.getline(buff,80).eof()) {
44 // Print the length of the line and its contents
45 cout << strlen(buff) << '\t' << buff << endl;
46 }
47
48 // Print the current position in the file
49 cout << fin.tellg() << endl;
50
51 // Move to byte 7 (the 8th byte) in the file
52 fin.seekg(7,ios_base::beg);
53
54 // Print the current position in the file
55 cout << fin.tellg() << endl;
56
57 // Set the boooalpha flag for cout
58 cout.setf(ios_base::boolalpha);
59
60 // Print the file's stream state and state bits
61 cout << "fin.rdstate()=" << fin.rdstate();
62 cout << " fin.bad()=" << fin.bad();
63 cout << " fin.fail()=" << fin.fail();
64 cout << " fin.eof()=" << fin.eof() <<endl;
65
66 // Clear the stream state
67 fin.clear();
68
69 // Print the file's stream state and state bits
70 cout << "fin.rdstate()=" << fin.rdstate();
71 cout << " fin.bad()=" << fin.bad();
72 cout << " fin.fail()=" << fin.fail();
73 cout << " fin.eof()=" << fin.eof() <<endl;
74
75 // Move to byte 7 (the 8th byte) in the file
76 fin.seekg(7,ios_base::beg);
77
78 // Print the current position in the file
79 cout << fin.tellg() << endl;
80
81 // Read the next "word" from the file
82 fin >> buff;
83
84 // Print the "word"
85 cout << buff << endl;
86
87 // Print the current position in the file
88 cout << fin.tellg() << endl;
89
90 // Read the next 10 characters
91 for (i = 0; i < 10; i++) {
92 ch = fin.get();
93 // Print counter, char (as int), char, and stream position
94 cout << i << '\t' << ch << '\t' << (char) ch << '\t'
95 << fin.tellg() << endl;
96 }
97
98 return 0;
99 }
****** Output (MS Visual C++ 2008) ******
15 Have a nice day
1 7
7 3.14159
6 7b 173
-1
37
fin.rdstate()=0 fin.bad()=false fin.fail()=false fin.eof()=false
fin.rdstate()=0 fin.bad()=false fin.fail()=false fin.eof()=false
7
nice
11
0 32 12
1 100 d 13
2 97 a 14
3 121 y 15
4 10
17
5 55 7 18
6 10
20
7 51 3 21
8 46 . 22
9 49 1 23
****** Output (GNU 4.32b) ******
15 Have a nice day
1 7
7 3.14159
6 7b 173
-1
-1
fin.rdstate()=6 fin.bad()=false fin.fail()=true fin.eof()=true
fin.rdstate()=0 fin.bad()=false fin.fail()=false fin.eof()=false
7
nice
11
0 32 12
1 100 d 13
2 97 a 14
3 121 y 15
4 10
16
5 55 7 17
6 10
18
7 51 3 19
8 46 . 20
9 49 1 21
What's the difference between the two outputs? Why?
Example 8-11 - File I/O – positioning, modes, and stream state
1 // File ex8-ll.cpp – positioning, modes, and stream state
2
3 #include <fstream>
4 #include <iostream>
5 #include <cstdlib>
6 using namespace std;
7
8 void print_file(istream&);
9
10 int main() {
11
12 // Declare and open an output file stream
13 ofstream fout("da_file");
14
15 // Check the file open
16 if (!fout) {
17 cerr << "I can't open \"da_file\"\n";
18 exit (EXIT_FAILURE);
19 }
20
21 // write 3 lines into a new file
22 fout << "Have a nice day.\n";
23 fout << "Have a great day.\n";
24 fout << "Have a totally excellent day.\n";
25
26 // close the file Why?
27 fout.close();
28
29 // re-open the file as an fstream object for input and output
30 fstream finout("da_file",ios_base::in|ios_base::out);
31
32 print_file(finout);
33
34 // clear the EOF bit
35 finout.clear();
36
37 // position to byte 7 in the file
38 finout.seekp(7,ios::beg);
39
40 // Are the get and put pointers the same?
41 cout<< "finout.tellg()=" << finout.tellg()<< endl;
42 cout<< "finout.tellp()=" << finout.tellp()<< endl;
43
44 // replace "nice" with "fine"
45 finout<< "fine";
46
47 // Are the get and put pointers still the same?
48 cout<< "finout.tellg()="<< finout.tellg()<< endl;
49 cout << "finout.tellp()=" << finout.tellp () << endl;
50
51 print_file(finout);
52
53 // close the file
54 finout.close();
55
56 // reopen the file in input/output/binary mode
57 finout.open("da_file",ios_base::in|ios_base::out|ios::binary);
58
59 // write hey into the file
60 finout<< "hey";
61
62 print_file(finout);
63
64 // try again to write hey into the file
65 finout.seekp(0,ios::beg);
66 finout<< "hey";
67 print_file(finout);
68
69 // try app mode
70 finout.clear();
71 finout.close();
72 finout.open("da_file",ios_base::in|ios_base::out|ios::app);
73 finout<< "hey";
74
75 print_file(finout);
76
77 return 0;
78 }
79
80
81 void print_file(istream& file)
82 {
83 cout<< "file.rdstate="<< file.rdstate() << endl;
84 char buffer[80];
85 file.seekg(0,ios::beg);
86 while (file.getline(buffer,sizeof(buffer))) {
87 cout<< buffer << endl;
88 }
89 cout << endl;
90 }
****** Output (MS Visual C++ 2008) ******
file.rdstate=0 (32)
Have a nice day.
