CODEWITHSUNDEEP
c++file-iobinary
c0da
c0da

Reputation: 1009

Writing unsigned chars to a binary file using write()

I was making a program that read binary files. So, I read the individual bytes into unsigned chars (actually reading the data as chars and casting them to unsigned chars for each character). Now I have to write back the unsigned chars to the binary file.

The problem is that now I am forced to write individual bytes after casting them to chars (because write() for binary files expects char* buffer). So, now i have to do the following:

for(int x=0; x<data_size; x++)
{
    ch=(char)data[x];
    outfile.write(&ch,1);
}

Is there any way to get around this thing so that the amount of I/O operations are reduced in case of reading and writing?

Upvotes: 18

Views: 33044

Answers (3)

Armen Tsirunyan
Armen Tsirunyan

Reputation: 132994

The type of outfile is ofstream, right? ofstream is a typedef for, 

typedef std::basic_ofstream<char, std::char_traits<char> > ofstream;

You need your own typedef, 

typedef std::basic_ofstream<unsigned char, std::char_traits<unsigned char> > uofstream;

And then, 

uofstream outfile(...);
outfile.write(data, data_size); //no need to cast

Upvotes: 6

6502
6502

Reputation: 114481

You can do the casting on a pointer...

outfile.write((char *)&data[0], data_size);

the same can be done for reading (i.e. just pass a pointer to the first element of an array of unsigned char casting it to a pointer to char).

Upvotes: 29

Nobody moving away from SE
Nobody moving away from SE

Reputation: 9547

When data is of type unsigned char* or unsigned char[] and you just want to write the bits into the file do a pointer cast:

for(int x=0; x<data_size; x++)
{
outfile.write((char*)data + x, 1);
}

As casting removes the issue of one at a time writing:

outfile.write((char*)data, data_size);

And you do it all at once. Note that this outdoes the type checking and therefore is not the best solution.

Upvotes: 2

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