A file problem

The code below is supposed to open an existing file to read, but when you enter a nonexisting file it creates that file! Why does this happen and how can I solve this problem.

Code:

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ifstream inFile;

inFile.open("u.txt" );

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Thanks...

I doubt it with an ifstream like that. From: http://wwww.dinkum.com/manuals/defau...e=fstream.html

Code:

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

    void open(const char *filename,
        ios_base::openmode mode = ios_base::in);

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And here's info about all the openmode's: https://www.dinkumware.com/manuals/?...#filebuf::open

Quote:

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Originally Posted by A person who is very smart

The member function endeavors to open the file with filename filename, by calling fopen(filename, strmode). Here strmode is determined from mode & ~(ate | binary):

* ios_base::in becomes "r" (open existing file for reading).
* ios_base::out or ios_base::out | ios_base::trunc becomes "w" (truncate existing file or create for writing).
* ios_base::out | ios_base::app becomes "a" (open existing file for appending all writes).
* ios_base::in | ios_base::out becomes "r+" (open existing file for reading and writing).
* ios_base::in | ios_base::out | ios_base::trunc becomes "w+" (truncate existing file or create for reading and writing).
* ios_base::in | ios_base::out | ios_base::app becomes "a+" (open existing file for reading and for appending all writes).

If mode & ios_base::binary is nonzero, the function appends b to strmode to open a binary stream instead of a text stream. It then stores the value returned by fopen in the file pointer fp. If mode & ios_base::ate is nonzero and the file pointer is not a null pointer, the function calls fseek(fp, 0, SEEK_END) to position the stream at end-of-file. If that positioning operation fails, the function calls close(fp) and stores a null pointer in the file pointer.

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In 'r', the file must exist. It will not create it.

Also you don't have to call open explicitely if you don't need to, you can pass the filename and flags to constructor of ifstream object:

Code:

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  ifstream inFile("u.txt", ios::in);

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But it happens I can't understand what is going on

Code:

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#include <fstream>
#include <iostream>

int main()
{
        std::ifstream in("thisdoesnotexist.txt");
        if(!in)
        {
                std::cout << "Failed. If it now exists, then "
                        "please give compiler / OS details." << std::endl;
        }

        in.clear();
        in.open("thisdoesnotexist.txt");

        if(in)
        {
                std::cout << "That is so so wierd. Definitely tell " \
                        "us compiler / OS details" << std::endl;
        }
}

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You heard the man. Move out!

Quote:

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Originally Posted by babyboomer

The code below is supposed to open an existing file to read, but when you enter a nonexisting file it creates that file! Why does this happen and how can I solve this problem.

Code:

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ifstream inFile;
 
inFile.open("u.txt" );

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Thanks...

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To Calculator
why Not main return 0 here on SuccessFully Completion .and Where you Close the File.

Noe Come to Main Question
Have a look on this Link to understand How to Do this in a Efficient Manner
http://www.codeguru.com/forum/showpo...84&postcount=2

Thanx

In C++, return 0 is not a requirement. Also, the ifstream's destructor is called automatically, which closes the file. His problem is that opening an ifstream that does not exist seems to create the file, which is not standard behavior.

Doing a small test under an old version of Visual C++ (ver 5),
it looks like if you use:

Code:

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#include <fstream.h>

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instead of the standard header

Code:

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#include <fstream>

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The file is created, even if it does not exist. I think there is a no create
option under fstream.h , but I do not remember what it is called.

EDIT : found it ...

Code:

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in.open("thisdoesnotexist.txt",ios::nocreate); // non-standard

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You should use the standard headers as shown by calculator.

Quote:

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Originally Posted by Calculator

In C++, return 0 is not a requirement. Also, the ifstream's destructor is called automatically, which closes the file. His problem is that opening an ifstream that does not exist seems to create the file, which is not standard behavior.

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There is no hard and fast rule .but for writing a Program in a Efficient manner all of this is compulsory.have a look here

Code:

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/* Include the necessary libraries for a basic program
Because this program is intended for C++ ,you should include the conio.h library that has the getch() function.
*/
 
#include <iostream>
#include <conio>
using namespace std;
#pragma hdrstop
//---------------------------------------------------------------------------
#pragma argsused
/* Every C++ function must have a main() function
The current C++ Standard recommends that the main() function
return a value. In this case, it will return an integer.
The main() function is the entry point of the program.
This version of main() takes two arguments. Sometimes the       
        arguments are not used at all, as is the case in this program.
*/
int main(int argc, char* argv[])
{
        // Display a simple sentence to the user
        cout << "Welcome to the wonderful world of C++ Programming!!!\n";
        /* Because we are using C++, we will explicitly       
                        prompt the user        to press an key in order to stop the
                program. Upon pressing a key, we will call the getch()
                function to receive it
        */
        cout << "\nPress any key to continue...";
        getch();
        // The program went alright, so return 0
        return 0;
}

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Thanx

The real gap in the library is that you cannot open a file in one go to read and write, creating the file if it doesn't exist but not truncating it if it does.

The only way to do that is open for append, then close then open again for read and write.

humptydumpty: <conio.h> is a non-standard header (not usually available on Unix for example), and I've never come across <conio>. Don't use them in portable code. In addition, what does your posted code have to do with "writing a Program in a Efficient manner"?

agreed .that's a Mistake. Never use conio.h use conio .but i forget to mention above example is in case of borland C++.Anyway where you found <conio.h> in my code. Because i am unable to see it.

Thanx:thumb:

Quote:

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Originally Posted by humptydumpty

Never use conio.h use conio

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That is the opposite.
conio.h is supported by several compilers under Windows and DOS.

Here, you rely on the fact that Borland C++ automatically adds ".h" if it is omitted and there is no matching file.