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From: lxg8906@yahoo.com To: gcc-gnats@gcc.gnu.org Subject: other/9693: sequence point consistency Date: Thu, 13 Feb 2003 17:06:00 -0000 [thread overview] Message-ID: <20030213170039.27334.qmail@sources.redhat.com> (raw) >Number: 9693 >Category: other >Synopsis: sequence point consistency >Confidential: no >Severity: non-critical >Priority: medium >Responsible: unassigned >State: open >Class: sw-bug >Submitter-Id: net >Arrival-Date: Thu Feb 13 17:06:01 UTC 2003 >Closed-Date: >Last-Modified: >Originator: Frank G >Release: g++ 3.2 >Organization: >Environment: Solaris 2.8 Sparc9 >Description: >How-To-Repeat: >Fix: >Release-Note: >Audit-Trail: >Unformatted: ----gnatsweb-attachment---- Content-Type: text/plain; name="bug_report.txt" Content-Disposition: inline; filename="bug_report.txt" Dear Sir/Madam, The version I am using is g++ 3.2, platform is Solaris 2.8. <program 1> int main() { int i=2; int j=(++i)+(++i); cout<<j<<endl; return 0; } The output is 7. It seems that (++i) is evaluated first, yields 3, increments, then 3+4=7. I am a little confused here. C++ std 5.1/4 reads: "Between the previous and next sequence point a scalar object shall have its stored value modified at most once by the evaluation of an expresion..." Should we have i modified twice here? Or this statement is just undefined? Solaris CC gives j=8 which I think is more reasonable. Consider the following <program 2> int& foo(int& i) { return ++i; } int main() { int i=2; int j=foo(i)+foo(i); cout<<j<<endl; return 0; } foo() is just an in-place increment function, g++ gives j=8. Is it that j=foo(i)+foo(i) is the same as j=(++i)+(++i) ? why they give two different results? If I wrap the integer in a class, as follows, <program 3> #include <iostream> using namespace std; class T { public: int m_b; T(int i=0): m_b(i) {} T& operator ++(void) { ++m_b; return *this;} T operator +(cosnt T& rhs) { m_b+=rhs.m_b; return *this;} void foo(void) {cout<<m_b<<endl;} }; int main() { T t1(2); T t2=(++t1)+(++t1); t2.foo(); return 0; } The result is j=8. In the program 2 and 3, if we change the return type in foo() and operator++ to T&, j=7. Is it reasonble to have program 1-3 have different results when they in fact are doing the same thing? Thank you. Frank G.
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