In article <6iff8g$i8q@engnews1.Eng.Sun.COM>#1/1,
  clamage@Eng.Sun.COM (Steve Clamage) wrote:
>
> The sample code looked like this:
>
> >output   : command (or parial cmd) that generated the output
> >---------:----------------------------------------------------------
> >(3,3,3)  :  Vector3f a(atof(str[++i]), atof(str[++i]), atof(str[++i]))
>
> The code attempts to modify a variable more than once between
> sequence points, and so the behavior is undefined. That has
> always been the case in C, and is still the case in C++.

I thought that this example yields unspecified behavior, not undefined
behavior.  The reason is the unspecified order of function argument
evaluation.  Isn't there a sequence point after each call to atof()
(assuming that atof() is a function call and not a macro) [intro.execution
p17]?  If so, then 'i' isn't modified more than once between sequence
points.

The arguments to Vector3f constructor will be evaluated in an un-specified
order, but there is a sequence point after each one.  If 'i' had the value
of 2, then there should be six possible outcomes:

   (3,4,5)
   (3,5,4)
   (4,3,5)
   (4,5,3)
   (5,3,4)
   (5,4,3)

Whether or not the behavior is classified as undefined or unspecified
is probably of little consequence to most programmers, as relying on either
one will lead to unreliable programs.


Aaron

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Can anyone tell me if the following are defined by the ANSI C++ standard,
and if so.  What the correct results should be?

I have included the test code below, and this is the results with
various compiliers on various OSes.

They all produce different output with the same test case.  I realize
the use of ++i in the same expression may be undefine, so implementors
are able to code it as desired.  However, I would like clarification
on this (if possible).

Thanks,

 Neil Buesing
 buesing@demaco1.demaco.com

SGI : IRIX6.3 : MIPSpro Compilers: Version 7.20 (n32)
%CC -n32 test.C
%./a.out
output   : command (or parial cmd) that generated the output
---------:----------------------------------------------------------
(3,3,3)  :  Vector3f a(atof(str[++i]), atof(str[++i]), atof(str[++i]))
(3,3,3)  :  Vector3f b(f[++i], f[++i], f[++i])
3 3 3    :  cout << ++i    <<  << ++i    <<  << ++i
1 2 3    :  cout << a1     <<  << a2     <<  << a3
3 3 3    :  cout << f[++i] <<  << f[++i] <<  << f[++i]
2 2 2    :  cout << i++    <<  << i++    <<  << i++
1 2 3    :  cout << b1     <<  << b2     <<  << b3


SGI : IRIX6.3 : MIPSpro Compilers: Version 7.20 (o32)
%CC -32 test.C
%./a.out
output   : command (or parial cmd) that generated the output
---------:----------------------------------------------------------
(1,2,3)  :  Vector3f a(atof(str[++i]), atof(str[++i]), atof(str[++i]))
(3,3,3)  :  Vector3f b(f[++i], f[++i], f[++i])
1 2 3    :  cout << ++i    <<  << ++i    <<  << ++i
1 2 3    :  cout << a1     <<  << a2     <<  << a3
1 2 3    :  cout << f[++i] <<  << f[++i] <<  << f[++i]
0 1 2    :  cout << i++    <<  << i++    <<  << i++
1 2 3    :  cout << b1     <<  << b2     <<  << b3

SGI : IRIX6.2 : Mongoose Compilers: Version 7.10 (n32)
%CC -n32 test.C
%./a.out 1 2 3 4
output   : command (or parial cmd) that generated the output
---------:----------------------------------------------------------
(1,2,3)  :  Vector3f a(atof(str[++i]), atof(str[++i]), atof(str[++i]))
(3,3,3)  :  Vector3f b(f[++i], f[++i], f[++i])
1 2 3    :  cout << ++i    <<  << ++i    <<  << ++i
1 2 3    :  cout << a1     <<  << a2     <<  << a3
1 2 3    :  cout << f[++i] <<  << f[++i] <<  << f[++i]
0 1 2    :  cout << i++    <<  << i++    <<  << i++
1 2 3    :  cout << b1     <<  << b2     <<  << b3


SUN : Solaris2.5.1 : CC: WorkShop Compilers 4.2 18 Sep 1997 C++ 4.2
%CC test.C
%./a.out
output   : command (or parial cmd) that generated the output
---------:----------------------------------------------------------
(3,3,3)  :  Vector3f a(atof(str[++i]), atof(str[++i]), atof(str[++i]))
(3,3,3)  :  Vector3f b(f[++i], f[++i], f[++i])
1 2 3    :  cout << ++i    <<  << ++i    <<  << ++i
1 2 3    :  cout << a1     <<  << a2     <<  << a3
3 3 3    :  cout << f[++i] <<  << f[++i] <<  << f[++i]
0 1 2    :  cout << i++    <<  << i++    <<  << i++
1 2 3    :  cout << b1     <<  << b2     <<  << b3




