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I am intending to do a Phone-book program. I am thinking of creating some 'Person' Class. I want to write the names to the hard-disk. I am thinking of using binary read/write.
I know that you have to use some 'read( )' / 'write( )' function.
But I don't have a clue on binary read/write. :confused:
Could some-one help me with binary read/writing of objects ?
It will be sufficient if you just refer some tutorial site.
In text mode you have fancy things like << operator, format strings (like those acepted by printf and so on), file read line by line, char* and std::string.
In binary mode, the content of the file is ignored. Simply read n bytes to a buffer with allocated memory, or write those n byte from the buffer to the file. I don't know binary io in iso c++, but in ansi c is simply something like
fread(buffer, size,1,file) and fwrite(buffer, size,1,file)
since you don't know what the flie can store (that is, only by looking at it's content), you need to control well where each piece of binary information will be placed (at which byte offset).
Sorry for not informing :(
This code is in c++.
and as for the byte offset it is obviously the size of the oject ('Person' here) is'nt it???Quote:
since you don't know what the flie can store (that is, only by looking at it's content), you need to control well where each piece of binary information will be placed (at which byte offset).
yes, maybe. If you store two obecjt of type Person in a file, the first will be a offset 0, the second at offset sizeof(Person), and so on. Note that if Person contains pointer to memory or classes that internally keep pointers, when loading the object from the file those pointer wont be valid for sure, and you'll get segmentation faults
Thanks I'll keep that in mind.
I'm only new and my 'person' is very primitive and contain ony two (char*)s.
Binary reading/writing in C++ is done with the appropriate stream's read/write member function:
Code:#include <fstream>
...
// Reading
std::ifstream in("input.txt",std::ios::binary|std::ios::in);
...
in.read(...);
// Writing
std::ofstream out("output.txt",std::ios::binary|std::ios::out);
...
out.write(...);
>I am thinking of creating some 'Person' Class.
Cool.
>I want to write the names to the hard-disk.
More power to ya.
>I am thinking of using binary read/write.
Erm, probably not a good idea. Unless your Person class matches the very strict rules of a POD type, you'll be invoking undefined behavior. A safer method would be to work out some way to serialize the object so that it can be safely written to file (with the corresponding unserialize function to read it back and create an object safely). It could be as simple as making a string with the members formatted nicely:
Then you can simply write the resulting string to file:Code:// Serialize the first and last name
std::string Person::toString() const
{
return std::string(first + '|' + last)'
}
But don't use read() and write() unless you know the pitfalls, which you don't seem to.Code:file << myperson.toString();
Just a question...what exactly are the pitfalls? I have used read and write quite often and I havent found any problems with that so far.
You have to learn sometime. If you want to learn how to do this given fair warning, thenhere's a reference. Start at C++ I/O and work your way through the options until you get to read() and write().
http://www.cppreference.com/
Some people will recommend you use C++ stlye casting rather than the C style casting that is demonstrated in the example provided at the above site, but that's a relatively minor problem.
Is there a particular reason why you want to read and write all bytes of a "person" at once rather than with serial reads of each member data? All data written to and read from file is eventually involve binary data. Since most non-computers have more difficulty reading binary/hex than text, many programs do the conversions for you. To my knowledge you don't gain any "efficiency" by writing to file in binary and as already indicated if any data member are or contain memory addresses rather than primary data, then you will not be able to utilize direct binary read without some conversion anyway.
First of all iused binary read / write beacause sinece every persons names are different, there would be no way of knowing where a person ends (everybody's names are different.)
But in binary read / write, every object has the same length (every bit is written).
so it is easier(or so I thought.)
now I'm stuck on some file size problem in the same program.
I'mposting it too...
Are'nt I Annoying? :D
Quote:
Originally Posted by Shakti
Tell me this, if your person object contains two char* members representing the first and last names of the person (as mentioned by the poster), then if you simply write out the data for both these members to a file how do you know how much data to read back in later? If my name is "Bill Clinton" and I simply write out "BillClinton" to a file, when it comes time for me to read this back in, how many of those characters do I allocate to the first name and how many do I consider to belong to the last name? Do I read "BillC" into the first name and "linton" into the last? This problem comes up with any data that may be of variable length. Char pointers, strings, vectors of objects, etc... Unless your object is of fixed length, properly reading and writing the data to the file becomes more complicated.
That's why I was forced for a binary read/write.
Really. Why can't a person's name end with a space or a newline?Quote:
First of all i used binary read / write beacause sinece every persons names are different, there would be no way of knowing where a person ends (everybody's names are different.)
I think you have that backwards. If you read and write in binary mode, you have no way of knowing how many bytes to read for a person's name. If you think you do, post how many bytes you are going to read for a name, and it will be trivial to show you a name that won't work.Quote:
Quote:
if you simply write out the data for both these members to a file how do you know how much data to read back in later?
Quote:
That's why I was forced for a binary read/write.
