Change a text file in random access - c++

Question

I have a text file and want to change it in some places, for example in byte range 4030 to 4060, in this way:
if there is a character 'C' or 'c' followed by 'G' or 'g' ,must be changed in 'B' character
The input file is a text file and I want to get a changed text output file. There is no random access in text file and so I must open file in binary form and make changes, but the output file will be binary and I don't have any idea to get a text output. The code is below:
int main()
{
string str, cstr;
ReadTextFile("in", 4030, 4060);
return 0;
}
string ReadTextFile(string path, int from, int to)
{
fstream fp(path.c_str(), ios::in|ios::out|ios::binary);
char *target;
string res, str;
target = new char[to - from + 1];
if (!target)
{
cout << "Cannot allocate memory." << endl;
return "";
}
fp.seekg(from);
fp.read(target, to - from);
target[to - from] = 0;
res = target;
str = changestring(res);
fp.seekg(from);
fp.write((char *)&str, to-from);
return res;
}
string changestring(string str)
{
int l = str.length();
l = l-1;
for (int i = 0; i <= l; i++)
{
if (str[i] == 'C' || str[i] == 'c')
{
int j = i+1;
if (str[j] == 'G' || str[j] == 'g')
str[i] = 'B';
}
}
return str;
}

Answer 1

You misunderstand text and binary. It's not true that if you open a file in binary mode then the output will be binary. All binary mode does in practice is stop \n being translated to \r\n on Windows systems. In other words it affects the line endings in a text file, nothing else.
You are getting binary in your output because you are writing the pointers internal to a string to your output, write characters to your file instead.
Change
fp.write((char *)&str, to-from);
to
fp.write(str.c_str(), to-from);
This would also work
fp << str;
To get text output, write text. It doesn't matter whether you do that with write or with <<, text is just text. Stop thinking in terms of two modes of output, binary and text, it's not accurate, its what you write not how you write it that determines whether the output is text or binary.