CODEWITHSUNDEEP
cstandardsarithmetic-expressionsinteger-arithmetic
ToBeen
ToBeen

Reputation: 69

What can bool type return in C99?

What is bool guaranteed to return as? Can I rely on a bool type to always be interpreted as 0 or 1? If bool's numerical interpretation isn't reliable in C99 is there a standard where it is? Are there any things I should look out for in LLVM, GCC, or Visual C++ when relying on bool for arithmetic?

Just for example what if I did this:

// An example of calculating a possible stride in bytes.
// hasData(), is8bit(), and is16bit() return as bool type

unsigned int stride = object->hasData() * object->size() * 
                      (object->is8bit()  * sizeof(uint8_t) + 
                       object->is16bit() * sizeof(uint16_t));

In the example I'm betting on bool type only returning 0 or 1.

Upvotes: 2

Views: 323

Answers (4)

haccks
haccks

Reputation: 106052

Boolean value of an expression is either 1 (true) or 0 (false) always and are of type int instead of _Bool.

C99 - 7.16:

  1. The header <stdbool.h> defines four macros.
  2. The macro
    • bool expands to _Bool.
  3. The remaining three macros are suitable for use in #if preprocessing directives. They are
    • true : which expands to the integer constant 1,
    • false: which expands to the integer constant 0, and
    • __bool_true_false_are_defined which expands to the integer constant 1.

Upvotes: 1

Mats Petersson
Mats Petersson

Reputation: 129374

The C99 standard requires the compiler to produce _Bool values that are either 0 or 1. C++ also has this requirement. No, I don't know the exact page of the standard document that describes this.

Of course, 16 years after the introduction, it's still possible that there are bugs in compilers, and more so if we have a compiler that is, say, 10 years old and wasn't well tested for C99 compatibility when it was produced. But I frequently write code that relies on the compiler producing 0 or 1 from conditional expressions, for example.

Upvotes: 1

Martin Bonner supports Monica
Martin Bonner supports Monica

Reputation: 28997

In C++ bool is a type which is guaranteed to contain either true or false. When converted to int (as in your example), these correspond to 1 and 0 respectively.

Upvotes: 1

ouah
ouah

Reputation: 145879

What is bool guaranteed to return as? Can I rely on a bool type to always be interpreted as 0 or 1?

In a strictly conforming program a _Bool (or bool from stdbool.h) expression is guaranteed to be evaluated to either 0 or 1.

If you get a different value from a bool expression it means you are invoking some undefined behavior upstream.

Upvotes: 5

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