ifstream

问题 On this website, someone writes: while (! myfile.eof() ) { getline (myfile,line); cout << line << endl; } This is wrong, read carefully the documentation for the eof() memberfunction. The correct code is this: while( getline( myfile, line)) cout << line << endl; Why is this? 回答1: There are two primary reasons. @Etienne has pointed out one: reading could fail for some reason other than reaching the end of the file, in which case your first version will go into an infinite loop. Even with no

问题 How do I read all .txt files in a certain directory? Let's say in my C:\ I have a foo.txt and a foo2.txt . Is there a way to read both of these two without having to do something like this? string text; string text2; ifstream myFile ("foo.txt"); ifstream myFile2 ("foo2.txt"); while(myFile << text){ }; while(myFile2 << text2){ }; In other words, is it possible to put a wildcard *.txt to instruct to read all .txt files? ifstream myFile ("*.txt"); 回答1: You'd typically do this by passing the name

问题 I'm opening a file, and getting lines from it. The first line should say how many variables there are, and what their names are. The second line should be a logic equation using these variables. The assignment is to have it print out a truth table for the variables and equation. The first line the program is taking in is not printing without me inserting a new line character. I tried converting to a string and using both printf and cout. Main file that inputs everything: #include "truthTable2

问题 In c++, how expensive is it to use the istream::seekg operation? EDIT: How much can I get away with seeking around a file and reading bytes? What about frequency versus magnitude of offset? I have a large file (4GB) that I am parsing, and I want to know if it's necessary to try to consolidate some of my seekg calls. I would assume that the magnitude of differences in file location play a role--like if you seek more than a page in memory away, it will impact performance--but small seeking is

问题 My code is simply as this: UPDATED : #include <iostream> #include <fstream> using namespace std; int main(int argc, char **argv) { ifstream r("foo.bin", ios::binary); ofstream w("foo.bin", ios::binary); int i; int ints[10] = {0,1,2,3,4,5,6,8,9}; w.write((char*)&ints, sizeof(ints)); int in_ints[10]; r.read((char*)&in_ints, sizeof(in_ints)); for(i = 0;i < 10;i++) { cout << in_ints[i] << " "; } cout << endl; return 0; } Now, the write portion appears to be successful, for example running the od

问题 This question already has answers here : Reading directly from an std::istream into an std::string (6 answers) Closed 3 years ago . How do I read exactly 128 bytes from an fstream into a string object? I wrote some code to read the first 128 bytes of a file and print it and then the last 128 bytes of the file and print that. The last part works, since you can easily iterate to EOF, but how do I get exactly 128 bytes from the front? The code below doesn't work since you can't add 128 to an

问题 I have some code that uses ifstream to read some data from a file and everything works. Now I wish, without modifying some code, read this data from a memory, actually I have a char * that contains the data... How can I put my char * data into a ifstream without reading effectively the file? 回答1: If the code that uses the ifstream& could be changed slightly to use an istream& then you could easily switch between ifstream and istringstream (for reading data from memory): void read_data(std:

问题 In a C++ program, using std::ifstream, I'm attempting to open a user-specified file -- so far so good. However, I accidentally entered a filename that's actually a directory, and I was quite surprised to see that attempting to open() that directory didn't generate any errors. Here's a minimal example: std::ifstream f; f.open(".."); if(!f.is_open() || !f.good() || f.bad() || f.fail()) { std::cout << "error bit set on open" << std::endl; return 1; } No sign of error here. If I go on and attempt

问题 I am trying to help a friend with a project that was supposed to be 1H and has been now 3 days. Needless to say I feel very frustrated and angry ;-) ooooouuuu... I breath. So the program written in C++ just read a bunch of file and process them. The problem is that my program reads files which are using a UTF-16 encoding (because the files contain words written in different languages) and a simple use to ifstream just doesn't seem to work (it reads and outputs garbage). It took me a while to

问题 I have a big file nearly 800M, and I want to read it line by line. At first I wrote my program in Python , I use linecache.getline: lines = linecache.getlines(fname) It costs about 1.2s. Now I want to transplant my program to C++. I wrote these code: std::ifstream DATA(fname); std::string line; vector<string> lines; while (std::getline(DATA, line)){ lines.push_back(line); } But it's slow(costs minutes). How to improve it? Joachim Pileborg mentioned mmap() , and on windows CreateFileMapping()