/////////////////
//test.C : BEGIN
//

#include <iostream.h>
#include <stdlib.h>

///////////////////////////////////////////////////////////////////////////////

class Vector3f {
 private:
  float _vector[3];
 public:
  Vector3f(float, float, float);
  friend ostream &operator<<(ostream &os,  const Vector3f &v);
  inline const float& operator[](int i) const;
  inline float& operator[](int i);
};

///////////////////////////////////////////////////////////////////////////////

Vector3f::Vector3f(float x, float y, float z) {
  _vector[0] = x;
  _vector[1] = y;
  _vector[2] = z;
}

///////////////////////////////////////////////////////////////////////////////

inline const float& Vector3f::operator[](int i) const {
  return _vector[i];
}

///////////////////////////////////////////////////////////////////////////////

inline float& Vector3f::operator[](int i) {
  return _vector[i];
}

///////////////////////////////////////////////////////////////////////////////

ostream &operator<<(ostream &os,  const Vector3f &v) {
  os << "(" << v[0] << "," << v[1] << "," << v[2] << ")";
  return os;
}

///////////////////////////////////////////////////////////////////////////////

int main(int argc, char *argv[])
{
 int i=0;

 float f[4] = {0, 1, 2, 3};
 const char *str[4] = { "0", "1", "2", "3" };

 //

 cout << "output   : command (or parial cmd) that generated the output" <<
endl;
 cout <<
"---------:----------------------------------------------------------"
      << endl;

 //Case #1

 i=0;
 Vector3f a(atof(str[++i]), atof(str[++i]), atof(str[++i]));

 cout << a
      << "  :  Vector3f a(atof(str[++i]), atof(str[++i]), atof(str[++i]))"
      << endl;

 //Case #2

 i=0;

 Vector3f b(f[++i], f[++i], f[++i]);

 cout << b;
 cout << "  :  Vector3f b(f[++i], f[++i], f[++i])"
      << endl;

 //Case #3

 i=0;

 cout << ++i << " " << ++i << " " << ++i
      << "    :  cout << ++i    << " " << ++i    << " " << ++i"
      << endl;

 //Case #4

 i=0;

 int a1 = ++i;
 int a2 = ++i;
 int a3 = ++i;

 cout << a1 << " " << a2 << " " << a3
      << "    :  cout << a1     << " " << a2     << " " << a3"
      << endl;

 //Case #5

 i=0;
 cout << f[++i] << " " << f[++i] << " " << f[++i]
      << "    :  cout << f[++i] << " " << f[++i] << " " << f[++i]"
      << endl;

 //Case #6

 i=0;
 cout << i++ << " " << i++ << " " << i++
      << "    :  cout << i++    << " " << i++    << " " << i++"
      << endl;

 //Case #7

 i=0;

 int b1, b2, b3;

 b1=++i,b2=++i,b3=++i;

 cout << b1 << " " << b2 << " " << b3;
 cout << "    :  cout << b1     << " " << b2     << " " << b3"
      << endl;

}

//
// test.C : END

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buesing@demaco1.demaco.com writes:

>Can anyone tell me if the following are defined by the ANSI C++ standard,
>and if so.  What the correct results should be?

>I have included the test code below, and this is the results with
>various compiliers on various OSes.

>They all produce different output with the same test case.  I realize
>the use of ++i in the same expression may be undefine, so implementors
>are able to code it as desired.  However, I would like clarification
>on this (if possible).

The sample code looked like this:

>output   : command (or parial cmd) that generated the output
>---------:----------------------------------------------------------
>(3,3,3)  :  Vector3f a(atof(str[++i]), atof(str[++i]), atof(str[++i]))
>(3,3,3)  :  Vector3f b(f[++i], f[++i], f[++i])
>3 3 3    :  cout << ++i    <<  << ++i    <<  << ++i
>3 3 3    :  cout << f[++i] <<  << f[++i] <<  << f[++i]
>2 2 2    :  cout << i++    <<  << i++    <<  << i++

The code attempts to modify a variable more than once between
sequence points, and so the behavior is undefined. That has
always been the case in C, and is still the case in C++.

See the C++ draft standard, chapter 5, paragraph 4, which even
contains examples like yours.

---
Steve Clamage, stephen.clamage@sun.com
---
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For the statement:
cout<<++i<<' '<<++i<<' '<<++i<<endl;
The output should be 4 5 6, rather than 3 3 3 if i was initialized as 3.

--
sweet boy
e-mail: anson@mail.com
ICQ UIN: 2056592
homepage: http://www.geocities.com/~foreverland



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"sweet boy" <anson@mail.com> writes:

>For the statement:
>cout<<++i<<' '<<++i<<' '<<++i<<endl;
>The output should be 4 5 6, rather than 3 3 3 if i was initialized as 3.

No, the results are undefined, as I explained in another article
with the same subject. You can't expect anything in particular
as program output.

--
Steve Clamage, stephen.clamage@sun.com
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