Have a great day.
Have a totally excellent day.
finout.tellg()=7 (41)
finout.tellp()=7 (42)
finout.tellg()=11 (48)
finout.tellp()=11 (49)
file.rdstate=0 (51)
Have a fine day.
Have a great day.
Have a totally excellent day.
file.rdstate=7 (62)
Have a fine day.
Have a great day.
Have a totally excellent day.
file.rdstate=7 (67)
file.rdstate=0 (75)
Have a fine day.
Have a great day.
Have a totally excellent day.
hey
****** Output (GNU g++ 4.32) ******
file.rdstate=0
Have a nice day.
Have a great day.
Have a totally excellent day.
finout.tellg()=7
finout.tellp()=7
finout.tellg()=11
finout.tellp()=11
file.rdstate=0 (51)
Have a fine day.
Have a great day.
Have a totally excellent day.
file.rdstate=0 (62)
heye a fine day.
Have a great day.
Have a totally excellent day.
file.rdstate=6 (67)
file.rdstate=0 (75)
heye a fine day.
Have a great day.
Have a totally excellent day.
hey
The following example illustrates binary file I/O. The example is a bit hokey, but it demonstrates input-output techniques that might be useful in a database application.
Example 8-12 – File I/O – read() and write()
1 // File: ex8-12.cpp - File I/O read() and write()
2
3 #include <fstream>
4 #include <iostream>
5 #include <iomanip>
6 #include <cstdlib>
7 #include <cstring>
8 #include <string>
9 using namespace std;
10
11 class Employee
12 {
13 public:
14 Employee() : age(0), salary(0.f) {}
15 private:
16 char empno[8];
17 char name[32];
18 unsigned short age;
19 float salary;
20 friend istream& operator>>(istream&, Employee&);
21 friend ostream& operator<<(ostream&, const Employee&);
22 };
23
24 istream& operator>>(istream& in, Employee& E)
25 {
26 in >> E.empno >> E.name >> E.age >> E.salary;
27 return in;
28 }
29
30 ostream& operator<<(ostream& out, const Employee& E)
31 {
32 out << setprecision(2) << fixed << showpoint << left;
33 out << setw(8) << E.empno
34 << setw(13) << E.name
35 << right << setw(3) << E.age
36 << setw(10) << E.salary;
37 return out;
38 }
39 class EmployeeFile
40 {
41 public:
42 EmployeeFile(string filename = "empfile");
43 void print(); // Why isn’t this const?
44 void open_for_read_write();
45 void close() { File.close(); }
46 bool operator!() const { return !File.rdstate(); }
47 private:
48 string Filename;
49 fstream File;
50 friend EmployeeFile& operator>>(EmployeeFile&, Employee&);
51 friend EmployeeFile& operator<<(EmployeeFile&, const Employee&);
52 };
53
54 // constructor opens file in output/binary mode
55 EmployeeFile::EmployeeFile(string filename)
56 : Filename(filename), File(filename.c_str(),ios::out|ios::binary)
57 {
58 }
59
60 void EmployeeFile::print()
61 {
62 Employee temp;
63
64 // position to the beginning of the file
65 File.seekg(0,ios_base::beg);
66
67 // read data from the file and print out each Employee record
68 while (!(*this>>temp)) {
69 cout << temp << endl;
70 }
71
72 // clear the File stream state (after reading EOF)
73 File.clear();
74 }
75
76 void EmployeeFile::open_for_read_write()
77 {
78 File.open(Filename.c_str(),ios::in | ios::out | ios::binary);
79
80 if (File.fail() ) {
81 cerr << "Unable to open Employee file: " << Filename << endl;
82 exit(-1);
83 }
84 }
85
86 EmployeeFile& operator>>(EmployeeFile& EF, Employee& E)
87 {
88 EF.File.read((char*) &E, sizeof E);
89 return EF;
90 }
91 EmployeeFile& operator<<(EmployeeFile& EF, const Employee& E)
92 {
93 EF.File.write((char*) &E, sizeof E);
94 return EF;
95 }
96
97 int main()
98 {
99 Employee temp;
100 EmployeeFile EmpFile("employee.dat");
101
102 for (int i = 0; i<4; ++i) {
103 cout << "Enter empno name age salary\n";
104 cin >> temp;
105 EmpFile << temp;
106 }
107
108 // close the file and reopen it in read-write mode
109 EmpFile.close();
110 EmpFile.open_for_read_write();
111
112 EmpFile.print();
113 return 0;
114 }
****** Sample Run ******
Enter empno name age salary
654321 Joe 35 80000
Enter empno name age salary
642731 Jim 55 85000
Enter empno name age salary
615787 Helen 60 90000
Enter empno name age salary
J00787 Susan 47 50000
654321 Joe 35 80000.00
642731 Jim 55 85000.00
615787 Helen 60 90000.00
J00787 Susan 47 50000.00
What is the purpose of the char* cast in the file.read() above?
