Why am I getting this output in C++? Explain the logic

Question

#include <stdio.h>
#include <iostream>
int main() 
{
  if(NULL)
    std::cout<<"hello";
  else
    std::cout<<"world";
  return 0;
}

The output to the above question is:

world

Kindly explain me why am I getting this output. I am not able to get the satisfactory answer even after referring to several different sources.

NULL == 0 == false (for a given definition of ==). Therefore the if statement becomes `if(false)` etc — Richard Critten, Aug 20 '17 at 13:36
I am learning C++, and I read somewhere that NULL and 0 are different. So, I am a bit confused. — Palak Jain, Aug 20 '17 at 13:37
[What is the difference between NULL, '\0' and 0](https://stackoverflow.com/questions/1296843/what-is-the-difference-between-null-0-and-0) — user7860670, Aug 20 '17 at 13:39
*I read somewhere that NULL and 0 are different*. Don't read anything from that place. The language requires NULL and 0 to be identical. — n. m. could be an AI, Aug 20 '17 at 13:39
I can't imagine any language where NULL is a "truthy" value. — David, Aug 20 '17 at 13:41
*I read somewhere that NULL and 0 are different.* - Conceptually, they are different. That is, you should not, for example, use `NULL` for numeric computations. But technically, when evaluated in a context where it is interpreted as a number `NULL` will always yield the value `0` (at least before C++11). — ComicSansMS, Aug 20 '17 at 13:41
@PalakJain you might have confused `NULL` and `nullptr`. `nullptr` is the one that has a separate type, but even that will be implicitly converted to a type that will evaluate to false. — PeterT, Aug 20 '17 at 14:01

score 1 · Accepted Answer · answered Aug 20 '17 at 13:49

NULL results in a false condition. You could imagine that NULL is a 0, so this:

if(NULL)

would be equivalent to this:

if(0)

thus your code would become:

#include <stdio.h>
#include <iostream>
int main() 
{
  if(0)
    std::cout<<"hello";
  else
    std::cout<<"world";
  return 0;
}

where is obvious that because 0 results to false, the if condition is not satisfied, thus the body of the if-statement is not executed. As a result, the body of the else-statement is executed, which explains the output you see.

PS: Correct way of defining NULL and NULL_POINTER?

Why am I getting this output in C++? Explain the logic

1 Answers1