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digitalmars.D.bugs - bool.d(5): '=' does not give a boolean result

reply Regan Heath <regan netwin.co.nz> writes:
void main()
{
   bool b;

   if (b = true) {}
}

ok, bad form I know, but the message is a bit.. odd.. right?

Regan

The real-world example read something like

function() {
   foreach() {
     if (!(r = function_call(a,b,c)) break;
   }
   return r;
}

-- 
Using M2, Opera's revolutionary e-mail client: http://www.opera.com/m2/
Oct 03 2004
next sibling parent reply J C Calvarese <jcc7 cox.net> writes:
Regan Heath wrote:

 void main()
 {
   bool b;
 
   if (b = true) {}


Did you mean...

if (b == true) {}

or did the point of this code fly right over my head?

-- 
Justin (a/k/a jcc7)
http://jcc_7.tripod.com/d/
Oct 03 2004
next sibling parent reply Derek Parnell <derek psych.ward> writes:
On Sun, 03 Oct 2004 23:34:42 -0500, J C Calvarese wrote:


 Regan Heath wrote:

 void main()
 {
   bool b;
 
   if (b = true) {}


 
 Did you mean...
 
 if (b == true) {}
 
 or did the point of this code fly right over my head?


Maybe, but the error message is 

"test.d(5): '=' does not give a boolean result"

The message is "odd" because the variable in question is a bool ;-)

-- 
Derek
Melbourne, Australia
4/10/2004 2:56:56 PM
Oct 03 2004
parent Regan Heath <regan netwin.co.nz> writes:
On Mon, 4 Oct 2004 14:58:42 +1000, Derek Parnell <derek psych.ward> wrote:

 On Sun, 03 Oct 2004 23:34:42 -0500, J C Calvarese wrote:


 Regan Heath wrote:

 void main()
 {
   bool b;

   if (b = true) {}


 Did you mean...

 if (b == true) {}

 or did the point of this code fly right over my head?


 Maybe, but the error message is

 "test.d(5): '=' does not give a boolean result"

 The message is "odd" because the variable in question is a bool ;-)


Yeah that's what I mean't. Assigning to a bool should give a boolean 
result, yes?
eg.

bool a,b;

a = (b = true);

if (b = true) wasn't a bool, then the above would give an error, right? 
But it doesn't, instead this

bool r;
bool fn();

if (r = fn()) break;

gives an error.

Basically something seems amiss to me.

Regan

-- 
Using M2, Opera's revolutionary e-mail client: http://www.opera.com/m2/
Oct 04 2004
prev sibling parent reply Regan Heath <regan netwin.co.nz> writes:
On Sun, 03 Oct 2004 23:34:42 -0500, J C Calvarese <jcc7 cox.net> wrote:

 Regan Heath wrote:

 void main()
 {
   bool b;

   if (b = true) {}


 Did you mean...

 if (b == true) {}

 or did the point of this code fly right over my head?


It was intended to be an assignment, the real-world code looks something 
like:

bool function() {
   foreach() {
     if (!(r = function_call(a,b,c)) break;
   }
   return r;
}

I just found it really odd that assigning '=' to a 'bool' doesn't give a 
boolean result.

Regan

-- 
Using M2, Opera's revolutionary e-mail client: http://www.opera.com/m2/
Oct 04 2004
parent reply Derek <derek psyc.ward> writes:
On Tue, 05 Oct 2004 10:38:19 +1300, Regan Heath wrote:


 On Sun, 03 Oct 2004 23:34:42 -0500, J C Calvarese <jcc7 cox.net> wrote:

 Regan Heath wrote:

 void main()
 {
   bool b;

   if (b = true) {}


 Did you mean...

 if (b == true) {}

 or did the point of this code fly right over my head?


 
 It was intended to be an assignment, the real-world code looks something 
 like:
 
 bool function() {
    foreach() {
      if (!(r = function_call(a,b,c)) break;
    }
    return r;
 }
 
 I just found it really odd that assigning '=' to a 'bool' doesn't give a 
 boolean result.
 


