Thread: Problem with .write() function

Hi dudes.

For some reason this works:

Code:

    ofstream out("filename.dat");
    char x[3]="ol";
    out.write(x, 1);

and this doesnt:

Code:

    ofstream out("filename.dat");
    unsigned char x[3]="ol";
    out.write(x, 1);

and this second code returns me the folowing error: invalid conversion from 'unsigned char *' to 'const char*'

but i dont know why, why cant i use an unsigned char instead of a char. and even in some books the examples use an unsigned char.

Im using the correct headers:

Code:

#include <iostream>
#include <fstream>
#include <ctype.h>

any ideas??

The first argument to the write method of ofstream is a const char *; therefore if you want to write data that is NOT a const char *, you must force it to be a const char *.

Code:

ofstream out("filename.dat");
unsigned char x[]="ol";
out.write(reinterpret_cast<const char *>(x), 1);

I thought that wouldn't matter.

The const here means write() promises it won't change the string. It's totally fine if your original string is non-const.

Why not just use char?

Unsigned/signed chars only make sense when you are using them to store numbers. For characters, the default signedness will always do the right thing.

Yes i know i can use just char, but what really drives me insane is that they use it in tutorials and books, then why doesnt it work for me?? for example, my code was taken from an example in a book.

Originally Posted by shiroaisu

Yes i know i can use just char, but what really drives me insane is that they use it in tutorials and books, then why doesnt it work for me?? for example, my code was taken from an example in a book.

Are you sure [it was like that in the book]? In that case, throw away the book, it's crap. You can't do that. Also, mention what book it is. Let's make a "C books not to buy" thread..

You should check the book's errata webpage, especially after you notice or otherwise encounter errors in book examples.

Not to argue with the idea of a bad books thread, but there are good, instructive books with errors, and bad ones with inexcusable problems.

It's better to not use write at all.

Unless you are using a stream in binary mode.

The the thing that the write function needs a cast is to discourage its use.

>_<

Could you not do that? If this poor newbie takes such complete crap to heart he'll have a bad start when he wants to operate on binary files.

Soma

o_O

So your argument for dishing out such trash is that one could use something different?

*shrug*

Okay. I can dig it.

It's better to not use C++ at all. You have the Ruby* language, so it's a good idea to actually use Ruby. It will ensure portable code without having to bother with casts.

The thing is that the template functions cause "bloat" to discourage their use.

The C++ formatted IO manipulators offer no advantage whatsoever over its C siblings. In fact, it's more cumbersome.

But with some manual labour, the inefficiencies of a system can be fixed or mitigated.

Awesome.

Soma

* picked randomly

Plus binary files aren't portable.

O_o

WTF!?

Plus binary files aren't portable.

o_O

That explains a lot.

Soma