Re: Why doesn't this code generate an inaccessible memory-error?

peter koch larsen <>
Tue, 9 Feb 2010 12:46:47 CST
On 9 Feb., 05:08, Seungbeom Kim <> wrote:

peter koch larsen wrote:

On 8 Feb., 14:35, Seungbeom Kim <> wrote:

CornedBee wrote:

On Feb 6, 10:19 pm, "Martin B." <> wrote:

Wait. Hmm. std::vector::operator[] is *allowed* to throw a std::exception?

Using [] with an invalid index is UB, so it is allowed to do whatever
it wants.
It's just a very bad idea for an implementation to do that, IMO.
Control flow should never differ between compilation modes, i.e. a
function that cannot throw an exception in release mode shouldn't do
so in any other mode either.


     template<typename T, ...>
     T& vector<T>::operator[](size_type i)
     #ifndef NDEBUG
         if (i >= size_) throw bad_index();
         return data_[i];


I'm not much in favour of different control flows across different
compilation modes, but in this case, the effect of an out-of-bounds
element access is undefined behaviour. Do you mean the behaviour still
has to be consistent between different compilation modes? Or was it
just a bad example?

I am not sure I understand your question as I believe that I have
already stated my opinion, but rephrasing might help. So here we go:
Since we are talking undefined behaviour, we can't demand any specific
behaviour but from a quality of implementation issue, I would expect a
compiler detecting this to stop the program or enter a debugger. What
I would not expect was letting the program continue.
I am well aware that this is likely to cause different behaviour
depending on your build settings. But then, one of those settings is
for testing the validity with a slower running program. If you need
the extra performance you compile without the checks and hope theat
your program will not run into undefined behaviour. Ideally, there
should be no difference between the two modes - only software bugs can
make a difference.


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