in below code i am trying to print message if file is empty and then throwing exception. Consider file Text.txt is empty.
ifstream inputFile;
try
{
inputFile.exceptions(ifstream::eofbit);
inputFile.open("Text.txt");
if (inputFile.is_open()) {
cout << "file opened"<<endl;
}
if (inputFile.peek()== ifstream::traits_type::eof())
{
cout << "file opened but it is empty or invalid
content" << endl;
}
}
catch (ifstream::failure &e)
{
cout << "Some issue with input file: " << e.what()<<endl;
_exit(1);
}
But it is throwing exception before going into the if block (if (inputFile.peek()== ifstream::traits_type::eof())).What I am missing?
This problem happens when the file exists and it's empty.
The problem is that you enabled exceptions upon eof in the line: inputFile.exceptions(ifstream::eofbit);
It opens the file and since it's empty, it immediately sets the eof flag, and you get an exception. If you comment out this line, it will work OK.
Anyway, I would write the same code it a little shorter way:
ifstream inputFile("Text.txt");
if (inputFile)
{
cout << "file opened" << endl;
inputFile.peek();
if (inputFile.eof())
{
cout << "file opened but it is empty or invalid content" << endl;
}
}
Related
I am writing a small application that modifies a text file. It first creates a copy of the file in case something goes wrong.
The following function creates this copy in the same directory. It takes the file's name as an argument and returns true if the copy is successfully created, and false if it fails.
#include <iostream>
#include <filesystem>
#include <fstream>
#include <string>
using std::ifstream;
using std::ofstream;
using std::string;
using std::cerr;
using std::cin;
using std::cout;
using std::endl;
bool backupFile(string FileName) {
cout << "Creating backup for " << FileName << "..." << endl;
try { // for debugging purposes
string NewName = "bkp_" + FileName;
string CurLine;
ifstream FileCopy(FileName);
ofstream FileBackup(NewName);
if (FileCopy.fail()) { // Could specify how file copy failed?
cerr << "Error opening file " << FileName << ".";
return false;
}
while (getline(FileCopy, CurLine)) { // Copy lines to new file
//cout << "Copying " << CurLine << "\" to " << NewName << "." << endl;
FileBackup << CurLine << "\n";
}
cout << "File successfully backed up to " << NewName << endl;
return true;
}
catch (const ifstream::failure& iE) {
cerr << "Exception thrown opening original file: " << iE.what() << endl;
return false;
}
catch (const ofstream::failure& oE) {
cerr << "Exception thrown outputting copy: " << oE.what() << endl;
}
catch (...) {
cerr << "Unknown exception thrown copying file." << endl;
return false;
}
}
I've used a few catch statements to indicate if there is an issue with the input (ifstream::failure), the output (ofstream::failure), or neither.
During compilation, however, the following error appears:
error C2312: 'const std::ios_base::failure &': is caught by 'const std::ios_base::failure &' on line 42
To me, the error implies that both ifstream::failure and ofstream::failure are caught on ifstream::failure, which seems strange. When I remove the catch for ofstream::failure, it runs fine.
Why is this the case?
ifstream::failure and ofstream::failure are both the same type defined in the std::ios_base base class std::ios_base::failure, you can't catch the same type in two separate catch clauses.
Note that neither of your streams will actually throw any exceptions, by default std::fstream doesn't throw any exceptions. You have to turn exceptions on by calling exceptions:
FileCopy.exceptions(f.failbit);
FileBackup.exceptions(f.failbit);
The above will cause an std::ios_base::failure to be thrown when the stream enters the failed state. As you are already checking for FileCopy.fail() you could just expand that checking to cover other failure cases (e.g. check that FileCopy doesn't fail during getline and that FileBackup also doesn't fail) rather than enabling exceptions.
I am trying to open a json file that I will be working with in C++. Code that I have used successfully before fails to open the file. I am using Visual Studio 2017 on Windows 10 Pro with JSON for Modern C++ version 3.5.0.
I have a very simple function, which is supposed to open a file as input to a json object. It appears to open the file, but aborts when writing it to the json object. Originally the file to be opened was in another directory, but I moved it into the same directory as the executable while testing...but it didn't help.
