boolean variables for loops vs conditions - c++

Question

Here is my code:
int main()
{
int input;
bool isZero = false;
while (!isZero)
{
...
if (0 == input)
{
isZero = true;
}
else
{
isZero = false;
}
}
return 0;
}
the program does what its supposed to, but I feel like the !isZero expression is not foolproof.
is
bool isZero = false;
while(!isZero);
{
....
}
the same thing as
bool isZero = false;
while(isZero == false)
{
...
}
why or why not?
Also, in which cases does true represent 1, and in which cases does it represent any nonzero number?

Answer 1

While requires the condition to be a Boolean and !isZero is already a Boolean. Equality operator generates a Boolean, so again isZero == false is a Boolean. So the only thing you changed is adding another operator and (possibly) slowing down loop a little (AFAIR comparison is slower than bit operators).
Operations on Boolean values are similar to operations on integer values. Your concerns can be then translated into integers: "Hey, x == 1 is not foolproof, I have to check, whether x - 1 == 0".
The bottomline is, that !isZero is absolutely foolproof enough.
As for int to bool conversions, the standard says,
A prvalue of arithmetic, unscoped enumeration, pointer, or pointer to member type can be converted to a prvalue of type bool. A zero value, null pointer value, or null member pointer value is converted to false; any other value is converted to true. For direct-initialization (8.5), a prvalue of type std::nullptr_t can be converted to a prvalue of type bool; the resulting value is false.

Answer 2

C++ is very lenient in its requirements for a conditional. It accepts booleans, and it will implicitly cast any integer to a boolean (where 0 is false and non-zero is true), and it will also implicitly cast any pointer to a boolean (since pointers are unsigned integers. In that case, NULL would be false, and any other pointer would be true). Thus, the following all evaluate to true, so the while will execute.
int i = 4;
char c = '0';
bool b = true;
void * p = 0xdeadbeef;
while (i) // ...
while (c) // ...
while (b) // ...
while (p) // ...
While the following won't evaluate to true, so the while won't execute.
int i = 0;
char c = '\0';
bool b = false;
void * p = NULL;
while (i) // ...
while (c) // ...
while (b) // ...
while (p) // ...
According to C++, all of these are allowed.
When you're looking at a boolean in your expression, it's redundant to put a == operator on it. Whether b above is true or false, it makes perfect sense to say while (b), and you don't need while (b == true). As for all of the others, each programmer makes their own choice. My personal preference is to explicitly compare all non-booleans. The statement int i = 5; while (i) may be legal in C++, but it just doesn't read clearly. I would much rather see int i = 5; while (i != 0). It's unnecessary, but doesn't cost you any extra processor operations, and is explicit as to its intent.