Re: Can a well-formed program incur undefined behaviour?
On Jan 18, 2:54 pm, "jord...@gmail.com" <jord...@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?
Consider:
#include <new>
void f(int *p)
{
*p = 7;
}
int main()
{
f(new (std::nothrow) int[1000000000];
return 0;
}
Depending on how much memory is available to your process, the nothrow
new may or may not return NULL. The accessing of *p in f() is
undefined
in the case where it returns NULL.
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