How do you open a file in C

Question

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

User · Answer

include  lt iostream gt   include  lt fstream gt  using namespace std   void main         ifstream in stream     fstream command to initiate  in stream  as a command      char filename 31      variable for  filename       cout  lt  lt   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  lt  lt   Could not open file to read    n      if the open file fails          return                   the rest of the text goes beneath

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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   lt  lt  and     or the read function to read write blocks of text   int nsize   10  char  somedata  ifstream myfile  myfile open   lt path to file gt     myfile read somedata 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

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include  lt iostream gt   include  lt fstream gt  using namespace std   int main        ofstream file    file open   codebind txt      file  lt  lt   Please writr this text to a file  n this text is written using C   n     file close      return 0

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include lt fstream gt    to use file  include lt string gt     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 lt  lt str

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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

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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  lt  lt   Can t open input file        file contents   vector lt string gt  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  lt  lt   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    lt iostream gt

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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

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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 amp  file  const char  fileName   m file file                 m file open fileName               FileOpeneer                  file close            private      std  fstream amp  m file       You can now use this class in your code like this   int nsize   10  char  somedata  ifstream myfile  FileOpener opener myfile    lt path to file gt     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

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include  lt fstream gt   ifstream infile  infile open   file path     while  infile eof         getline infile data     infile close

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