![]() Ifstream f ( "file" ) while (getline (f, line ) ) if (f. When reaching the end of the file, it usually is the goal to treat the trailing data as a healthy line even if it is not terminated by a newline character (there is no standard saying whether a line is defined by special character separation or termination).Īfter the investigation described below, I am pretty sure that the simplest rock-solid language construct for above-specified task is: Of course, we want to call process(line) only if the preceding getline() was able to extract meaningful data and store it in line. The goal is to process the data read from the file, line by line, via the fictitious call to process(line). For attempting to get a line from the file, we use std::getline(s, line), where line is a std::string to store the data to. Therefore, we first try to open a file by invoking ifstream s ("file"). This is the task: iteratively process the lines read from a file by means of an ifstream ( why ifstream?). Note: all code shown in this post is contained in this HG repository, and can also be downloaded in a tarball. Otherwise, before continuing, you should make yourself briefly familiar with eofbit, failbit, badbit of the ios class. If you just want to have a look at the results of this small investigation, I recommend scrolling down to the ideal solutions section. Update (July 7th, 2011): I revised the article after an important insight provided by Alexandre Duret-Lutz (confer comments). It is one of the most-visited articles on my website. Update (January 18th, 2015): In the mean time, this article has made it into the top Google search results for “c++ read file ifstream”. We all know that re-using established recipes saves time and improves software quality in the long-term. To name, explain, and share those with the community is the goal of this article. As expected, for writing reliable code, there also best practices or “recipes” to follow. ![]() Of course there are many obvious and non-obvious ways to write unreliable code. In order to understand the relation between the language and operating system constructs involved, I performed quite some research and testing. We for example expect that when fopen() returns NULL, errno is set to something meaningful. libc) - they all are documented in different places and to a different extent. POSIX) and low-level APIs (provided by e.g. The exact behavior of C++ code with respect to file handling and stream manipulation is defined by an intertwining of language specification (C++ in this case), operating system interface (e.g. From these discussions we learn that most of the related uncertainty comes from a lack of centralized documentation or even missing documentation. Proper evaluation of errno, respectively perror(), in response to the stream error bits is not a trivial task as can be inferred from discussions like this and this. When it comes to the idea of providing meaningful error messages, things become quite complicated. It is worth mentioning that although is a convenient reference, it does not provide us with a rock-solid solution for the above-stated problem and also does not mention all the important details. Proper handling of the stream error bits eofbit, failbit, and badbit requires a tremendous amount of care, as discussed for example here, here, and here, and finally at. ![]() However, in C++ this turned out to be a rather complex topic. In a high-level programming language such as Python this level of reliability and usability is not difficult to obtain. While doing so, the goal was to handle all underlying stream errors as well as file opening errors, and to emit as precise error messages as possible. Motivated by this POV-Ray issue I tried to find a reliable way to read a file line by line in C++ using std::ifstream in combination with std::getline().
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