I open this file to read data, but if file doesn't exist it throws error. So how can I check if file exists? In console project I can use ifstream, where I can check it by writing if(!file) {}, and StreamReader doesn't allow me to do this.
StreamReader^ data = gcnew StreamReader("data.txt");
You need to read the documentation.
The exception thrown is FileNotFoundException if the file cannot be found.
Related
I am developing a study project. It includes registration, authorization, and saving records of customer. In my project reading the information from file using fstream is not working so I use FILE* method. And I need to clear all data in file to rewrite it for saving new records.
There is a question. How to clear it? I found a lot of examples how to do this but all of their are for using fstream. Is it real to do using FILE* ?
Here is some code:
FILE* Usrs = fopen("Users.dat", "rb");
StructForReg Array[100];
StructForReg Perenos;
int i = 0;
while (!!(feof(Usrs) == 0))
{
fread(&Perenos, sizeof(StructForReg), 15, Usrs);
Array[i] = Perenos;
i += 1;
}
fclose(Usrs);
//here must be a code with clearing and rewriting
How to clear a file using FILE*?
And I need to clear all data in file to rewrite it for saving new records.
Just open with using "w" (or "wb") mode. From cppreference the w mode destroys file contents if the file exists. If the file does not exists, it will create a new one.
See the manpage for fopen. "w" or "w+" in the flags causes the file to get truncated.
I have an application for which GUI is written in C# UWP and accessing the file logic is written in C++ DLL. The DLL should open a file to read data from it. I am trying to access the file from its location. When I call inFile.open("D:\\File\\readFile.txt", ios::in)
the value returned is NULL.
To check if there is any problem with the file path, I have created a console application to access the file using the same way and it worked. What could be the problem?
fstream inFile;
inFile.open(filePath, ios::in);
if (!inFile.is_open())
{
/* Display unable to read file*/
return;
}
/* Perform operation on file */
inFile.close();
I have some c++ code in a node.js function that opens a file like this and reads it into a buffer and then closes it.
ifstream inputFile(source.c_str(), ios::in | ios::binary);
inputFile.read(buffer, results.st_size);
inputFile.close();
Then I manipulate the file in some way and attempt to write it back to the same location. I get the following error code and message: "Text file busy". I know there is no other process touching the file. If I rename the file to some random name, and rename it back, then I am able to overwrite the file.
I am reading a file from some directory using visual c++. How do i check if that file exists or not.
If i use:
file.open("file.txt", ios::in);
where file is a member of fstream. This creates a file ifthat file is not present.
How can i check if that file is present or not.
Thank you
Your code creates no file if that file isn’t already present (on most systems, assuming file is a std::ifstream).
Due to inherent concurrency in file system access, there is no reliable way to check whether a file exists. The best way is simply to open the file for read access and test whether that was successful:
if (file.good()) …
// or simply
if (file) …
ifstream my_file("file.txt");
if (my_file.good())
{
// read away
}
If you are working on Windows, you can use Windows API to do the job for you. Here is a brief code snippet for you... You have to pass the complete path of file.txt in order for this to succeed. For E.g. D:\MyFolder\file.txt.
WIN32_FIND_DATA FindFileData;
HANDLE hFind = INVALID_HANDLE_VALUE;
hFind = FindFirstFile("D:\MyFolder\file.txt", &FindFileData);
GetLastError();
if (hFind == INVALID_HANDLE_VALUE)
// File not found
just calling file like that should not create a file if it is missing. It should fail and succeeding calls to file.is_open() would return false.
Your code could implement the following:
ifstream file("text.text");
if(!file){
//handle error / throw exception here
}
I am opening a file with ifstream to check if it exists. Then I close it and open it with ofstream to write to it, and I think setting ios::trunc flag allows me to overwrite it.
However I'd like the ability to keep the file open if it exists, but I used an ifstream to open it so does that mean I can't write to the file till I close and re-open using fstream or ofstream? I didn't use fstream to begin with because that wouldn't tell me if the file was already there or not.
Just open a read-write fstream on the file. You can test if the file previously existed (and was non-empty) by seeking to the end and seeing if you're at a non-zero offset. If so, the file existed, and you can do whatever with it. If not, the file didn't exist or was empty. Assuming you don't need to distinguish between those two cases, you can then proceed as if it did not exist.
For example:
// Error checking omitted for expository purposes
std::fstream f("file.txt", std::ios::in | std::ios::out);
f.seekg(0, std::ios::end)
bool didFileExist = (f.tellg() > 0);
f.seekg(0, std::ios::beg);
// Now use the file in read-write mode. If didFileExist is true, then the
// file previously existed (and has not yet been modified)
The setting ios::trunc erases previous contents of the file.
Try opening the file without this setting; with only the 'write' setting.
this is touching very serios problem - race conditions - what if somebody manages to do something with this file between closing and reopening? unfortunately iostream does not provide any means of resolving that issue - you can use cstdio FILE. If you want to turncate file if exists or create new one if not use fopen(name, "w"). If you want to turncate file if it exists or fail otherwise, then it seems standard library has nothing to offer, and you should go to other libraries or platform specific functions like OpenFile in windows.h