C++ File IO Question

[westco]

The code below attempts to read in a file. It first asks the user to enter the name of the file.

Here is the code:

#include <iostream>
#include <string>
#include <vector>
#include <fstream> //file io support
#include <cstdlib> //exit() support

int main(void)
{
using namespace std;

string data_in = "";
string filename = "";

cout << "Enter filename: ";
getline(cin,filename);

ifstream file_in (filename); //create input file type
if(file_in.is_open()) //if the file does not open properly
{
while(! file_in.eof())
{
getline(file_in,data_in);
cout << endl << data_in;
}
file_in.close();
}
else
{
cout << "Unable to open file";
}

return 0;
}

The problem I am having is the compiler is complaining about the following line:
ifstream file_in (filename);

If I replace this line with the following, all is well:
ifstream file_in ("filename.txt");

Why is this? Why can't I use a variable name here? How can I get this to work?

[Etherous]

You need to convert your string object to a char array (cstring) by calling c_str() on your filename object.

[westco]

So the ifstream class only supports a char type?

[westco]

Isn't "filename.txt" a string? Why does this work?

[Etherous]

Isn't "filename.txt" a string? Why does this work?

No, "filename.txt" in not a string; it is an array of char primitives. Above when you created a string and assigned it the filename, what is happening is behind the scenes, the string object is converting that array of chars into a string object representation of the same data. Latter, we may need to convert it back, so the c_str() method is provided

[Etherous]

So the ifstream class only supports a char type?

ifstream ( );
explicit ifstream ( const char * filename, ios_base:: openmode mode = ios_base::in );

It appears so

[westco]

Thanks Etherous.

I think I understand now.