Overflow

[Gonçalo Rodrigues]

Hi all,

Suppose I have:

#include <climits>

const int val = INT_MAX;
val += 1;

Does the standard prescribe what happens with the above overflowing?
And if not, is there any *portable* way of discovering what happens?

Note: I've tested in my platform and the value val wrapps around.

TIA, with my best regards,
G. Rodrigues

 

[Tim Love]

Hi all,

Suppose I have:

#include <climits>

const int val = INT_MAX;
val += 1;

Does the standard prescribe what happens with the above overflowing?
And if not, is there any *portable* way of discovering what happens?

Note: I've tested in my platform and the value val wrapps around.

Did you mean the const to be there? I get

Error 337: "foo.cc", line 5 # The left side of '+=' must be a modifiable lvalue.
val += 1;
^^^

 

[Stefan Strasser]

Hi all,

Suppose I have:

#include <climits>

const int val = INT_MAX;
val += 1;

Does the standard prescribe what happens with the above overflowing?
And if not, is there any *portable* way of discovering what happens?

Note: I've tested in my platform and the value val wrapps around.

undefined behaviour
the standard even mentions systems which raise exceptions on
overflow(like x86 does on division by zero).

 

[Alf P. Steinbach]

* Gonçalo Rodrigues:

Suppose I have:

#include <climits>

const int val = INT_MAX;
val += 1;

Does the standard prescribe what happens with the above overflowing?

No.

It does however specify overflow behavior for unsigned integer types.

Where the value is reduced modulo 2^n, n the number of value representation
bits.

And if not, is there any *portable* way of discovering what happens?

Partially. std::numeric_limits<int>::is_modulo tells you whether the type
has wrapping behavior or not. However, if it doesn't (on nearly all modern
C++ implementation it will be wrapping, but _if_ it doesn't wrap) then that
still leaves the question of what happens on overflow, open.

Note: I've tested in my platform and the value val wrapps around.

Not very surprising; google for "two's complement".

 

[Gonçalo Rodrigues]

Did you mean the const to be there? I get

Error 337: "foo.cc", line 5 # The left side of '+=' must be a modifiable lvalue.
val += 1;
^^^

Of course not, bad cut and paste.

Regards,
G. Rodrigues