fstream.write()

**agerealm** · 09-30-2002

im currently studying how to manipulate files using seekp, seekg, etc, and i am attempting to use the command fstream.write. heres the snippet of code i've currently got:

Code:

....
  fstream fileObject("data.dat",ios::out);
....

  cout << "enter id: ";
  cin >> temp->ID;
  cout << "enter name: ";
  cin >> temp->name;
  cout << "enter birthdate: ";
  cin >> temp->birthdate;
  cout << "enter position: ";
  cin >> temp->position;

  fileObject.write(reinterpret_cast<const char *>(temp),sizeof(temp));

and heres what i get in the data.dat file:

Code:

i have a feeling its got something to do with the reinterpret_cast<const char*> bit, and i was hoping someone could please give me any insight into this peculiar problem. thanks a lot in advance! (borland 5.02)

**FillYourBrain** · 09-30-2002

is temp a struct or a class? you shouldn't do that sort of thing with a class. A struct might be fine.

aside from that, you are writing this stuff out as binary so the name is the only thing that might be readable to you (that's if it isn't dynamic memory).

**agerealm** · 09-30-2002

temp is a class. whats wrong with using a class over a struct? thanks.

**FillYourBrain** · 09-30-2002

Well, I suppose it's not entirely that way. struct can be just as bad. The problem comes when your data location is unsure. I would prefer to make a call like this:

temp->Save(fileobject);

or something. This would allow an object like temp to control how it saves.

if you used a std::string for "name" it will not work cause you'll get memory addresses instead of string data. Things like that are why you shouldn't just write out a class that way.

**hk_mp5kpdw** · 09-30-2002

Can't you just do something like:

Code:

fileObject << temp->ID << endl
           << temp->name << endl
           << temp->birthdate << endl
           << temp->position << endl;

**agerealm** · 09-30-2002

i could, but i want to figure out how using the write() command

**FillYourBrain** · 09-30-2002

write() is just bytes. You have to make sure you have all of the bytes you really want to save is all.

Shadow12345 · 09-30-2002

This is irrelavent to anything, but
fstream fileObject("data.dat",ios: out);
could be written as
ofstream fout;
just declares an instance of output file stream.
and then you just call
fout.open("data.dat")

I dunno, it isn't important, and your way works fine

**hk_mp5kpdw** · 09-30-2002

write() should only be used with character data. If any part of whatever you are planning to output with write() contains binary data, as in any integers/floats or the like, you will need to use some other method. One problem is that binary data written out by this function could possibly be interpretted as an EOF character which would effectively close the file even though you think you are still writing more data to it.

**elad** · 09-30-2002

temp is a pointer to a class object but you want to save the data in the class object being pointed to. Therefore I wouldn't try to write() temp, I'd try to write() *temp.

fileobject.write(reinterpret_cast<char*> *temp, sizeof(*temp));

**FillYourBrain** · 09-30-2002

that's incorrect hk_mp5kpdw. write is what you use for binary specifically.

**FillYourBrain** · 09-30-2002

elad, I hadn't caught that. dang that sucks. either way, there's no way of knowing what's going on inside a given class, you should never just write out the entire thing as binary.

**hk_mp5kpdw** · 09-30-2002

that's incorrect hk_mp5kpdw. write is what you use for binary specifically.

Sorry bout that... brain fart. I was thinking of one thing but typing something else.

**Sang-drax** · 09-30-2002

Actually you're right in a way..

The problem lies in the reinterpret_cast.

The class contains binary data, when converted to a char* the wite method prints what the class contains, byte for byte.

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