This is a text-only version of the following page on https://raymii.org:
---
Title : Only zero is false, everything else is true in C++
Author : Remy van Elst
Date : 12-07-2019
URL : https://raymii.org/s/snippets/Cpp_Only_zero_is_false.html
Format : Markdown/HTML
---
Some of the code I work on does the following, mostly because it's older C style
code now used in a C++ context:
* Have a value that can be changed by a user.
* If that value is not set, it is set to `-1`
* That variable, when set to a number, is what a function will use
Zero in this context means that it can be changed but is set to 0. `-1` sometimes
means it can not be edited, but sometimes means it's off. Most of the cases I find
where this is used do it this way to save memory.
(This gives a host of other problems, like, how to preserve the value when
(turning it off and later just turning it back on?)
Sometimes this variable is checked for true-ness by using a boolean conversion,
like this:
if (variable) {
do_stuff();
}
Only if the variable is zero, this check will not execute. If it's set to `-1`,
the boolean conversion will convert to `true`, which might not be what you meant.
What you want to check for is:
if (variable > 0) {
do_stuff();
}
But better would be to have a seperate variable for the `on/off` and a seperate
variable for the actual value to use.
This is oversimplified and for seasoned programmers this will be nothing new,
however I found it interesting.
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### Implicit conversion rules to booleans
The rules for implicit conversion, which is what's happening when you use
something else as a boolean, are described [here][1].
[1]: https://en.cppreference.com/w/cpp/language/implicit_conversion
Quoting:
> A prvalue of integral, floating-point, unscoped enumeration, pointer, and
pointer-to-member types can be converted to a prvalue of type bool. The
value zero (for integral, floating-point, and unscoped enumeration) and the
null pointer and the null pointer-to-member values become false. All other
values become true.
Here is my example code:
#include <iostream>
int main () {
bool boolMinOne;
bool boolPlusOne;
bool boolZero;
bool boolnullptr;
bool boolPtr;
int intPlusOne { 1 };
int intMinOne { -1 };
int intZero { 0 };
int* intPtr { &intPlusOne };
int* nullPtr { nullptr };
boolMinOne = intMinOne;
boolPlusOne = intPlusOne;
boolZero = intZero;
boolPtr = intPtr;
boolnullptr = nullPtr;
std::cout << "boolMinOne: " << boolMinOne << "\n";
std::cout << "boolPlusOne: " << boolPlusOne << "\n";
std::cout << "boolZero: " << boolZero << "\n";
std::cout << "boolNullptr: " << boolnullptr << "\n";
std::cout << "boolPtr: " << boolPtr << "\n";
return 0;
}
Result:
boolMinOne: 1
boolPlusOne: 1
boolZero: 0
boolNullptr: 0
boolPtr: 1
---
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