Here is the very short function that fails:
json baselineOpenAndRead(string fileName) //passed string used for filename
{
json baseJObject;
cout << "we have a baseJObject" << endl;
//ifstream inFileJSON("test_file.json"); // Making this explicit made no difference
ifstream inFileJSON;
inFileJSON.open("test_file.json", ifstream::in);
cout << "we have opened json inFileJSON" << endl; // get here
inFileJSON >> baseJObject;
cout << " Can direct inFileJSON into baseJObject" << endl; //never get here; the app aborts.
inFileJSON.close();
return baseJObject;
}
This seems basically identical to the example on the nlohmann site:
// read a JSON file
std::ifstream i("file.json");
json j;
i >> j;
I just expected this to open the json file, load it into the object, and return the object. Instead, it just quits.
Thanks for any thoughts...i.e., what am I doing wrong? (I'm going to ignore that it worked before...maybe I missed something).
--Al
As requested, here is a minimal reproducible example, but it will require nlohmann's json.hpp in order to compile:
#include <iostream>
#include <fstream>
#include "json.hpp"
using json = nlohmann::json;
using namespace std;
string fileName;
json baselineOpenAndRead(string);
int main(int argC, char *argV[])
{
json baseJObject;
if (argC != 2) // check to make sure proper number of arguments are given.
{
cout << "\n\nFilename needed...";
exit(1); // number of arguments is wrong - exit program
}
else
{
fileName = argV[1];
baseJObject = baselineOpenAndRead(fileName); // opens and reads the Base Line JSON file
cout << "baseJObject returned" << endl;
}
return 0;
}
json baselineOpenAndRead(string fileName) //
{
cout << "File name: " << fileName << endl;
json baseJObject;
cout << "we have a baseJObject" << endl;
ifstream inFileJSON(fileName);
if (inFileJSON.is_open())
{
cout << "file open..." << endl;
if (nlohmann::json::accept(inFileJSON))
{
cout << "valid json" << endl;
try { inFileJSON >> baseJObject; }
catch (const std::exception &e) { std::cout << e.what() << '\n'; throw; }
}
else
{
cout << "not valid json" << endl;
}
}
else
{
cout << "file not really open" << endl;
}
inFileJSON >> baseJObject;
cout << " We can echo inFileJSON into baseJObject" << endl;
inFileJSON.close();
return baseJObject;
}
I tested it with this json file:
{
"people": [{
"name": "Scott",
"website": "stackabuse.com",
"from": "Nebraska"
},
{
"name": "Larry",
"website": "google.com",
"from": "Michigan"
},
{
"name": "Tim",
"website": "apple.com",
"from": "Alabama"
}
]
}
When I run this passing it the json above as data.json, I get the following output and then it quits:
./Test_json data.json
File name: data.json
we have a baseJObject
file open...
valid json
[json.exception.parse_error.101] parse error at line 1, column 1: syntax error while parsing value - unexpected end of input; expected '[', '{', or a literal
Without the try, it just quits. It never gets past inFileJSON >> baseJObject;
Another try that seems to work, but why?
OK. I tried this with the same main (the only changes are in the function):
json baselineOpenAndRead(string fileName) //
{
json baseJObject;
string filePath = "../baselines/" + fileName;
cout << "filePath: " << filePath << endl;
ifstream inFileJSON(fileName);
//baseJObject = json::parse(inFileJSON);
inFileJSON >> baseJObject;
cout << baseJObject << std::endl;
return baseJObject;
}
This looks basically the same to me. I tried making it ifstream inFileJSON(fileName.c_str()) on both the original and in this one. The original continued to fail, this one continued to work. Sorry this is getting so long, but I can't get decent formatting out of comments... Should I just try answering my own question instead?
I think I've got this. I believe my initial problem was caused by an errant ',' in one of my json test files. Subsequently, the if (inFileJSON.is_open) worked, but the if (nlohmann::json::accept(inFileJSON) was failing and causing the same (or perhaps a similar) error. I thought that I needed the c_str() for file paths outside of the executable's directory, but it doesn't seem to make a difference one way or the other. I took out the accept(), and this code seems to work consistently:
json baselineOpenAndRead(string fileName) //
{
json baseJObject;
cout << "we have a baseJObject" << endl;
string filePath = "../baselines/" + fileName;
cout << "filePath: " << filePath << endl;
//ifstream inFileJSON(filePath.c_str());
ifstream inFileJSON(filePath);
if (inFileJSON.is_open())
{
cout << "File is open." << endl;
inFileJSON >> baseJObject;
cout << baseJObject << std::endl;
inFileJSON.close();
return baseJObject;
}
else
{
cout << "File not open." << endl;
exit(1);
}
}
Thanks to everyone for your help. I appreciate it.
