I want to open a file for reading, the C++ way. I need to be able to do it for:
text files, which would involve some sort of read line function.
binary files, which would provide a way to read raw data into a
char*buffer.
- 2C++: Input/Output with filesmreggen– mreggen2008-08-11 15:53:34 +00:00Commented Aug 11, 2008 at 15:53
- Anyone interested in RAII, should check out The Official Resource Management Page by Bartosz Milewski.titanae– titanae2008-09-08 12:43:12 +00:00Commented Sep 8, 2008 at 12:43
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9 Answers
You need to use an ifstream if you just want to read (use an ofstream to write, or an fstream for both).
To open a file in text mode, do the following:
ifstream in("filename.ext", ios_base::in); // the in flag is optional To open a file in binary mode, you just need to add the "binary" flag.
ifstream in2("filename2.ext", ios_base::in | ios_base::binary ); Use the ifstream.read() function to read a block of characters (in binary or text mode). Use the getline() function (it's global) to read an entire line.
3 Comments
foraidt
+1 for noting that the global getline() function is to be used instead of the member function.
rogerdpack
I assume the binary flag is only needed in a windows environment?
Derek Park
roger, I've never found a case where I needed the binary flag on Windows or Unix. In theory, though, it should be used to avoid any implicit conversions. stdcxx.apache.org/doc/stdlibug/30-4.html
There are three ways to do this, depending on your needs. You could use the old-school C way and call fopen/fread/fclose, or you could use the C++ fstream facilities (ifstream/ofstream), or if you're using MFC, use the CFile class, which provides functions to accomplish actual file operations.
All of these are suitable for both text and binary, though none have a specific readline functionality. What you'd most likely do instead in that case is use the fstream classes (fstream.h) and use the stream operators (<< and >>) or the read function to read/write blocks of text:
int nsize = 10; std::vector<char> somedata(nsize); ifstream myfile; myfile.open("<path to file>"); myfile.read(somedata.data(), nsize); myfile.close(); Note that, if you're using Visual Studio 2005 or higher, traditional fstream may not be available (there's a new Microsoft implementation, which is slightly different, but accomplishes the same thing).
1 Comment
swdev
Wouldn't you get a segfault on the read? You didn't allocate any space for the data. Should be
char somedata[10], right?To open and read a text file line per line, you could use the following:
// define your file name string file_name = "data.txt"; // attach an input stream to the wanted file ifstream input_stream(file_name); // check stream status if (!input_stream) cerr << "Can't open input file!"; // file contents vector<string> text; // one line string line; // extract all the text from the input file while (getline(input_stream, line)) { // store each line in the vector text.push_back(line); } To open and read a binary file you need to explicitly declare the reading format in your input stream to be binary, and read memory that has no explicit interpretation using stream member function read():
// define your file name string file_name = "binary_data.bin"; // attach an input stream to the wanted file ifstream input_stream(file_name, ios::binary); // check stream status if (!input_stream) cerr << "Can't open input file!"; // use function that explicitly specifies the amount of block memory read int memory_size = 10; // allocate 10 bytes of memory on heap char* dynamic_buffer = new char[memory_size]; // read 10 bytes and store in dynamic_buffer file_name.read(dynamic_buffer, memory_size); When doing this you'll need to #include the header : <iostream>
Comments
#include <iostream> #include <fstream> using namespace std; int main () { ofstream file; file.open ("codebind.txt"); file << "Please writr this text to a file.\n this text is written using C++\n"; file.close(); return 0; } 
1 Comment
mech
Could you add a short explanation on how/why this code snippet answers the question?
#include <iostream> #include <fstream> using namespace std; void main() { ifstream in_stream; // fstream command to initiate "in_stream" as a command. char filename[31]; // variable for "filename". cout << "Enter file name to open :: "; // asks user for input for "filename". cin.getline(filename, 30); // this gets the line from input for "filename". in_stream.open(filename); // this in_stream (fstream) the "filename" to open. if (in_stream.fail()) { cout << "Could not open file to read.""\n"; // if the open file fails. return; } //.....the rest of the text goes beneath...... } 
Comments
Follow the steps,
- Include Header files or name space to access File class.
- Make File class object Depending on your IDE platform ( i.e, CFile,QFile,fstream).
- Now you can easily find that class methods to open/read/close/getline or else of any file.
CFile/QFile/ifstream m_file; m_file.Open(path,Other parameter/mood to open file);
For reading file you have to make buffer or string to save data and you can pass that variable in read() method.
Comments
**#include<fstream> //to use file #include<string> //to use getline using namespace std; int main(){ ifstream file; string str; file.open("path the file" , ios::binary | ios::in); while(true){ getline(file , str); if(file.fail()) break; cout<<str; } }** 
Comments
#include <fstream> ifstream infile; infile.open(**file path**); while(!infile.eof()) { getline(infile,data); } infile.close(); 
2 Comments
Michael
You should explain your answer either before the code or with comments so other people can get a sense of what you're trying to do.
fstream are great but I will go a little deeper and tell you about RAII.
The problem with a classic example is that you are forced to close the file by yourself, meaning that you will have to bend your architecture to this need. RAII makes use of the automatic destructor call in C++ to close the file for you.
Update: seems that std::fstream already implements RAII so the code below is useless. I'll keep it here for posterity and as an example of RAII.
class FileOpener { public: FileOpener(std::fstream& file, const char* fileName): m_file(file) { m_file.open(fileName); } ~FileOpeneer() { file.close(); } private: std::fstream& m_file; }; You can now use this class in your code like this:
int nsize = 10; char *somedata; ifstream myfile; FileOpener opener(myfile, "<path to file>"); myfile.read(somedata,nsize); // myfile is closed automatically when opener destructor is called Learning how RAII works can save you some headaches and some major memory management bugs.
1 Comment
Flame
File stream objects get closed in their destructor, therefore this new class is useless.