The fact that you don't know the exact number of bytes to read in seems to me to scream out DON'T USE BINARY MODE. Instead, your situation calls for using text mode and the insertion and extraction operators( << and >>) for formatted stream input and output.
In my experience, you would do well to follow any advice Prelude gives you.
Okay...that site wasn't exactly helpful. Good reference, but it didn't say anything about "pitfalls".Quote:
Originally Posted by elad
Write it out as pascal strings instead of C-strings.Quote:
Originally Posted by hk_mp5kpdw
I was unaware of any hidden pitfalls of using read() and write(). As long as you are careful to serialize everything so that it makes sense when you read it back in, it works fine. Am I missing something?
>If you read and write in binary mode, you have no way of knowing how many bytes to read for a person's name.
Well, you could just use a plain ole char array, and if it's part of a struct, read and write sizeof(struct).
>I was unaware of any hidden pitfalls of using read() and write().
They aren't hidden, just subtle. Here's a taste of some of the issues you need to consider when using a binary file.
Pitfall #1 (the one I was referring to): Put simply, unless you can directly convert an object to a pointer to unsigned char without invoking undefined behavior, binary I/O using that object is also undefined. Any POD type can be safely cast to a pointer to unsigned char; this is guaranteed by the standard. If you don't know whether your class is POD or not then it probably isn't, and binary I/O is undefined.
Pitfall #2: Binary I/O is a shallow copy. If your object (assuming it's POD) has pointers to dynamic memory then you'll be copying addresses, not the content of those addresses.
Pitfall #3: Binary I/O is not portable at all. You can write to a file on one platform and fail to read it on another platform (even a different version of the same platform). Even worse, differen implementations or versions of the same implementation can fail to handle the file properly if you mix them.
Of course, if your object is not POD, but you can convert it to some representation using only POD types, then you can get around pitfalls #1 & #2. For example, as has been said already, converting a C++ string to its length and its character data before outputting that information in binary, and then converting it back into a string after reading it in, is a legitimate way to read/write a string object in binary.
Why not just write the char array in text mode and read it back in text mode?Quote:
Well, you could just use a plain ole char array, and if it's part of a struct, read and write sizeof(struct).
Because the question was how to do it in binary mode.Quote:
Originally Posted by 7stud
>is a legitimate way to read/write a string object in binary
Yes, as I said in my first post. But the OP apparently wanted to do it directly, without the intermediate step of converting the object to something safe, which is all kinds of wrong.
I have have decided to avoid all this ambiguity and arguments simply by going back to the normal read / write.
P.S.(I have to since I'm sTuck on a filesize problem)
I take back what I just said!!!
I got help in correcting a function whose argument I mixed up.
Binary is allmost all OK now. :D :D :D
>Binary is allmost all OK now.
How are you doing it now? Just to get a concrete view, can you show us your class declaration and how you write objects to the file? It's possible that it really is okay, but it's also possible that it's still undefined, but hasn't broken loudly yet.
My code isn't very pretty; in fact you might just pass out seeing its horrible state.
Any way I will post it afte I finish a certain function(Which obviously creates problems!).
I edited this post cause I found a stupid mistake in my person class.
This is my program. not all functions have been implemnted.
Don't take this thing very seriously, since I have abandoned the same project tons of times due I/O problems. Whenever theres a cin any where in the program before a cin.getline() it is ignored by the program, ie, the program does not wait for input. also the name is not copied on to the object; only the number is copied.
don't know why.
You were right; binary isn't my cup o' tea.
The class.
program.Code:class Person{
private:
char name[30], numb[15];
public:
Person(){
name[0] = '\0';
numb[0] = '\0';
}
char* get_name(){ return name; }
char* get_numb(){ return numb; }
void set(char*, char*);
};
void Person::set(char* na, char* n1){
for(int i=0; na[ i] !='\0'; i++)name[ i] = na[ i]; //This was edited and the next line added.
name[ i] = '\0';
for(i=0; n1[ i] !='\0'; i++)numb[ i] = n1[ i];
numb[ i] = '\0';
}
Code://Note the size of buffer used to read all the records.
//It might need expansion or some thing.
//deletion of entries not implemented.
//edit of entries not implemented.
const char *substr(const char *match, const char *s); //remember me?
void list_record(Person*, int);
void search_record(Person*, int);
void add_record(Person*, int&);
void delete_record(Person*, int&);
void edit_record(Person*, int);
void refresh_file(Person*, int, fstream &);
long read_from_file(Person*, fstream &); //transfer all records to Person*.
int main(){
clrscr();
fstream myfile;
myfile.open("Numbers.imp", ios::in | ios::binary | ios::app);
if(myfile.fail())cout<<"E R R O R Opening File !\n";
else{
Person buffer[30];
int size = read_from_file(buffer, myfile); //every thing is in buffer.
int n = size/sizeof(Person);
char ch;
cout<<"No: of saved records :"<<n<<endl;
myfile.close(); //Let's close it for now...
do{
cout<<"\n<< M E N U >>\n"
<<"1) List ('l')"<<endl
<<"2) Search ('s')"<<endl
<<"3) Add New Entry ('n')"<<endl
<<"4) Delete an entry ('d'){not implemented yet}"<<endl
<<"5) Edit an entry ('e') {n i y}"<<endl
<<"6) Quit ('q')"<<endl
<<"Your Choice :";
cin>>ch; //I think this cin causes cin.getline() problems.
