Strange behavior of .toStdString()

[HaHoHSO]

Hi all,

I just came across the following subtlety: The code

QString a("121");

std::string s = a.toStdString();
const char* p = s.c_str();
const char* q = a.toStdString().c_str();

for (int i = 0; i < 4; i++) std::cout << (int)p[i] << " "; std::cout << std::endl;
for (int i = 0; i < 4; i++) std::cout << (int)q[i] << " "; std::cout << std::endl;

results in

49 50 49 0
0 50 49 0

Why does the first byte differs depending on using .toStdString() and .c_str() in one resp. two different lines?

Thanks a lot in advance!

[VRonin]

because accessing q is undefined behaviour.
a.toStdString() returns a temporary (std::string) object, you take its address and then the object gets deleted.

It's equivalent to:

const char* q;
{
std::string p = a.toStdString();
q = p.c_str();
}

[HaHoHSO]

Got it! Thanks a lot!