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Fstream Error Handling

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Ofstream Error Checking

Overflow Community Stack Overflow is a community of 4.7 million programmers, just like you, helping each other. Join them; it only takes a minute: Sign ifstream::failure up C++ ifstream Error Checking up vote 12 down vote favorite 3 I am new to C++ and want to add error checking to my code plus I want to make sure I'm using good coding practices. I read a line ifstream open fail from an ASCII file into a string using: ifstream paramFile; string tmp; //open input file tmp.clear(); paramFile >> tmp; //parse tmp How can I error check to make sure the input file read was successful? I'm seeing much more complicated ways of reading from an ASCII file out there. Is the way I'm doing it "safe/robust"? c++ error-handling ifstream share|improve this question edited Nov 19 '12 at 1:51 Lightness Races in Orbit 217k35337583 asked Nov 19 '12 at 1:36 slowmotionfred 68117 1 std::string's

C++ Ifstream Exceptions

default constructor makes an empty string. No need for the clear(). Also, if you want a line, use std::getline. –chris Nov 19 '12 at 1:38 Thanks for the link lightness –slowmotionfred Nov 19 '12 at 1:59 add a comment| 1 Answer 1 active oldest votes up vote 8 down vote accepted paramFile >> tmp; If the line contains spaces, this will not read the whole line. If you want that use std::getline(paramFile, tmp); which reads up until the newline. Basic error checking is done by examining the return values. For example: if(paramFile>>tmp) // or if(std::getline(paramFile, tmp)) { std::cout << "Successful!"; } else { std::cout << "fail"; } operator>> and std::getline both return a reference to the stream. The stream evaluates to a boolean value which you can check after the read operation. The above example will only evaluate to true if the read was successful. Here is an example of how I might make your code: ifstream paramFile("somefile.txt"); // Use the constructor rather than `open` if (paramFile) // Verify that the file was open successfully { string tmp; // Construct a string to hold the line while(std::getline(paramFile, tmp)) // Read file line by line { // Read was successful so do something with the line } } else { cerr << "File could not be opened!\n"; // Report error cerr << "Error code: " << strerror(errno); // Get some info as to why } share|improve this answer answered Nov 19 '12 at 1:58 Jesse Good 30.9k

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C++ Strerror

programmers, just like you, helping each other. Join them; it only takes a minute: Sign up Try-Catch Block For C++ File-IO Errors Not Working up vote 6 down vote favorite I'm very new to the world of http://stackoverflow.com/questions/13446593/c-ifstream-error-checking C++ error handling, but I was told here: http://stackoverflow.com/questions/3622030/checking-for-file-existence-in-c ...that the best way to checks for file existence was with a try-catch block. From my limited knowledge on the topic, this sounds like sound advice. I located this snippet of code: http://www.java2s.com/Tutorial/Cpp/0240__File-Stream/Readafileintrycatchblock.htm #include #include using namespace std; int main () { try{ char buffer[256]; ifstream myfile ("test.txt"); while (! myfile.eof() ) { myfile.getline (buffer,100); cout << buffer << endl; } }catch(...){ cout http://stackoverflow.com/questions/3629321/try-catch-block-for-c-file-io-errors-not-working << "There was an error !\n"; } return 0; } ...but when I compile it using g++ -Wall -pedantic -o test_prog main.cc And run the program in a directory where test.txt does not exist, the prog keeps spitting out empty lines to the terminal. Can anyone figure out why? Also is this a good way to check for file existence for a file you actually want to open and read from (versus just something where your indexing a bunch of files and checking them over)? Thanks! c++ file-io error-handling filesystems try-catch share|improve this question asked Sep 2 '10 at 16:54 Jason R. Mick 2,14842347 2 Your file reading logic is wrong. And use std::string. And streams don't use exceptions, by default. –GManNickG Sep 2 '10 at 17:02 It's not mine... I got the code from a tutorial site, as I mentioned ;) so if streams don't use exceptions, how should I catch errors if file doesn't exist??? –Jason R. Mick Sep 2 '10 at 17:19 Ah. The website sucks then, not a surprise. You should get a book if you really want to learn. –GManNickG Sep 2 '10 at 19:48 add a comment| 3 Answers 3 active oldest votes up vote 10 down vote accepted In C++ iostreams do not throw exeptions by

