Re: Some errors in MIT's intro C++ course

James Kanze <>
Sat, 18 Sep 2010 03:56:13 -0700 (PDT)
On Sep 11, 10:44 pm, Christian Hackl <> wrote:

Jorgen Grahn ha scritto:

On Sat, 2010-09-11, Christian Hackl wrote:


But that's what I said right from the beginning: I'd tell
them that formally you cannot count on operator[] detecting
an error, but in the context of the course and with a
quality implementation used with correct compiler settings
it will work as expected,

I don't see how that helps the students. If they want such
detection, they can run their programs under valgrind or
Purify (which they should be taught to do anyway, to catch a
dozen other common newbie errors).

I don't understand... actually, this was just about operator[]
vs. at(), and in both cases, the resulting crash won't
automatically point students to the wrong line of code, if
that's what you mean.

But the stack trace from the core dump will contain the wrong
line of code. Where as with an uncaught exception, it might not

The crash first of foremost helps students in that they
immediately see that their code is wrong and must be fixed.

And it can't be caught:-). Which is an advantage where students
are involved.

and it is also the commonly preferred form in "real" code,
not only in exercises at university.

I have never, ever used a bounds-checking
std::vector<T>::operator[]. And I have never seen others
use it, either. I rely on it to be as fast as C array

I meant "preferred form" in the sense that it is preferred
over at().

If the profiler says that you can't afford bounds checking (and
on most systems, it can only be turned on or off globally, with
a lot of other, far more expensive tests), then you can't afford
it, and you have to turn it off. In that case, however, you
generally can't use at() either.

There is a real issue that these checks must be turned on or off
globally, for the entire program. Which means that one critical
loop, and you're without bounds checking everywhere.

I have heard rumors that such checks are enabled by default in the
Microsoft world ... but then I have also heard people complaining
"ooh, the standard containers are too slow for me, I must use raw
C-style arrays!" from the same world.

This is indeed a rumor.

#include <vector>

int main()
   std::vector<int> v;
   v[0] = 1;

If you compile this just with "cl test.cpp", then you probably
won't get a crash.

But if you use the IDE, with the default settings, you probably
will. (I don't have any compilers installed on my machne here,
so I can't check.)

Note that if you compile with /MDd, and don't have bounds
checking turned on, you'll get some occasional problems
(crashes, etc.) with std::string (pre-VS 2010, at least---I've
been told by people at Microsoft that this problem has been
fixed in VS 2010). And in practice, you do have to specify
either /MDd or /MD is you want your code to work with user
written DLLs.

James Kanze

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