C++ writing string to file = extra bytes

Question

Hello, I am using c++ to look through 256 counts and write the ASCII representative to a file.

If i use the method of generating a 256 character string then write that string to the file, the file weighs 258bytes.

string fileString = "";

//using the counter to attach the ASCII count to the string.
for(int i = 0; i <= 256; i++)
{
 fileString += i;
}

file << fileString;

If i use the method of writing to the file withing the loop, the file is exactly 256bytes.

//using the counter to attach the ASCII count to the string.
for(int i = 0; i <= 256; i++)
{
 file << (char)i;
}

Whats going here with the string, what extra information from the string is being written to the file?

Answer 1

Both of these create a 256 byte file:

#include <fstream>
#include <string>

int main(void)
{
    std::ofstream file("output.txt", std::ios_base::binary);
    std::string fileString;

    for(int i = 0; i < 256; i++)
    {
     fileString += static_cast<char>(i);
    }

    file << fileString;
}

And:

#include <fstream>
#include <string>

int main(void)
{
    std::ofstream file("output.txt", std::ios_base::binary);
    std::string fileString;

    for (int i = 0; i < 256; ++i)
    {
     file << static_cast<char>(i);
    }

    file.close();
}

Note, before you had an off-by-one error, as there is no 256th ASCII character, only 0-255. It will truncate to a char when printed. Also, prefer static_cast.

If you do not open them as binary, it will append a newline to the end. My standard-ess is weak in the field of outputs, but I do know text files are suppose to always have a newline at the end, and it is inserting this for you. I think this is implementation defined, as so far all I can find in the standard is that "the destructor can perform additional implementation-defined operations."

Opening as binary, of course, removes all bars and let's you control every detail of the file.

---

Concerning Alterlife's concern, you can store 0 in a string, but C-style strings are terminated by 0. Hence:

#include <cstring>
#include <iostream>
#include <string>

int main(void)
{
    std::string result;

    result = "apple";
    result += static_cast<char>(0);
    result += "pear";

    std::cout << result.size() << " vs "
     << std::strlen(result.c_str()) << std::endl;
}

Will print two different lengths: one that is counted, one that is null-terminated.

Answer 2

Hi, im not too gud with c++ but did you try initializing the filestring variable with null or '/0' ?? Maybe then it will give 256byte file..

N yea loop should be < 256

PS: im really not sure but then i guess its worth trying..