Re: debug assert error in a std::sort of vector<int>
<lists@givemefish.com> wrote in message
news:1161170928.176129.21350@i3g2000cwc.googlegroups.com...
I'm getting a debug assert error when performing a sort.
The platform / compiler is Windows XP Pro / MSVC 8.0 (Professional
Edition), in debug configuration.
Here is the code fragment:
[code]
std::vector<int> v1;
v1.push_back(3);
v1.push_back(4);
v1.push_back(8);
v1.push_back(5);
v1.push_back(6);
v1.push_back(7);
v1.push_back(5);
std::sort(v1.begin(), v1.end(), not2(std::greater<int>()));
[/code]
[I realize that I could use std::less<int> in this case, but I was
experimenting with not2.]
I get the assertion from within the sort function that says:
Expression: invalid operator <
The code appears to work fine if I ignore the assertion. Also, the
code compiles and runs under gcc.
Am I really making an error?
Yes. Your predicate is effectively !(x < y) which is the same as
x >= y which is *not* a strict weak ordering. The debugging code
in our library detects any predicate call where pred(x, y) and
pred(y, x) are both true -- a sure sign that pred is not a strict
weak ordering.
Sometimes sort rattles around and survives being called with
a bad predicate; sometimes it gets the wrong answer; and
sometimes it stores off the end of the sorted sequence. So
just because it "appears to work fine" or "runs" does not
mean it's safe/valid code.
P.J. Plauger
Dinkumware, Ltd.
http://www.dinkumware.com
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