Perhaps they were trying to block people from using the C string functions that allow all the viruses that go beyond the read buffer length.jtbell said:
It's really very simple. It is triggered by the error of trying to read data beyond the end of the valid data in the file. The valid data in the file includes any '\0' there indicating the end of a string.yungman said:
yungman said:
You will not understand what is happening until you realize that "Michael" is only being read once.pbuk said:
#include <iostream>
#include <fstream>
using namespace std;
int main()
{//Create a file and write int 10 in it, only one integer data.
int num = 10;
fstream testFile;
testFile.open("demofile.txt", ios::out);
cout << " Writing 10 " << num << " to file.\n";
testFile << num;
cout << " testFile.eof() = " << testFile.eof() << "\n\n";
testFile.close();// close file
//Read from file that has only one integer data.
testFile.open("demofile.txt", ios::in);//open for read
cout << " Read file. \n";
testFile >> num; //read file to num
cout << " The value " << num << " was read.\n\n";
cout << " testFile.eof() = " << testFile.eof() << "\n\n";
return 0;
} do
{
people.read(reinterpret_cast<char*>(&person), sizeof(person));
} while (!people.eof());Yes. one single number 10.FactChecker said:
Thanks, I read the program in the link, seems it's like what the eof checking program in post 40...That after reading the last byte, the pointer advances and pointing to '\0', thereby the eof bit is set right away as shown in post 40. this is the copy of program from cplusplus you linked:FactChecker said:
// ios::eof example
#include <iostream> // std::cout
#include <fstream> // std::ifstream
int main () {
std::ifstream is("example.txt"); // open file
char c;
while (is.get(c)) // loop getting single characters
std::cout << c;
if (is.eof()) // check for EOF
std::cout << "[EoF reached]\n";
else
std::cout << "[error reading]\n";
is.close(); // close file
return 0;
}#include <iostream>
#include <fstream>
int main () {
std::ifstream is("example.txt");
char c;
while (is.get(c)) // When does this loop condition become false?
std::cout << c;
if (is.eof()) // eof is reached after the loop condition became false
std::cout << "[EoF reached]\n";
else
std::cout << "[error reading]\n";
is.close();
return 0;
}https://en.cppreference.com/w/cpp/io/basic_ios/eof
The program you're referring to is not working with C-strings. As I already said, the null character, '\0', is used to indicate the end of a string. The null character is NOT used to mark the end of a file.yungman said:
Post #44pbuk said:
That name was NOT read twice!yungman said:
No, the two programs are not alike. In the first (post #28, I think), you opened the file in binary mode, and used read() to extract the data. The more recent program, in post #40, works with a text file, and uses the stream extraction operator, >>, to get the data.yungman said:
#include <iostream>
#include <fstream>
using namespace std;
int main()
{ //Create a file and write an int value into it.
int num = 10;
int val;
fstream testFile;
testFile.open("demofile.txt", ios::out|ios::binary); // Open for writing in binary mode
cout << " Writing " << num << " to file.\n";
//testFile << str; // From earlier program
testFile.write((char*)&num, sizeof(num));
cout << " testFile.eof() = " << testFile.eof() << "\n\n";
testFile.close();// close file
testFile.open("demofile.txt", ios::in|ios::binary); //Open for reading in binary mode
cout << " Read file. \n";
//testFile >> str2; // From earlier program
testFile.read((char*)&val, sizeof(val));
cout << " The value " << val << " was read.\n\n";
cout << " testFile.eof() = " << testFile.eof() << "\n\n";
testFile.close();
} Writing 10 to file.
testFile.eof() = 0
Read file.
The value 10 was read.
