Solved can not print correctly after convert QString to char *

[Christian Ehrlicher]

@Mozzie said in can not print correctly after convert QString to char *:

where is the temp object in memory, stack or heap or somewhere else.

It's on the stack since you did not allocate it with new

if it is on stack, it can not remain until the stack is finished

No, this is not allowed since it's unnamed.

It's also not c++ specific - you can do the same (in a little bit more obvious way) in C:

int *myPtr = nullptr;
{
  int a = 3;
  myPtr = &a;
  printf("%d\n", *myPtr);   // works fine
}
printf("%d\n", *myPtr);   // works on garbage and may eat kitten

[JonB]

@Christian-Ehrlicher said in can not print correctly after convert QString to char *:

int *myPtr = nullptr;

Never heard of nullptr in C ;-) NULL was much nicer to read anyway.

[jsulm]

This post is deleted!

[Mozzie]

@Christian-Ehrlicher
thanks, it helped a lot.
and i have a hunch

{//main stack
	QString s = "hello world";
	char* p = nullptr;
	{// toUtf8()
		QByteArray b = s.toUtf8();
		{// data();
			p = b.data();
			qDebug() << p; // does this is same as "qDebug() << s.toUtf8().data();"
		}
	}
	// b is freed
	qDebug() << p; // this is same as "char * p = s.toUtf8().data(); qDebug() << p;"
}

does this right?

[Mozzie]

@JonB
nullptr is a c++11 key word, you can still use NULL, but NULL is defined as 0, sometimes it may cause some problem.

such as:

void test(int *p)
{
	qDebug() << "int *";
}
void test(int i)
{
	qDebug() << "int";
}
test(NULL);
test(nullptr);

output

int
int *

[JonB]

@Mozzie
I know this :) That's why I was picking @Christian-Ehrlicher on his use of nullptr in his C program, it was just intended for amusement ;-)

[Christian Ehrlicher]

@Mozzie said in can not print correctly after convert QString to char *:

does this right?

Yes, exactly.

[Mozzie]

@Christian-Ehrlicher
thanks.