for a quick overview of the site Help Center Detailed answers to any questions you might have Meta Discuss the workings and policies of this site About Us Learn more http://codereview.stackexchange.com/questions/57829/better-option-than-errno-for-file-io-error-handling about Stack Overflow the company Business Learn more about hiring developers or posting ads with us Code Review Questions Tags Users Badges Unanswered Ask Question _ Code Review Stack Exchange is a question and answer https://ubuntuforums.org/showthread.php?t=1243086 site for peer programmer code reviews. Join them; it only takes a minute: Sign up Here's how it works: Anybody can ask a question Anybody can answer The best answers are voted up and rise fstream error to the top Better option than “errno” for file IO error handling up vote 10 down vote favorite 3 I have the following method for opening a file: void TankFile::OpenForReading(const std::string & filename) { assert(!filename.empty()); errno = 0; file.exceptions(0); // Don't throw file.open(filename, (std::fstream::in | std::fstream::binary)); if (!file.is_open() || !file.good()) { const char * errorStr = strerror(errno); throw TankFileError(Format("Failed to open Tank file \"%s\": '%s'", filename.c_str(), errorStr)); } } fstream error handling The objective here is to attempt to open a file and throw TankFileError with a proper error description on failure. The caller will be expecting this exception type. Everything works fine and I get a nice error message like this if the exception is thrown: Failed to open Tank file "unexistent_file": 'No such file or directory' But what I don't like in that block is having to use the errno global and strerror(). A way around it would be to let the stream throw an exception, then catch it, get the error message from the what() member and re-throw with TankFileError, but I find this solution also a bit hackish, plus, in the tests I did, the resulting error message from std::fstream::failure was pretty cryptic: void TankFile::OpenForReading(const std::string & filename) { assert(!filename.empty()); try { file.exceptions(std::fstream::failbit); file.open(filename, (std::fstream::in | std::fstream::binary)); } catch (std::fstream::failure & err) { throw TankFileError(Format("Failed to open Tank file \"%s\": '%s'", filename.c_str(), err.what())); } } Produced the error message: Failed to open Tank file "unexistent_file": 'ios_base::clear: unspecified iostream_category error'. Is there a better way to implement this? I was hoping that the new C++11 system_error library would provide a way to query this kind of error messages, but from what I've seen, you still h

Get Kubuntu Get Xubuntu Get Lubuntu Get UbuntuStudio Get Mythbuntu Get Edubuntu Get Ubuntu-GNOME Get UbuntuKylin Ubuntu Code of Conduct Ubuntu Wiki Community Wiki Other Support Launchpad Answers Ubuntu IRC Support AskUbuntu Official Documentation User Documentation Social Media Facebook Twitter Useful Links Distrowatch Bugs: Ubuntu PPAs: Ubuntu Web Upd8: Ubuntu OMG! Ubuntu Ubuntu Insights Planet Ubuntu Activity Page Please read before SSO login Advanced Search Forum The Ubuntu Forum Community Ubuntu Specialised Support Development & Programming Programming Talk C++ file IO exceptions Having an Issue With Posting ? Do you want to help us debug the posting issues ? < is the place to report it, thanks ! Results 1 to 7 of 7 Thread: C++ file IO exceptions Thread Tools Show Printable Version Subscribe to this Thread… Display Linear Mode Switch to Hybrid Mode Switch to Threaded Mode August 18th, 2009 #1 Volt9000 View Profile View Forum Posts Private Message Cookies and cream Join Date Apr 2007 Beans 402 DistroUbuntu 10.04 Lucid Lynx C++ file IO exceptions I come from a C# background and so I was a bit disappointed to find there aren't many "built-in" exceptions in C++, particularly for file IO. So after a bit of digging I found I have out how to handle these exceptions. However, I'm confused about the return value of of the .what() member function. Here's a sample file: PHP Code: #include
#include

usingnamespacestd;

intmain()
{
charc;
ifstreamfile("test.txt",ifstream::in);

file.exceptions(ifstream:: 

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