--Al
string mapFile;
cout << "Enter the file name : ";
cin >> mapFile;
ifstream mapfh;
mapfh.open(mapFile.c_str());
if(mapfh.is_open()) { ... }
else //if board file did not open properly
{
throw;
}
mapfh.close();
I am compiling with g++ in the command line. Whenever I put a file input (even with a full path i.e. /User/...etc./file.txt) it throws an error. I know the input is good, but for whatever reason the open always fails.
This isn't fully portable, but you'll get a more informed output if you interpret the errno,
#include <cerrno>
#include <cstring>
...
if(mapfh.is_open()) { ... }
else //if board file did not open properly
{
std::cout << "error: " << strerror(errno) << std::endl;
throw;
}
And if your policy is to communicate the errors as exceptions then use iostreams native support for the exceptions:
ifstream mapfh;
mapfh.exceptions(std::ios::failbit);
try {
mapfh.open(mapFile.c_str());
...
mapfh.close();
} catch (const std::exception& e) {
std::cout << e.what() << " : " << std::strerror(errno) << std::endl;
}
I am trying to read a las file larger then 2GBs (about 15GBs) but ios::fail() flag becomes true in 345th byte. Here is the code below.
void Foo()
{
char* filename = "../../../../../CAD/emi/LAS_Data/AOI.las";
ifstream m_file (filename);
char c;
int count = 0;
if (m_file.is_open())
{
while ( m_file.good() )
{
m_file.get(c);
cout << c << endl;
count++;
}
// Check State
if(m_file.fail())
cout << "File Error: logical error in i/o operation." << endl;
if(m_file.eof())
cout << "Total Bytes Read: " << count << endl;
m_file.close();
}
else
{
cout << "File Error: Couldn't open file: " << endl;
}
}
And the output is:
...
File Error: logical error in i/o operation.
Total Bytes Read: 345
What am I missing?
I'm going to guess that you're using Windows. Windows has a quirk that a Control-Z marks the end of a text file, no matter how large the file actually is. The solution is to open the file in Binary mode.
ifstream m_file (filename, std::ios::binary);
For some reason, Xcode will not take input from a file, while Visual C++ will.
When I run this program in xcode, the variables numberRows and numberCols stay 0 (they are initialized to 0 in the main function).
When I run it in Visual C++ they become 30 and 30 (the top line of maze.txt is "30 30" without the quotes).
Any ideas why this is happening?
void readIn(int &numberRows, int &numberCols, char maze[][100]){
ifstream inData;
inData.open("maze.txt");
if (!inData.is_open()) {
cout << "Could not open file. Aborting...";
return;
}
inData >> numberRows >> numberCols;
cout << numberRows << numberCols;
inData.close();
return;
}
There is something else wrong.
Unfortunately it is hard to tell.
Try flushing the output to make sure you get the error message:
void readIn(int &numberRows, int &numberCols, char maze[][100])
{
ifstream inData("maze.txt");
if (!inData) // Check for all errors.
{
cerr << "Could not open file. Aborting..." << std::endl;
}
else
{
// Check that you got here.
cerr << "File open correctly:" << std::endl;
// inData >> numberRows >> numberCols;
// cout << numberRows << numberCols;
std::string word;
while(inData >> word)
{
std::cout << "GOT:(" << word << ")\n";
}
if (!inData) // Check for all errors.
{
cerr << "Something went wrong" << std::endl;
}
}
}
interesting, so I followed the following suggestion from this post http://forums.macrumors.com/showthread.php?t=796818:
Under Xcode 3.2 when creating a new
project based on stdc++ project
template the target build settings for
Debug configuration adds preprocessor
macros which are incompatible with
gcc-4.2:
_GLIBCXX_DEBUG=1
_GLIBXX_DEBUG_PEDANTIC=1
Destroy them if you want Debug/gcc-4.2
to execute correctly.