Re: Type of @true in Obj-C vs Obj-C++
Re: Type of @true in Obj-C vs Obj-C++
- Subject: Re: Type of @true in Obj-C vs Obj-C++
- From: Jean-Daniel Dupas <email@hidden>
- Date: Wed, 10 Jul 2013 09:51:54 +0200
Le 9 juil. 2013 à 20:56, Jens Alfke < email@hidden> a écrit : On Jul 9, 2013, at 11:31 AM, Scott Ribe < email@hidden> wrote: I don't see what @true/@false have to do with true/false.
@true is a shortcut for @(true), so the compiler first evaluates ‘true’ and then boxes the result.
I think the issue is that the auto-boxing code has a special case for the Obj-C YES/NO constants that outputs boolean-type NSNumbers instead of 0/1, but it doesn’t have the same special case for the C99 ‘bool’ type.
On Jul 9, 2013, at 02:48 , Jean-Daniel Dupas < email@hidden> wrote: No, unless you redefine the true and false macros.
The C standard specifies that true and false are macro that expand to integer constant with value 1 and 0 respectively.
In C99 ‘true’ and ‘false’ are builtin constants of type ‘bool’, not macros. (Right? IANALL.)
C99 defines a _Bool type, but it did not introduced new builtin constants for true and false. true and false are expected to be defined by <stdbool.h>. If you don't include this header, trying to use true or false will result in an undefined identifier error.
And to quote the C11 standard:
7.18 Boolean type and values <stdbool.h>
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.
4 Notwithstanding the provisions of 7.1.3, a program may undefine and perhaps then
redefine the macros bool, true, and false.259)
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