Overload cin to take ifstream instead of char* fname? [closed] - c++

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Closed 10 years ago.
I am trying to create a file manipulator so when I cin >> filename it will read the file and cout the value... of course I am going add more stuffs but I would like to proceed with the basic of getting the content of the file.
What I wanna achieve is by cout prompting user for a textfile name, then when he enter, I will do another cin >> readfile on next line of code then the compiler will run the operator overload and cout the content.
How do I achieve it. Below is my code.
Question is instead of taking in char* fname, can I send ifstream in instead
so I can do something like
ifstream infile(filename.c_str());
cin >> infile
Below is my current code
#include <iostream>
#include <fstream>
#include <string>
#include "point2d.cpp"
#include "line2d.cpp"
#include <iomanip>
using namespace std;
struct myself
{
string myname;
};
istream &operator>> (istream &stream, myself &myself )
{
cout << "Enter my name";
stream >> myself.myname;
return stream;
}
istream &operator>> (istream &in, char* fname)
{
ifstream inFile;
inFile.open(fname);
char ch1;
while (inFile.get(ch1))
{
cout << ch1;
}
}
int main()
{
string filename;
cout << "Enter file name: ";
cin >> filename;
// was thinking of
// ifstream infile(filename.c_str());
// then read infile with cin
//how do i cin filename and read its content.
return 0;
}

Answer 1

You are messing up some concepts here. Your question is so vague, I'm not convinced that you are clear on what you want to do.
As far as I understand, you want
cin >> istream;
To do the following:
Read a file name
Open the file for reading
replace cin with the input stream for the file
Is that correct?
IMHO this is a bad idea, because by conventions cin >> should not modify cin in any significant manner. Sure, it will advance the stream, and there are hacks to change the parsing mode.
If you replace the stream with a new stream, make this obvious, don't try to hide it in some >> construct.
Programming is not a sport of saving characters. Your program won't be faster if you make it more cryptic. Good programmers write code that is very clear on what it does, to avoid errors, and make it easier to find them.
istream* input = &cin; // Default input is stdin
*input >> filename; // read filename
input = new istream(filename, istream::in);
// Continue to use *input
// Clean up input, if you replaced it:
// You might want to use a boolean flag instead of this hack.
if (input != &cin) {
delete input;
}

Answer 2

I'm afraid you are quite confused here. cin does not read anything; you read from cin. So you might want to overload operator >> somehow to read the contents of cin into a file, but not the other way around.
What you can do is overload operator << so it outputs the contents of a file into an output stream, such as cout. The following, nevertheless, is invalid:
ostream& operator<<(ostream& out, char* filename);
You cannot define this operator because it is already defined. You need to write a class that represents a file name and overload the operator on that. A simple version:
struct Filename
{
Filename(const string& filename) : filename_(filename) {}
string filename_;
};
ostream& operator<<(ostream& out, const Filename& name)
{
// read the file whose name is name.filename_
}
And you can call it this way:
cout << Filename("filename.ext");

Answer 3

You are doing several bad practices all at once.
First, cin is an istream, so you are looking to overload an istream operator to take an istream. That will be incredibly confusing for anyone trying to read your code.
Second, a function should do 1 job (that is, you should not try to hide functionality by lumping several distinct tasks into 1 function). istream reads data from an input stream (in this case, the file name you want to open). You will then need to pass that file name to an ifstream instance to actually read the file. Presumably you want to output it, in which case you would use an ostream (e.g. cout).
The entire logic you are looking to implement can be done in roughly 5 lines of code or less:
string filename = "";
cin >> filename;
ifstream fin(filename.c_str(), ifstream::in);
copy(istreambuf_iterator<char>(fin), istreambuf_iterator<char>(), ostreambuf_iterator<char>(cout));
Which is much shorter and more readable than what you are trying to do.