This syntax format is a special case that D checks for. Its designed to
prevent a common source of bugs in C/C++. The more correct way to code what
you want is ...

 bool function() {
    foreach() {
      if ( (r = function_call(a,b,c)) == true) break;
    }
    return r;
 }
 
In general, the syntax form ...

   if ( a = b )

needs to be coded as ...

   if ( (a = b) != 0 ) 

The reason is that *most* of the time, what people mean when they code "if
(a = b)" is really "if (a == b)". Walter decided to prevent this common
coding error. So now if you really really mean to do an assignment /and/
test the result of that assignment, as if it was a boolean test, you must
explicitly code the test part of it.

-- 
Derek
Melbourne, Australia
Oct 04 2004
parent Derek Parnell <derek psych.ward> writes:
On Tue, 5 Oct 2004 07:56:19 +1000, Derek wrote:


 On Tue, 05 Oct 2004 10:38:19 +1300, Regan Heath wrote:
 

 On Sun, 03 Oct 2004 23:34:42 -0500, J C Calvarese <jcc7 cox.net> wrote:

 Regan Heath wrote:

 void main()
 {
   bool b;

   if (b = true) {}


 Did you mean...

 if (b == true) {}

 or did the point of this code fly right over my head?


 
 It was intended to be an assignment, the real-world code looks something 
 like:
 
 bool function() {
    foreach() {
      if (!(r = function_call(a,b,c)) break;
    }
    return r;
 }
 
 I just found it really odd that assigning '=' to a 'bool' doesn't give a 
 boolean result.
 


 
 This syntax format is a special case that D checks for. Its designed to
 prevent a common source of bugs in C/C++. The more correct way to code what
 you want is ...
 
  bool function() {
     foreach() {
       if ( (r = function_call(a,b,c)) == true) break;
     }
     return r;
  }
  
 In general, the syntax form ...
 
    if ( a = b )
 
 needs to be coded as ...
 
    if ( (a = b) != 0 ) 
 
 The reason is that *most* of the time, what people mean when they code "if
 (a = b)" is really "if (a == b)". Walter decided to prevent this common
 coding error. So now if you really really mean to do an assignment /and/
 test the result of that assignment, as if it was a boolean test, you must
 explicitly code the test part of it.


Well I suppose it could also be written out thus ...

   a = b;
   if (a) ...

so ...

 bool function() {
     foreach() {
       r = function_call(a,b,c);
       if (r) break;
     }
     return r;
  }

-- 
Derek
Melbourne, Australia
5/10/2004 10:00:29 AM
Oct 04 2004
prev sibling parent reply "Valéry Croizier" <valery freesurf.fr> writes:
 void main()
 {
    bool b;

    if (b = true) {}
 }

 ok, bad form I know, but the message is a bit.. odd.. right?

 Regan


It's not a bug, it's a feature described in the specs : "Assignments do not
yield boolean results."
Oct 04 2004
parent Regan Heath <regan netwin.co.nz> writes:
On Mon, 4 Oct 2004 23:36:53 +0200, Valéry Croizier <valery freesurf.fr> 
wrote:

 void main()
 {
    bool b;

    if (b = true) {}
 }

 ok, bad form I know, but the message is a bit.. odd.. right?

 Regan


 It's not a bug, it's a feature described in the specs : "Assignments do 
 not yield boolean results."


Well I'll be.. I didn't see that. :)

While I can understand why this 'feature' is a good idea, it stops 
mistakes like "if (a=b)" where you meant "if (a==b)" it is a little weird 
when applied to boolean assignments.

I guess I'll have to re-write:

bool function() {
   foreach() {
     if (!(r = function_call(a,b,c))) break;
   }
   return r;
}

as:

bool function() {
   foreach() {
     r = function_call(a,b,c);
     if (!r) break;
   }
   return r;
}

which is probably a GoodThing(TM) anyway.

Regan

-- 
Using M2, Opera's revolutionary e-mail client: http://www.opera.com/m2/
Oct 04 2004