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Re: problem in C++ pointer
- To: "swe sd" <ccwork at hotmail dot com>, <cygwin at sourceware dot cygnus dot com>
- Subject: Re: problem in C++ pointer
- From: "Jay Krell" <jay dot krell at cornell dot edu>
- Date: Sat, 11 Mar 2000 16:31:28 -0800
Your code might be triggering undefined behavior in C++, because of the
++X[0] in the same statement where you otherwise read X[0]. Definitely
something like
printf("%d%d", X[0], ++X[0]);
is undefined but I've read something along the lines that when the
operator's are actually overloaded, function calls, that the order of
evaluation becomes defined..
> line 9: cout<<A[0]<<" "<<&A<<" "<<&A[0]<<endl
> line 10: <<*X<<endl
> line 11: <<*X+5<<endl
> line 12: <<*X<<" "<<A[0]<<endl
> line 13: <<5+X[0]<<endl
> line 14: <<( X[0]==0 ? "X[0]=0" : "X[0]!=0")<<endl
> line 15: <<++X[0]<<endl
something like, skipping endl.
cout.<<(A[0]).<<(" ").<<(&A).<<(" ").<<(&A[0]).<<(*X).<<(*X+5).<<(*X).<<("
").<<A[0].<<(5+X[0]).<<(( X[0]==0 ? "X[0]=0" : "X[0]!=0")).<<(++X[0]);
- Jay
-----Original Message-----
From: swe sd <ccwork@hotmail.com>
To: cygwin@sourceware.cygnus.com <cygwin@sourceware.cygnus.com>
Date: Saturday, March 11, 2000 9:31 AM
Subject: B20: problem in C++ pointer
> I compiled the following program (attachment test.cc):
> line 1: #include <iostream>
> line 2: void main()
> line 3: { const int size=10;
> line 4: int A[size];
> line 5: int *X=NULL, i;
> line 6: for (i=0;i<size;i++) A[i]=i;
> line 7: cout<<A[0]<<" "<<&A<<" "<<&A[0]<<endl;
> line 8: X=A;
> line 9: cout<<A[0]<<" "<<&A<<" "<<&A[0]<<endl
> line 10: <<*X<<endl
> line 11: <<*X+5<<endl
> line 12: <<*X<<" "<<A[0]<<endl
> line 13: <<5+X[0]<<endl
> line 14: <<( X[0]==0 ? "X[0]=0" : "X[0]!=0")<<endl
> line 15: <<++X[0]<<endl
> line 16: }
> and executing it gives output:
> $./a.out
> 0 0x259fd7c 0x259fd7c
> 1 0x259fd7c 0x259fd7c
> 1
> 6
> 1 1
> 6
> X[0]!=0
> 1
> Obviously, the output of line 9 is different from line 7 which should
be
>the same indeed. Is there anything wrong ? Thanks.
>
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