How can I detect if a file is open in C++?
I am trying to use a code like this:
int main()
{
ifstream file("file.txt");
if ( /*here comes the check if file is open*/ ) cout<<"File open successfully"; else cout<<"File couldn't be opened. Check if the file is not used by another program or if it exists";
}
You are looking for the function is_open()
if(file.is_open()){}
if(file.is_open())
You are calling ifstream::is_open() function
Related
ifstream myfile;
myfile.open("FileTest");
string line;
if(myfile.is_open())
{
cout<<"Reading from file...";
getline(myfile,line);
}
if(myfile.fail())
{
cout<<"Unable to open file"<<endl;
}
myfile.close();
C++ tries to open the file in the current directory with the exact name FileTest. Check to see if the file is in the current directory? Maybe you spelled the name incorrectly? Maybe you forgot to write FileTest.txt? You are using ifstream, which will fail if the file you're trying to open does not exist or is corrupted.
first time here. I am a student doing some c++ coding for end year project. The programme that I coded does not read the text file even though everything seems to be in order. Some helps would be fantastic!
void transactionRecords(double total, char answer, string nameT, int HpNo, string address)
{
fstream myFile;
string name;
char idStatus;
double amt, sumAll=0;
myFile.open("transaction.txt",fstream::in);
if (!myFile) cout<<"Unable to Open File under Input Mode";
else
{
while (!myFile.eof())
{
myFile>>name>>idStatus>>address>>HpNo>>amt;
if (myFile.fail()) break;
}
myFile.close();
myFile.open("transaction.txt",fstream::app);
if (!myFile) cout<<"Unable to Open File under App Mode";
else
{
myFile<<nameT<<" "<<answer<<" "<<address<<" "<<HpNo<<" "<<total<<endl;
if (myFile.fail()) cout<<"Error encountered while adding data!\n";
}
}
myFile.close();
}
this is whats in the text file
Johns Y pasir_ris 81231211 4.14
First, I suggest you use the RAII principle when creating files that is:
void myfunction() {
ifstream f{"file.txt"};
// your logic here
// NB -- no need to manually close file, prevents resource leaks
}
There is no need to manually close or open such a file, it is opened by the constructor and closed when the destructor is invoked upon exiting the current scope. This prevents leaks of file handles and is a pervasive technique in C++.
Second, use the standard stream read loop in C++:
while (myFile>>name>>idStatus>>address>>HpNo>>amt) {
// your logic here, the read has succeded
// TODO process myFile, name, idStatus etc.
}
With these changes, your example should look something like:
void transactionRecords(double total, char answer, string nameT, int HpNo, string address)
{
ifstream myFile{"transaction.txt"};
string name;
char idStatus;
double amt, sumAll=0;
if (!myFile) cout<<"Unable to Open File under Input Mode";
return;
while (myFile>>name>>idStatus>>address>>HpNo>>amt) {
// TODO do something here??
}
ofstream tFile{"transaction.txt"};
if (!myFile) cout<<"Unable to Open File under App Mode";
return;
if (!(tFile<<nameT<<" "<<answer<<" "<<address<<" "<<HpNo<<" "<<total<<endl)) {
cout<<"Error encountered while adding data!\n";
}
}
}
You should probably do something in the TODO block, currently you are only storing the last values read? If you only mean to process one line from the file, swap the while loop with an if.
So I am working on a program for class in which we have to open two different text files to retrieve the appropriate text to be displayed in the console. My code is not opening the file and keeps outputting the else statement ".txt file cannot be open". I've tried several different ways to open the file but with no luck. Any help here would be greatly appreciated.
//
// main.cpp
// PunchLine program
// Page 896 Problem 3
//
//
#include <iostream>
#include <fstream>
#include <string>
using namespace std;
int main()
{
//File stream objects
fstream inFile;
string line;
//Open joke file to read lines to console
inFile.open("joke.txt", ios::in);
if (inFile.is_open())
{
//Read lines from file to console
while (getline(inFile, line))
{
cout << line << endl;
inFile.close();
}
}
else
cout << "joke.txt file cannot be open.\n";
//Open punchline file to read last line joke to console
inFile.open("punchline.txt", ios::in);
if (inFile.is_open())
{
//Read last line from file to console
inFile.seekp(-52L, ios::end);
getline(inFile, line);
}
else
cout << "punchline.txt file cannot be open.\n";
return 0;
}
When declaring an input file use
ifstream inFile;
Also make sure the input file is in the same folder as your .exe
Edit: http://www.cplusplus.com/doc/tutorial/files/ also, this link should help with working with files.
Edit 2: I already posted this in a comment, but I'll just add it to the official answer: "Change your while loop as well. Instead of the if test, use while(inFile.is_open()) and then use your getline statement inside the loop. Because right now your code reads like while get this line from the file is true cout line. So it might not even be doing the while loop."