testFile.eof() = 0Thanks, this is very helpful. The book just doesn't say anything about this. You would think for files, eof() means the same thing. I've been doing more experiments:Jarvis323 said:
#include <iostream>
#include <fstream>
#include <string>
using namespace std;
int main()
{
char name[100];
fstream people;
people.open("people.dat", ios::out | ios::binary);
people << "alan ";
people << "Paul ";
people << "john ";
people << "michael";
people.close();
people.open("people.dat", ios::in | ios::binary);
do
{
people >> name;
cout << " Name: " << name << " !people.eof() = " << !people.eof() << "\n\n";
} while (!people.eof());
people.close();
}I KNOW what you said before, I am not convinced, see the post I reply to Jarvis. I don't think the book will ignore this if that is true. I think some else is going on.Mark44 said:
I think it's time for me to move on. Someone gave me a link that really solve the problem, I cannot find that post anymore. Basically it do the read first in line 2 before enter the do-while loop and it works.Mark44 said:
people.open("people.dat", ios::in | ios::binary);
people.read(reinterpret_cast<char*>(&person), sizeof(person));
do
{
cout << " Name: " << person.name << "\n";
cout << " Age: " << person.age << "\n";
cout << " Enter address line 1: " << person.address1 << "\n\n";
cout << " Enter address line 2: " << person.address2 << "\n\n";
cout << " Enter phone number: "<< person.phone << "\n\n";
people.read(reinterpret_cast<char*>(&person), sizeof(person));
index++;
cout << " index = " << index << " !people.eof() = " << !people.eof() << "\n\n";
} while (!people.eof());What do you mean typed language?sysprog said:
I mean that when you pass values, the 'type' of the value, for example 'character', is enforced ##-## if you try to do an arithmetic operation on character data your program will fail ##-## e.g. you can't multiply L by Myungman said:
The original program was using read() to do the extraction from the file. The behavior with eof() has nothing to do with the string being in a struct.yungman said:
yungman said:
What's going on is that read() extracts a specific number of bytes, and the extraction operator >> extracts characters of a string until it reaches the null character.yungman said:
What's the rush? Skipping over things you don't understand will almost certainly come back to bite you in the butt. And "hurrying up" has caused many problems for you in the past couple of months.yungman said:
#include <iostream>
#include <fstream>
using namespace std;
int main()
{
char testStr[] = "cats";
char num = 65;
char val;
char testStr2[5];
char testStr3[5];
char ch;
fstream testFile;
testFile.open("demofile.txt", ios::out|ios::binary);
cout << " Writing the string \"cats\"" <<" and a byte to file.\n";
testFile.write(testStr, sizeof(testStr));
testFile.write((char*)&num, sizeof(num));
testFile.close();
testFile.open("demofile.txt", ios::in|ios::binary);
cout << " Read file in binary mode using read(). \n";
testFile.read(testStr2, sizeof(testStr2));
testFile.read((char*)&val, sizeof(val));
cout << " String: " << testStr2 << " and value: " << val << "\n";
cout << " testFile.eof() = " << testFile.eof() << "\n";
ch = testFile.get();
cout << " After get(), testFile.eof() = " << testFile.eof() << "\n\n";
testFile.close();
testFile.clear(0); // Clear all flags - eofbit, failbit, badbit
ch = 'Z';
val = 0x7F;
testFile.open("demoFile.txt", ios::in);
cout << " Read file in text mode using extraction operator. \n";
testFile >> testStr3;
testFile >> ch;
cout << " After extracting ch, testFile.eof() = " << testFile.eof() << "\n";
testFile >> val;
cout << " After extracting val, testFile.eof() = " << testFile.eof() << "\n";
cout << " String: " << testStr3 << " and value: " << val << "\n";
ch = testFile.get();
cout << " After get(), testFile.eof() = " << testFile.eof() << "\n";
testFile.close();
} Writing "cats" and a byte to file.
Read file in binary mode using read().
String: cats, value: A // Both values are read
testFile.eof() = 0 // After read() of the string, and read() of the byte, we're not at end of file
After get(), testFile.eof() = 1 // One more read, using get() and eof() is true
Read file in text mode using extraction operator.
After extracting ch, testFile.eof() = 0 // String extracted, and null (ch - with value '\0') extracted - no eof.
After extracting val, testFile.eof() = 0 // val extracted - no eof.
String: cats, value: A // Both values are read
After get(), testFile.eof() = 1 // Final read, using get() and eof() is true.Mark44 said:
yungman said:
No, you put a space after the first three names, but not the fourth.yungman said:
No, it's not according to the datatype that will be read -- it's according to the number of bytes that are supposed to be read. If the call to read() indicates that N bytes should be read, and read processes N bytes, then eof() is false. If read() is able to read only M bytes, where M < N, the eof() returns true.yungman said:
testFile.read(testStr2, sizeof(testStr2));The 2nd argument to read() is the number of bytes to read.yungman said:
Goals are based on plans, and plans are subject to some events that are difficult to forecast correctly. When that happens, the wise thing to do is to reassess the plans and possibly adjust the timeline for the goals.yungman said:
And does it ever happen that it turns out that your goals are unrealistic and you have to do an inadequate and rushed job to meet them instead of just taking longer and doing a good job? Maybe you set the wrong goal. You set a TIME goal. Why not set a LEARNING goal instead?yungman said:
phinds said:
Yeah, but he's setting very brief time goals.Vanadium 50 said:
Reminds me of:Vanadium 50 said: