Don't understand cplusplus.com example for istream::read

Question

On cplusplus.com an example is given:

// read a file into memory
#include <iostream>     // std::cout
#include <fstream>      // std::ifstream

int main () {

  std::ifstream is ("test.txt", std::ifstream::binary);
  if (is) {
    // get length of file:
    is.seekg (0, is.end);
    int length = is.tellg();
    is.seekg (0, is.beg);

    char * buffer = new char [length];

    std::cout << "Reading " << length << " characters... ";
    // read data as a block:
    is.read (buffer,length);

    if (is)
      std::cout << "all characters read successfully.";
    else
      std::cout << "error: only " << is.gcount() << " could be read";
    is.close();

    // ...buffer contains the entire file...

    delete[] buffer;
  }
  return 0;
}

Can someone please explain why the last if (is) can determine if all characters have been read? It's the same if statement we're already in and the way I interpreted it (probably too simplistically and false at that) we only check if is exists, but wasn't this already established?

Answer 1

std::ifstream has a conversion operator to bool, which returns whether or not badbit or failbit is set on the stream.

It is essentially shorthand for if (!is.fail()) {/*...*/}.