CODEWITHSUNDEEP
c++
Sharma harsh
Sharma harsh

Reputation: 220

File Handling:What is the use of peek() function in c++?

As I have so much problem while dealing with the eof of a file, whenever I code with fstream and the eof appears I have to clear the stream in order to work with that stream. Although I have searched a lot about the eof and I got the result that I should start using:

fstream file("Filename.txt",ios::in|ios::ate|ios::out);
char str[80];
while(file>>str)
{
    //do the required stuff
}
//clear the stream and reuse it
file.clear();
file.seekp(0);

But I have also read about a function called peek() which is also used for such purposes but I am a little confused in its working and I am not able to apply it in the code. So if anyone could guide me through this.

And I have also heard about a function called putback() what's that??

Edit-1

fstream file("Filename.txt",ios::in|ios::ate|ios::out);
char str[80];
while(file>>str)
{
    //do the required stuff
}
//clear the stream and reuse it
file.clear();
file.seekp(0);
//Now do the required writing operation after reading the whole file wherever is required
//I also want to perform writing operations and if this pattern seems most suitable for me

Upvotes: 2

Views: 6277

Answers (2)

Mats Petersson
Mats Petersson

Reputation: 129374

Say you want to write a parser for C or C++ and your code does something like this:

 char c = source.get();
 switch(c)
 {
    ... 
    case '<':
        // May be < or <=
        if (source.peek() == '=')
        {
           source.get();
           return less_or_equal;
        }
        // Ok, not <= so:
        return less;
   ...
}

[I ignored that it may be part of a template, a shift, or something else like that]

The need for putback() is very little when you have peek(), but it does allow code that "normally consumes" the character to put it back "if it got it wrong". Say you know that <= is more common than <, then you could do:

 char c = source.get();
 switch(c)
 {
    ... 
    case '<':
        // May be < or <=
        c = source.get();
        if (c == '=')
        {
           source.get();
           return less_or_equal;
        }
        source.putback(c);
        // Ok, not <= so:
        return less;
   ...
}

because it only does putback on the rare case [as per the assumed statistics above].

One can imagine cases where the common case is to get and the rare case is mismatch, e.g. if we want to read a number:

  int number = 0;
  do
  {
      char c = input.get();
      if (isdigit(c))
      {
           number *= 10;
           number += c - '0';
      }
      else
      {
           input.putback(c);
      }
while( isdgit(c) );

Since, most numbers have more than one digit in them, the more common case is that the first and the subsequent character is a digit, and the unusual case is that we need to call putback(). [Of course, reading numbers "properly" will require a bit more stuff...]

Upvotes: 2

Sergey Kalinichenko
Sergey Kalinichenko

Reputation: 726619

But I have also read about a function called peek() which is also used for such purposes

peek() was created for a different purpose - it's there to let your program process two characters at a time - essentially emulating ungetc() functionality from the portion of the library based on the C standard library.

Using peek to see if you are about to hit eof is valid, but the approach that you show in the code, i.e. while(file>>str), is more idiomatic to C++ than the one based on peek.

I have also heard about a function called putback() what's that?

The std::putback() function lets you do the same thing as ungetc() does for FILE* streams.

Upvotes: 2

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