I don't think you should close the file inside the while loop. Otherwise, your file gets closed after only the first line is read in. Move the close statement outside the loop. Same for the second block.
if (inFile.is_open())
{
//Read lines from file to console
while (getline(inFile, line))
{
cout << line << endl;
}
inFile.close();
}
else
cout << "joke.txt file cannot be open.\n";
Check that your file exist. If it does, check whether you have the correct path when you open it (check if your .txt files are in the same directory as your .exe file, or specify the full path in your code). If yes, check if the files are read-only.
use
if(!infile)
{
cout<<"cannot open file";
}
I think you need to flush the screen. Once you have flushed and closed the stream. The next time you run an application it should open the file.
e.g.
inFile.flush();
inFile.close();
I'm using fstream to open up .txt files with C++. So far it's working great. However, I'd like my console to display an error message if the input_file can't be opened. How should I go about this?
Snippet:
cin >> in_file_name ;
ifstream in_file(in_file_name.c_str());
in_file_str.assign(istreambuf_iterator<char>(in_file),
istreambuf_iterator<char>());
ifstream in_file(in_file_name.c_str());
if( in_file.fail() ) {
cerr << "Error!" << endl;
}
Use fail() to determine whether the ifstream was successfully opened or not.
you can use is_open method
cin >> in_file_name ;
ifstream in_file(in_file_name.c_str());
if(!in_file.is_open())
{
cout<<"Can't open the file";
}
Use operator! to test whether file open was succesful. Example:
std::ifstream fs(...);
if (!fs) {
std::cerr << "Could not open file.\n";
}
All standard iostreams support a conversion to void*, which allows for testing. This means that you can do if(!in_file) { ... }.
I've been all over the ifstream questions here on SO and I'm still having trouble reading a simple text file. I'm working with Visual Studio 2008.
Here's my code:
// CPPFileIO.cpp : Defines the entry point for the console application.
//
#include "stdafx.h"
#include <fstream>
#include <conio.h>
#include <iostream>
#include <string>
using namespace std;
int _tmain(int argc, _TCHAR* argv[])
{
ifstream infile;
infile.open("input.txt", ifstream::in);
if (infile.is_open())
{
while (infile.good())
cout << (char) infile.get();
}
else
{
cout << "Unable to open file.";
}
infile.close();
_getch();
return 0;
}
I have confirmed that the input.txt file is in the correct "working directory" by checking the value of argv[0]. The Open method just won't work.
I'm also having trouble debugging- should I not be able to set a watch on infile.good() or infile.is_open()? I keep getting
Error: member function not present.
EDIT: Updated code listing with full code from .CPP file.
UPDATE: The file was NOT in the Current Working Directory. This is the directory where the project file is located. Moved it there and it works when debugging in VS.NET.
Try using the bitwise OR operator when specifying the open mode.
infile.open ("input.txt", ios::ate | ios::in);
The openmode parameter is a bitmask. ios::ate is used to open the file for appending, and ios::in is used to open the file for reading input.
If you just want to read the file, you can probably just use:
infile.open ("input.txt", ios::in);
The default open mode for an ifstream is ios::in, so you can get rid of that altogether now. The following code is working for me using g++.
#include <iostream>
#include <fstream>
#include <cstdio>
using namespace std;
int main(int argc, char** argv) {
ifstream infile;
infile.open ("input.txt");
if (infile)
{
while (infile.good())
cout << (char) infile.get();
}
else
{
cout << "Unable to open file.";
}
infile.close();
getchar();
return 0;
}
Sometimes Visual Studio puts your exe file away from your source code. By default VS may only look for the file starting from your exe file. This process is a simple step for getting the input txt file from the same directory as your source code. Should you not want to fix your IDE setup.
using namespace std;
ifstream infile;
string path = __FILE__; //gets source code path, include file name
path = path.substr(0,1+path.find_last_of('\\')); //removes file name
path+= "input.txt"; //adds input file to path
infile.open(path);
Hopefully this helps other people for a quick solution. It took me a while to find this setup myself.
I've found two problems in your code:
a) syntax error in "ios::ate || ios::in" => should be "ios::ate | ios::in"
b) "ios::ate" sets the cursor to the end of file - so you get nothing when you start reading
So just remove "ios::ate" and you are fine :)
ciao,
Chris
infile.open ("input.txt", ios::ate || ios::in);
|| is the logical or operator, not the bitwise operator (as Bill The Lizzard said).
so i guess you are doing the equivalent to:
infile.open ("input.txt", true);
(assuming neither ios::ate or ios::in are 0)
Try using:
ifstream fStm("input.txt", ios::ate | ios::in);
I'm also having trouble debugging- should I not be able to set a watch on "infile.good()" or "infile.is_open()"? I keep getting "Error: member function not present."
and the proper includes:
#include <fstream>
etc.
If you use the default Vs code setup, place the text file that you want to read from in the same folder as your executable, I know it is not pretty but yeah it works