CODEWITHSUNDEEP
c++c
AndersK
AndersK

Reputation: 36092

fread equivalent with fstream

In C one can write (disregarding any checks on purpose)

const int bytes = 10;
FILE* fp = fopen("file.bin","rb");
char* buffer = malloc(bytes);
int n = fread( buffer, sizeof(char), bytes, fp );
...

and n will contain the actual number of bytes read which could be smaller than 10 (bytes).

how do you do the equivalent in C++ ?

I have this but it seems suboptimal (feels so verbose and does extra I/O), is there a better way?

  const int bytes = 10;
  ifstream char> pf("file.bin",ios::binary);
  vector<char> v(bytes);

  pf.read(&v[0],bytes);
  if ( pf.fail() )
  {
    pf.clear();
    pf.seekg(0,SEEK_END);
    n = static_cast<int>(pf.tellg());
  }
  else
  {
    n = bytes;
  }

  ...

Upvotes: 7

Views: 8124

Answers (3)

Duck
Duck

Reputation: 27572

pf.read(&v[0],bytes);

streamsize bytesread =  pf.gcount();

Upvotes: 1

Jonathan Leffler
Jonathan Leffler

Reputation: 754800

According to http://www.cplusplus.com/reference/iostream/istream/read/:

istream& read(char* s, streamsize n);

Read block of data
Reads a block of data of n characters and stores it in the array pointed by s.

If the End-of-File is reached before n characters have been read, the array will contain all the elements read until it, and the failbit and eofbit will be set (which can be checked with members fail and eof respectively).

Notice that this is an unformatted input function and what is extracted is not stored as a c-string format, therefore no ending null-character is appended at the end of the character sequence.

Calling member gcount after this function the total number of characters read can be obtained.

So pf.gcount() tells you how many bytes were read.

Upvotes: 3

Benjamin Lindley
Benjamin Lindley

Reputation: 103741

Call the gcount member function directly after your call to read.

pf.read(&v[0],bytes);
int n = pf.gcount();

Upvotes: 8

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