Re: std::map, does the standard define what to do here?

Richard Herring <junk@[]>
Tue, 7 Apr 2009 12:08:30 +0100
In message
rishabh <> writes

On Apr 7, 3:01?am, peter koch <> wrote:

On 6 Apr., 23:39, Zachary ?Turner <> wrote:

std::map<int, int> test;
bool eq1 = false;
bool eq2 = false;

test[5] = 7;
std::map<int, int>::iterator iter = test.begin();
std::map<int, int>::iterator iterEnd = test.end();

eq1 = (iter == iterEnd);
eq2 = (iter == iterEnd);

I suspect the answer is that no, decrementing the begin iterator is
undefined, but just looking for confirmation. ?In Visual C++, after
this code executes eq1 is false and eq2 is true(!), and in fact in
Visual C++ you can decrement this iterator indefinitely and it will
just cycle through the tree forever.

You are correct - it is undefined behaviour, so don't do it.

eq2 is never true even you decrease it more than 1 time.

What part of "undefined behaviour" don't you understand?


Expression: --r
pre: there exists s such that r == ++s.

Richard Herring

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