C++ Tutorial/File Stream/ofstream
Содержание
- 1 Create a sequential file.
- 2 Demonstrate peek(), unget(), and ignore().
- 3 ofstream: seek from beginning
- 4 Read char array from a file
- 5 Save and read double array in a binary file
- 6 Save and read structure
- 7 Save char array to a file
- 8 Save chars to a binary file
- 9 Writing numbers to a file
- 10 12. 9. 3. Writing on a text file
Create a sequential file.
<source lang="cpp">#include <iostream> using std::cerr; using std::cin; using std::cout; using std::endl; using std::ios;
- include <fstream>
using std::ofstream;
- include <cstdlib>
using std::exit; int main() {
ofstream outClientFile( "clients.dat", ios::out );
if ( !outClientFile )
{
cerr << "File could not be opened" << endl;
exit( 1 );
}
int account = 12;
char name[80] = "This Is A Test";
double balance = 123.123;
outClientFile << account << " " << name << " " << balance << endl;
return 0;
}</source>
Demonstrate peek(), unget(), and ignore().
<source lang="cpp">#include <iostream>
- include <fstream>
- include <cctype>
using namespace std; int main() {
char ch;
char idnum[5];
idnum[4] = 0;
ofstream fout("test.dat");
if(!fout) {
cout << "Cannot open test.dat for output.\n";
return 1;
}
fout << "A #pending\n, B, #8875\n";
fout.close();
if(!fout.good()) {
cout << "Error creating data file.";
return 1;
}
ifstream fin("test.dat");
if(!fin) {
cout << "Cannot open test.dat for input.\n";
return 1;
}
fin.exceptions(ios::badbit | ios::failbit);
try {
do {
fin.ignore(40, "#");
if(fin.eof()) {
fin.clear(); // clear eofbit
break;
}
ch = fin.peek();
if(isdigit(ch)) {
fin.read((char *)idnum, 4);
cout << "ID #: " << idnum << endl;
} else {
cout << "ID not available: ";
ch = fin.get();
while(isalpha(ch)) {
cout << ch;
ch = fin.get();
};
fin.unget();
cout << endl;
}
} while(fin.good());
} catch(ios_base::failure exc) {
cout << "Error reading data file.\n";
}
try {
fin.close();
} catch (ios_base::failure exc) {
cout << "Error closing data file.";
return 1;
}
return 0;
}</source>
ofstream: seek from beginning
<source lang="cpp">#include <iostream.h>
- include <fstream.h>
- include <stdlib.h>
main(int argc,char *argv[]) {
ofstream out("test.txt");
if(!out)
{
cout << "Cannot open file";
return 1;
}
out.seekp(2,ios::beg);
out.put("X");
out.close();
return 0;
}</source>
Read char array from a file
<source lang="cpp">#include<iostream.h>
- include<fstream.h>
main() {
ofstream out("test.txt");
if(! out)
{
cout<<"Cannot open output file\n";
return 1;
}
char nums[4]={"a","b","c","d"};
out.write((char *)nums,sizeof(nums));
out.close();
ifstream in("test.txt");
if(! in)
{
cout<<"Cannot open input file\n";
return 1;
}
in.read((char *)& nums,sizeof(nums));
for(int i=0;i<4;i++)
cout<<nums[i]<<" ";
cout<<"\n";
cout<<in.gcount()<<"characters read\n";
in.close();
return 0;
}</source>
a b c d 4characters read
Save and read double array in a binary file
<source lang="cpp">#include <iostream>
- include <fstream>
using namespace std; int main() {
double fnum[4] = {9.5, -3.4, 1.0, 2.1};
int i;
ofstream out("numbers", ios::out | ios::binary);
if(!out) {
cout << "Cannot open file.";
return 1;
}
out.write((char *) &fnum, sizeof fnum);
out.close();
for(i=0; i<4; i++) // clear array
fnum[i] = 0.0;
ifstream in("numbers", ios::in | ios::binary);
in.read((char *) &fnum, sizeof fnum);
// see how many bytes have been read
cout << in.gcount() << " bytes read\n";
for(i=0; i<4; i++) // show values read from file
cout << fnum[i] << " ";
in.close();
return 0;
}</source>
32 bytes read 9.5 -3.4 1 2.1 "
Save and read structure
<source lang="cpp">#include <iostream>
- include <fstream>
- include <cstring>
using namespace std; struct MyRecord {
char name[80]; double balance; unsigned long account_num;
}; int main() {
struct MyRecord acc;
strcpy(acc.name, "R");
acc.balance = 1.3;
acc.account_num = 34;
ofstream outbal("balance", ios::out | ios::binary);
if(!outbal) {
cout << "Cannot open file.\n";
return 1;
}
outbal.write((char *) &acc, sizeof(struct MyRecord));
outbal.close();
ifstream inbal("balance", ios::in | ios::binary);
if(!inbal) {
cout << "Cannot open file.\n";
return 1;
}
inbal.read((char *) &acc, sizeof(struct MyRecord));
cout << acc.name << endl;
cout << "Account # " << acc.account_num;
cout.precision(2);
cout.setf(ios::fixed);
cout << endl << "Balance: $" << acc.balance;
inbal.close();
return 0;
}</source>
R Account # 34 Balance: $1.30"
Save char array to a file
<source lang="cpp">#include<iostream.h>
- include<fstream.h>
main() {
ofstream out("test.txt");
if(! out)
{
cout<<"Cannot open output file\n";
return 1;
}
char nums[4]={"a","b","c","d"};
out.write((char *)nums,sizeof(nums));
out.close();
ifstream in("test.txt");
if(! in)
{
cout<<"Cannot open input file\n";
return 1;
}
in.read((char *)& nums,sizeof(nums));
for(int i=0;i<4;i++)
cout<<nums[i]<<" ";
cout<<"\n";
cout<<in.gcount()<<"characters read\n";
in.close();
return 0;
}</source>
a b c d 4characters read
Save chars to a binary file
<source lang="cpp">#include <iostream>
- include <fstream>
using namespace std; int main() {
int i;
ofstream out("test.dat", ios::out | ios::binary);
if(!out) {
cout << "Cannot open output file.\n";
return 1;
}
// write all characters to disk
for(i=0; i<256; i++) out.put((char) i);
out.close();
return 0;
}</source>
Writing numbers to a file
<source lang="cpp">#include <fstream>
- include <iomanip>
int main() {
std::ofstream outFile("test.txt");
for(int i = 0 ; i < 100 ; i++) {
if(i 5 == 0)
outFile << std::endl;
outFile << std::setw(10) << i;
}
return 0;
}</source>
12. 9. 3. Writing on a text file
<source lang="cpp">
- include <fstream.h>
int main () {
ofstream examplefile ("test.txt");
if (examplefile.is_open())
{
examplefile << "This is a line.\n";
examplefile << "This is another line.\n";
examplefile.close();
}
return 0;
}</source>