Why is the EmployeeFile File stream opened by the constructor in write mode, then later closed and re-opened in read-write mode?
Example 8-13 - A DOS grep command
The following example is used to create a grep command for DOS. The UNIX grep command is used to search a file, or a list of files for the existence of a desired target string. This example was written for DOS, not UNIX. It demonstrates conditional compilation to allow for compiler differences.
After the program listing, sample compile commands are demonstrated and there is a note about executing the program under Windows XP.
1 // File: ex8-13.cpp - A grep command for DOS
2
3 #include <fstream>
4 #include <iostream>
5 #include <cstdlib>
6 #include <cstring>
7 using namespace std;
8
9 // The macro __GNUG__ is set for GNU C++
10 #ifdef __GNUG__
11 #include <unistd.h> // for access()
12 #else
13 #include <io.h> // for _access()
14 #endif
15
16 int main(int argc, char* argv[])
17 {
18 char filename[64],
19 buffer[1024],
20 command[128],
21 tempFilename[9];
22 int lineno,
23 hits = 0,
24 files = 0,
25 system_command_status;
26 ifstream fin1,
27 fin2;
28
29 // Check command-line syntax
30 if (argc != 3) {
31 cerr << "Syntax error\ngrep [target text] [target file(s)]
32 \n";
33 exit (-1);
34 }
35
36 // create a temporary file to hold the filenames to be searched
37 strcpy(tempFilename,"tempa"); // first possible filename
38
39 // The access() or _access() functions are used to status the
40 // existence of a file
41
42 // If you're not using a GNU C++ compiler, use _access()
43 #ifdef __GNUG__
44 while (access(tempFilename,0) == 0 && tempFilename[4] <= 'z')
45 // If you're not using a GNU C++ compiler, use _access()
46 #else
47 while (_access(tempFilename,0) == 0 && tempFilename[4] <= 'z')
48 #endif
49 tempFilename[4]++;
50
51 // If you've tried all filenames from "tempa" to "tempz",
52 // then give up
53 if (tempFilename[4] > 'z') {
54 cerr << "Unable to create a temp* file. Please cleanup\n";
55 exit (-2);
56 }
57
58 // Create the command: "dir /b [filename(s)] > [tempfilename]
59 strcpy(command,"dir /b ");
60 strcat(command,argv[2]);
61 strcat(command," > ");
62 strcat(command,tempFilename);
63
64 // system() allows you to issue operating system commands
65 system_command_status = system(command);
66
67 // check the status of the system command
68 if (system_command_status == -1) {
69 cerr << "Error with system command: " << command << endl;
70 exit(-3);
71 }
72
73 // open temporary file (containing names of files to be searched)
74 fin1.open(tempFilename);
75
76 // Make sure the tempFilename is not empty
77 if (fin1.peek() == EOF) {
78 cerr << "Error: target file(s) do(es) not exist, dummy\n";
79 exit (-4);
80 }
81
82 // for each file in tempFilename, search for the target string
83 while (fin1.getline(filename,sizeof(filename))) {
84
85 // open the next file to be searched
86 fin2.open(filename);
87
88 // increment file count
89 files++;
90
91 // initialize line counter
92 lineno = 0;
93
94 // clear errors in fin2 stream
95 fin2.clear();
96
97 // position at the beginning of the file to be searched
98 fin2.seekg(0L, ios:: beg);
99
100 // read each line from the file into buffer
101 while (fin2.getline(buffer,sizeof(buffer))) {
102
103 // increment line counter
104 lineno++;
105
106 // does buffer contains the target string?
107 if (strstr(buffer,argv[1])) {
108
109 // If so, increment hit counter
110 hits++;
111
112 // display filename, line count, contents of line
113 cout << filename << '[' << lineno << "] "
114 << buffer << '\n';
115 }
116 }
117
118 // close the file to be searched
119 fin2.close();
120 }
121
122 // close the temporary file
123 fin1.close();
124
125 // Print summary information
126 cout << "Found " << hits << " occurrence(s) in " << files
127 << " file(s)\n";
128
129 // create the DOS command to erase the tempFilename
130 strcpy(command,"erase ");
131 strcat(command,tempFilename);
132
133 // Issue system erase command
134 system_command_status = system(command);
135
136 // status system command
137 if (system_command_status == -1) {
138 cerr << "Error with system command: " << command << endl;
139 exit(-5);
140 }
141
142 return 0;
143 }
