digitalmars.D.bugs - [Issue 2532] New: '=' does not give a boolean result
- d-bugmail puremagic.com (24/24) Dec 21 2008 http://d.puremagic.com/issues/show_bug.cgi?id=2532
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- d-bugmail puremagic.com (10/10) Jun 13 2011 http://d.puremagic.com/issues/show_bug.cgi?id=2532
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http://d.puremagic.com/issues/show_bug.cgi?id=2532
Summary: '=' does not give a boolean result
Product: D
Version: 2.022
Platform: PC
OS/Version: Linux
Status: NEW
Severity: normal
Priority: P2
Component: DMD
AssignedTo: bugzilla digitalmars.com
ReportedBy: adolf.mathias googlemail.com
Using a boolean assignment in an if condition e.g. as follows gives IMHO an
unjustified error message. When a new variable is declared in the if()
condition, everything is OK:
// tst.d
void main()
{ bool b;
if(b=false) { b = true; }
if(bool c=false) { c = true; }
}
$ dmd tst
tst.d(4): Error: '=' does not give a boolean result
--
Dec 21 2008
d-bugmail puremagic.com
http://d.puremagic.com/issues/show_bug.cgi?id=2532
= does not give a boolean result for good reason: it's to avoid the common
mistake in C/C++ where:
if(x = 5) { ... }
is used when:
if(x == 5) { ... }
was meant.
--
Dec 21 2008
http://d.puremagic.com/issues/show_bug.cgi?id=2532
2korden gmail.com changed:
What |Removed |Added
----------------------------------------------------------------------------
Status|NEW |RESOLVED
Resolution| |INVALID
It works as designed. It is designed to avoid mistakes.
"if (var = expr) {" is disallowed on purpose to avoid mistakes where you
inteded to write "if (var == expr) {":
int a = ...;
if (a = 42) { // oops, what a typo! I ment if (a == 42) here...
Bool is no different from int in this expression.
However, "if (type var == expression) {" is not a valid construct, so there is
no chance to make a mistake and thus "if (type var = expression) {" is allowed.
There is no 'special cases' for bools.
--
Dec 21 2008
http://d.puremagic.com/issues/show_bug.cgi?id=2532 The text of the message is not true if the variable being assigned to is ALREADY a bool. I know that the syntax form 'if (a = b)' is to avoided, but to say that, IN THIS CASE, the result is not a boolean is plainly wrong. bool a; bool b; b = (a = true); // A boolean expression (a = true) is used to assign to 'b'. As we know that (a = b) is a boolean expression, it seems wrong to say that it is a boolean EXCEPT in an 'if' construct. We do this to no other data type. So maybe the wording of the message can be changed to reflect what is actually being disallowed. Something like ... "The syntax form 'if (a = b)' is not allowed". That takes the argument away from the datatype issue and makes it a syntax issue instead. --
Dec 21 2008
http://d.puremagic.com/issues/show_bug.cgi?id=2532 Might "expression is not a valid condition" be better? --
Dec 21 2008
http://d.puremagic.com/issues/show_bug.cgi?id=2532
2korden gmail.com changed:
What |Removed |Added
----------------------------------------------------------------------------
Severity|normal |enhancement
Status|RESOLVED |REOPENED
Resolution|INVALID |
Reopening as an enhancement.
--
Dec 22 2008
http://d.puremagic.com/issues/show_bug.cgi?id=2532
gide nwawudu.com changed:
What |Removed |Added
----------------------------------------------------------------------------
Keywords| |diagnostic
Added keyword diagnostic.
--
Dec 22 2008
http://d.puremagic.com/issues/show_bug.cgi?id=2532 Hmm... After all D has switched to strict booleans? --
Dec 22 2008
http://d.puremagic.com/issues/show_bug.cgi?id=2532
yebblies <yebblies gmail.com> changed:
What |Removed |Added
----------------------------------------------------------------------------
CC| |schveiguy yahoo.com
*** Issue 4163 has been marked as a duplicate of this issue. ***
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Jun 13 2011
http://d.puremagic.com/issues/show_bug.cgi?id=2532
yebblies <yebblies gmail.com> changed:
What |Removed |Added
----------------------------------------------------------------------------
Keywords| |patch
CC| |yebblies gmail.com
https://github.com/D-Programming-Language/dmd/pull/122
A better message for all?
Error: assignment cannot be used as a condition
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Jun 13 2011
http://d.puremagic.com/issues/show_bug.cgi?id=2532
Walter Bright <bugzilla digitalmars.com> changed:
What |Removed |Added
----------------------------------------------------------------------------
Status|REOPENED |RESOLVED
CC| |bugzilla digitalmars.com
Resolution| |FIXED
00:46:56 PST ---
https://github.com/D-Programming-Language/dmd/commit/6279cb917ba926d2b2a14c28b23ff8e00871be27
https://github.com/D-Programming-Language/dmd/commit/772ff9a6009731cd509385d1d365d91398d39f7c
https://github.com/D-Programming-Language/dmd/commit/2f69b157fe06218cab0ebe6cf93d2f474d50a32f
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Nov 08 2011