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Only zero is false, everything else is true in C++

Published: 12-07-2019 | Author: Remy van Elst | Text only version of this article


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Some of the code I work on does the following, mostly because it's older C style code now used in a C++ context:

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.

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
Tags: c , c++ , conversion , cpp , development , snippets