c++ - testing an istream object -

when use std::istream object (in example below cplusplus.com, std::ifstream) in test : "if (myistreamobject)", object, automatically allocated in stack never null, right ?... in example below, using same test check if bytes read file... , that's strange code, use style when i'm dealing pointers...

i want know mechanism used in std::istream return value in tests, , value means... (the success/failure of last operation ??) overloading of bool cast (like const char* operator cast in mfc class cstring) or technique ?

because object never null, putting in test return true.

// read file memory #include <iostream>     // std::cout #include <fstream>      // std::ifstream  int main () {    std::ifstream ("test.txt", std::ifstream::binary);   if (is) {     // 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 block:     is.read (buffer,length);      if (is) // <== odd       std::cout << "all characters read successfully.";     else       std::cout << "error: " << is.gcount() << " read";     is.close();      // ...buffer contains entire file...      delete[] buffer;   }   return 0; } 

the

operator bool()  

returns true if stream has no errors, false otherwise.

the "no error" concept related previous operation done on stream itself.

for example: after invoke constructor

std::ifstream ("test.txt", std::ifstream::binary); 

a internal status flag in stream object set. when invoke operator bool check whether construction operation fails or not.

moreover method

is.read(...) 

also set internal status flag, can see in reference:

errors signaled modifying internal state flags: eofbit, failbit, badbit.

so after method call, if stream reaches eof (end-of-file) state bit set, , operator bool return positive value.

that means in case when test stream with

if (is) { ... } 

and status bit set, condition verified , if-branch taken.