Is it poosible to open the same file using wfstream and fstream - c++

Question

Actually i have a requirement wherein i need to open the same file using wfstream file instance at one part of the code and open it using fstream instance at the other part of the code. I need to access a file where the username is of type std::wstring and password is of type std::string. how do i get the values of both the variables in the same part of the code?
Like you can see below i need to get the values for username and password from the file and assign it to variables.
type conversion cannot be done. Please do not give that solution.
......file.txt.......
username-amritha
password-rajeevan
the code is written as follows:
#include <iostream>
#include <fstream>
#include <sstream>
#include <string>
int main()
{
std::string y;
unsigned int l;
std::wstring username;
std::wstring x=L"username";
std::wstring q;
std::string password;
std::string a="password";
std::cout<<"enter the username:";
std::wcin>>username;
std::cout<<"enter the password:";
std::cin>>password;
std::wfstream fpp("/home/aricent/Documents/testing.txt",std::ios::in | std::ios::out );
std::getline(fpp,q);
if (q.find(x, 0) != std::string::npos) {
std::wstring z=q.substr(q.find(L"-") + 1) ;
std::wcout<<"the username is:"<<z;
fpp.seekg( 0, std::ios::beg );
fpp<<q.replace(x.length()+1, z.length(), username);
}
fpp.close();
std::fstream fp("/home/aricent/Documents/testing.txt",std::ios::in | std::ios::out );
std::getline(fp,y);
if (y.find(a, 0) != std::string::npos)
{
unsigned int len=x.length()+1;
unsigned int leng=username.length();
l=len+leng;
fp.seekg(l+1);
std::string b=y.substr(y.find("-") + 1) ;
fp<<y.replace(a.length()+1, b.length(), password);
}
fp.close();
}

Answer 1

It's not recommended to open multiple streams to a same file simultaneously. On the other hand, if you don't write to the file, but only read (and thus, would be using ifstream and wifstream), that's probably safe.
Alternatively, you can simply open a wfstream, read the username, close the stream, open a fstream and read the password.
If you have the choice, avoid mixed encoding files entirely.

Answer 2

You should not try to open the same file with two descriptors. Even if it worked (read only mode for example), both descriptors would not be synchronised, so you would read first characters on one, and next same characters on second.
So IMHO, you should stick to one single solution. My advice is to use a character stream to process the file, and use a codecvt to convert from a narrow string to a wide wstring when you need it.
An example conversion function could be (ref: cplusplus.com: codecvt::in):
std::wstring wconv(const std::string& str, const std::locale mylocale) {
// define a codecvt facet for the locale
typedef std::codecvt<wchar_t,char,std::mbstate_t> facet_type;
const facet_type& myfacet = std::use_facet<facet_type>(mylocale);
// define a mbstate to use in codecvt::in
std::mbstate_t mystate = std::mbstate_t();
size_t l = str.length();
const char * ix = str.data(), *next; // narrow character pointers
wchar_t *wc, *wnext; // wide character pointers
// use a wide char array of same length than the narrow char array to convert
wc = new wchar_t[str.length() + 1];
// conversion call
facet_type::result result = myfacet.in(mystate, ix, ix + l,
next, wc, wc + l, wnext);
// should test for error conditions
*wnext = 0; // ensure the wide char array is properly null terminated
std::wstring wstr(wc); // store it in a wstring
delete[] wc; // destroy the char array
return wstr;
}
This code should test for abnormal conditions, and use try catch to be immune to exceptions but it is left as exercise for the reader :-)
A variant of the above using codecvt::out could be used to convert from wide string to narrow string.
In above code, I would use (assuming nconv is the function using codecvt::out to convert from wide string to narrow string):
...
#include <locale>
...
std::cin>>password;
std::locale mylocale;
std::fstream fp("/home/aricent/Documents/testing.txt",std::ios::in | std::ios::out );
std::getline(fp,y);
q = wconv(y, mylocale);
...
fp<<nconv(q.replace(x.length()+1, z.length(), username));
}
std::getline(fp, y);
...