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From: Daniel T. on 8 Aug 2010 06:42 Anubhav <rkldabs(a)gmail.com> wrote: > int& f(){ > static int x = 0; > x++; > return x; > } > > int main(){ > int x = 0; > f() += 1; > f() = f() + 1; > > cout << f(); > } > > What should be the output of the code shown? > > VS gives 5 and gcc gives 6. The output is undefined. The problem here is that "f() = f() + 1" doesn't specify which f() will be resolved first and which second. -- [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ]
From: CornedBee on 8 Aug 2010 06:45 On Aug 7, 3:18 am, Anubhav <rkld...(a)gmail.com> wrote: > int& f(){ > static int x = 0; > x++; > return x; > > } > > int main(){ > int x = 0; > f() += 1; > f() = f() + 1; > > cout << f(); > > } > > What should be the output of the code shown? > > VS gives 5 and gcc gives 6. Both are correct. Everything is simple up to the expression that calls f twice. The order of evaluation around assignment isn't specified, so the compiler could execute the lhs call, increasing x to 3 and yielding a reference, then execute the rhs call, increasing x to 4, fetch x, add 1 and assign back to the reference to yield 5. Or it could execute the rhs call, increasing x to 3 and yielding a reference, then fetch the value there, add 1 to get 4, then execute the lhs call, increasing x to 4, and assign the 4 for the previous computation. There are some other valid execution orders, but those two should be enough to illustrate the issue. Sebastian -- [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ]
From: Prasoon Saurav on 8 Aug 2010 06:52 On Aug 8, 8:07 am, "Johannes Schaub (litb)" <schaub-johan...(a)web.de> wrote: > Anubhav wrote: > > int& f(){ > > static int x = 0; > > x++; > > return x; > > } > > > int main(){ > > int x = 0; > > f() += 1; > > f() = f() + 1; > > > cout << f(); > > } > > > What should be the output of the code shown? > > > VS gives 5 and gcc gives 6. > > The output is unspecified, because the orderof value computation of the > operands of op= is unspecified. Lvalue computation of the two f()'s are > unsequenced. So how the "f()" value in "f() + 1" gets computed to can be > either > > f() += 1; // after which f.x == 2 > f() = f() + 1 // after which f.x == 5 or 4 > cout << f() // after which f.x == 6 or 5 { quoted clc++m banner removed -mod } Don't you think the terms 'sequenced' and 'unsequenced' should be used in the context of C++0x? -- [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ]
From: Bo Persson on 8 Aug 2010 23:52 Prasoon Saurav wrote: > On Aug 7, 3:18 pm, Anubhav <rkld...(a)gmail.com> wrote: >> int& f(){ >> static int x = 0; >> x++; >> return x; >> >> } >> >> int main(){ >> int x = 0; >> f() += 1; >> f() = f() + 1; >> >> cout << f(); >> >> } >> >> What should be the output of the code shown? >> >> VS gives 5 and gcc gives 6. >> >> Regards, >> Dabs. >> > > As per my opinion your code invokes Undefined Behaviour because the > order of evaluation of arguments of assignment operator is > unspecified and both right hand side and left hand side f() refer > to the same static variable. So indirectly you are trying to modify > the variable more than once between two sequence points. > > Somebody please correct me if I am wrong. Slightly. :-) There are sequence points in the function calls, so the behavior is not undefined. But, like you say, it is unspecified in which order the functions are called. That makes more than one result possible, and we don't know which one we get. Bo Persson -- [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ]
From: Nick Maclaren on 9 Aug 2010 00:48
In article <fce17895-95ff-4f2b-84e1-3f16780d88dc(a)i18g2000pro.googlegroups.com>, Prasoon Saurav <prasoonsaurav.nit(a)gmail.com> wrote: >On Aug 7, 3:18 pm, Anubhav <rkld...(a)gmail.com> wrote: >> int& f(){ >> static int x = 0; >> x++; >> return x; >> >> } >> >> int main(){ >> int x = 0; >> f() += 1; >> f() = f() + 1; >> >> cout << f(); >> >> } >> >> What should be the output of the code shown? >> >> VS gives 5 and gcc gives 6. >> > >As per my opinion your code invokes Undefined Behaviour because the >order of evaluation of arguments of assignment operator is unspecified >and both right hand side and left hand side f() refer to the same >static variable. So indirectly you are trying to modify the variable >more than once between two sequence points. > >Somebody please correct me if I am wrong. Since you ask :-) The problem isn't in the assignment itself, but in the x++. There is a specific exception for assignments, but the x++ is a side-effect within one. Otherwise, you are correct. Regards, Nick Maclaren. -- [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] |