Re: Can a well-formed program incur undefined behaviour?
On 18/01/2011 22:54, jordigh@gmail.com wrote:
Reading 1.3.12 of the current standard and comparing it with 1.3.13,
it's not clear to me if the standard allows a well-formed program
(1.3.14) to incur undefined behaviour. In Freenode's ##c++ channel,
people suggested that
#include<vector>
int main(){
std::vector<int> x; x[2] = 5;
}
is a well-formed program that causes undefined behaviour, but doesn't
this contradict the requirement of a well-formed program that it is
"constructed according to [...] diagnosable semantic rules"?
The definition of undefined behaviour talks of erroneous programs, but
the wording doesn't seem to exclude the possibility of undefined
behaviour from a well-formed program. Am I under the correct
impression?
Thanks,
- Jordi G. H.
The answer is 'yes' And here is a simple minimalist example (#includes
omitted)
int main(){
Int i;
std::cin >> i;
std::cout << i + i - i << std::endl;
}
Any undefined behaviour will be completely benign on almost every
exiting implementation but nonetheless if the input to i is sufficiently
large so that i + i overflows you have UB.
Effectively, Well-formed means that no diagnostic is required.
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