CODEWITHSUNDEEP
c
onemasse
onemasse

Reputation: 6584

Are negative boolean values defined?

In C, at least every positive value except for 0 is treated as a boolean true. But what about a negative value? I did some tests and it seems that also negative values are treated as a boolean true. Is this a defined behaviour or implementation specific?

(I came to think about this when I saw in a question, someone promoting declaring "true" and "false" in an enum as 1 and 0.)

Upvotes: 10

Views: 10957

Answers (5)

SiegeX
SiegeX

Reputation: 140267

This is defined behavior. I'll look for the C99 standard paragraph stating as such

§ 6.3.1.2
When any scalar value is converted to _Bool, the result is 0 if the value compares equal to 0; otherwise, the result is 1.

Upvotes: 24

Chris Lutz
Chris Lutz

Reputation: 75389

C defines 0 as false and everything else as true. Positive, negative, whatever.

I believe I recently advocated the use of typedef enum { false, true } bool; so I'll own up to it. (If my original code didn't have a typedef involved, that was an error of judgement on my part.) All nonzero values are true, so I wouldn't advocate using an enumerated bool type for things like this:

if(x == true) // not what you want
if(!x == false) // works, but why so much effort?

I generally perfer simply if(x) or if(!x) to explicit tests against boolean values. However, sometimes it's good to have a boolean type:

bool is_something(int x)
{ // assume for the sake of an argument that the test is reasonably complex
    if(/* something */) return false;
    if(/* something else */) return true;
    return false;
}

This is no better than having the type be int, but at least you're being explicit with what the result is meant for.

Also, as per someone else above, a better bool might be:

typedef enum { false, true = !false } bool;

I believe ! is guaranteed to return 0 or 1, but I could be wrong, and the above works well either way.

Upvotes: -1

SubniC
SubniC

Reputation: 10317

This is the correct behavior, in C 0 is False and everything else is True

Upvotes: 2

posdef
posdef

Reputation: 6532

I believe 0 is false and everything else is true.

See @casper's reply here: thread

I would take a hint from C here, where false is defined absolutely as 0, and true is defined as not false. This is an important distinction, when compared to an absolute value for true. Unless you have a type that only has two states, you have to account for all values within that value type, what is true, and what is false.

Upvotes: 2

tdammers
tdammers

Reputation: 20721

In C, there is no boolean type; 0 and 0.0f are considered "false" in boolean contexts, everything else is "true".

Declaring "true" and "false" in an enum is wrong, because then the following code will break:

if (2 == TRUE)

(2 should evaluate as "true", but if TRUE has been defined as 1, the two values aren't considered equal).

Upvotes: 1

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