question about opening a file

[Coolguy123]

Ok.

So I'm doing a tutorial, and it says you can write:

// reading a text file
#include <iostream.h>
#include <fstream.h>
#include <stdlib.h>

int main () {
  char buffer[256];
  ifstream examplefile ("example.txt");
...

this works.

but you can also declare a pointer to a constant char array and put that pointer in the brackets.

i.e.

// reading binary file
#include <iostream.h>
#include <fstream.h>

const char * filename = "example.txt";

int main () {
  char * buffer;
  long size;
  ifstream file (filename, ios::in|ios::binary|ios::ate);
...

so in the first case, you can place the string directly, in the other case you can place the pointer to the string without specifying that you want the content of the string.

How does this work?

Does the compiler automatically distinguish that it's a pointer and in that case of the open function of ifstream/ofstream/fstream, treat it differently?

[redhead]

It's part of the deffinition when beeing called with either the string itself or a const char pointer, you won't conflict with the standard declaration of the ifstream constructor.
Since basicaly it's the same so the rolled back code will in your first case be:

• temporary const char pointer (temp) assigned value "example.txt"
• ifstream default constructor called with (temp) relying on default open flags
• file pointer assigned to (file)

Where in your second case it would be:

• static constant char pointer (filename) assigned value "example.txt"
• ifstream constructor taking (const char*, ios::flags) called with (filename, ios::in|ios::binary|ios::ate)
• file pointer assigned to (file)

As you see the implied code is virtualy the same, you just did a bit more coding in order to achieve the same result.

On a side note, if you're planing on reading into your

char * buffer;

once you've opened the file, you will experience errors, the buffer has to have the needed memory assigned, either by usign malloc()/new() or by specifying the size needed as in your first example.

[Coolguy123]

thanx

[Coolguy123]

Q, what is this referring to?

file pointer assigned to (file)

[redhead]

When the ifstream() constructor is called it returns a FILE* of intrinsic type, it is through this every read/write/search/close handling on the file is carried out. I was just pointing out, as the

ifstream file (filename);

exits the FILE* created by the ifstream constructor is assigned to the variable file, so you can read/write/etc. by accessing file.
Another thing that's good to know, is that youy most likely want to check if the opening was ok ie:

ifstream file(filename, flags);
if(file.bad() || file.fail()){
    std::cout << "Error opening file: " << filename << std::endl;
    return -1;
}