//cause when I changed it to getche(), the program waited for input
//but name was'nt copied to object still.
switch(ch){
case 'l':list_record(buffer, n);
break;
case 's':search_record(buffer, n);
break;
case 'n':add_record(buffer, n);
break;
case 'd':delete_record(buffer, n);
break;
case 'e':edit_record(buffer, n);
break;
case 'q':refresh_file(buffer, n, myfile);
cout<<"Records updated...\n\n\n"
<<"Exiting...\n\n\nPress any key...";
getch();
exit(0);
default :cout<<"Invalid Choice !!!\n";
}
}while(1);
}
return 0;
}
//Is there a file size function???below
long read_from_file(Person* buff, fstream &file){
file.seekg(0, ios::beg); file.seekp(0, ios::beg);
long start = file.tellg(), end, size;
file.seekg(0, ios::end);
end = file.tellg();
size = end-start;
file.seekg(0, ios::beg);
file.read((unsigned char*) &buff, size); //Copies records to buffer.
return size;
}
void list_record(Person* buff, int n){
if(n){
cout<<endl;
for(int i=0; i<n; i++)
cout<<i+1<<") "<<buff[ i].get_name()<<"\t\t\t"<<buff[ i].get_numb()<<endl;
for(i=0; i<20; i++)cout<<"==";
cout<<endl;
}
}
void search_record(Person* buff, int n){
char na[30];
cout<<"Enter the name or a part of the name :";
cin.getline(na, 30); //This isn't working...
for(int i=0; i<n; i++)
if(substr(na, buff[ i].get_name()) )
cout<<i+1<<") "<<buff[ i].get_name()<<"\t\t\t"<<buff[ i].get_numb()<<endl;
for(i=0; i<20; i++)cout<<"==";
cout<<endl;
}
void add_record(Person* buff, int& n){
char name[30], number[15];
cout<<"Enter the name :";
cin.getline(name, 30); //This isn't working... program not waiting for input.
cout<<"Enter the number :"; //The program jumps to this line instead
cin.getline(number, 15); //But here the machine DOES wait for input.
//I think the problem is in the 'cin>>ch;' in the
//previous 'switch'. but dont know why.
Person a;
a.set(name, number);
buff[ n]=a;
n++;
cout<<"Record added :"<<endl;
}
Mixing getline() and >> in the same program often causes this problem. The problem arises because, for the most part, input in C++ is buffered. That is, input goes from the keyboard to a buffer before it is placed into variables. Istream methods extract the information from the input buffer and attempt to place into the variable(s) indicated in the code. Each istream method works a little differently, giving you a lot of flexibility, as well as a few headaches as you try to figure them all out. In this case, >> doesn't remove the terminating whitespace char. The newline char is considered a whitespace char. >> is frequently terminated by newline char (when you push the enter key a newline char is entered into the input buffer). The default terminating char is also the newline char. Therefore if you call >> before getline() and the call to >> is terminated by the newline char, the newline char will remain in the input buffer. When getline() goes to retrieve information from the input buffer the first thing it sees is the newline char and ends up not putting anything except the terminating null char in the desired char array. You don't even have to enter anything because the newline char is already there. The program just "magically" skips right over the getline() call. To deal with this you need to call ignore() before each call to getline(). The best way is to go all out and ignore as many char as could possibly be stored in the input buffer, although usually just ignoring 1 char is enough.
There are functions in the cstring (or string.h if your compiler doesn't have cstring) header file that allows you to copy strings one to another without your having to loop through the string to copy copying, one char at a time. You could look them up if you want. A common one to use is strcpy(). Your loop works fine, but most people will use strcpy.
Try this. Put a line like this:
cout << "sizeof Person" << endl;
before this line:
int n = size/sizeof(Person);
and see what you get. I think it might surprise you. Ithink you would be better served by making name and numb member variables of Person, where they belong, rather than global variables, and then writing their value to file with << and reading from file with >> rather than trying to use binary read/write.
Thanks a lot for the buffer info.
I did'nt use strcpy because then i would have include the complete header file containing other functions. I would then be wasting filesize for a measly srtring copy function. Any way I was looking for a file size returning function.
To get rid of this buffer thingi, cant I use cout<<flush; or cin>>flush; (I heard that this empties the buffer; but I'm not sure).
I'm trying to build another version which uses the normal( '>>', '<<' ) file I/O.
How can that line you suggested help?Quote:
Try this. Put a line like this:
cout << "sizeof Person" << endl;
before this line:
int n = size/sizeof(Person);