STL Queue Data Lifetime

[Scott Kilpatrick]

Maybe I'm going crazy here. I thought that a queue deletes its copied
elements when the queue is destructed. Can someone explain why the
following code works?
--------------------------
string *p;
{

queue<string> q;
string x = "hello";
q.push(x);
p = &q.front(); // p points to "hello" stored in the queue
cout << "p, *p: " << p << ", " << *p << endl;
q.pop(); // shouldn't this invalidate p?
cout << "p, *p: " << p << ", " << *p << endl;

}
// the queue is dead, why does this work?
cout << "p, *p: " << p << ", " << *p << endl;

 

[Gianni Mariani]

Maybe I'm going crazy here. I thought that a queue deletes its copied
elements when the queue is destructed. Can someone explain why the
following code works?

It may "work" but it is undefined. Consider yourself "lucky".

Here is another example ...

#include <queue>
#include <string>
#include <iostream>

using namespace std;

struct Stuff
{
Stuff( const string & ix )
: mx( ix )
{
}

~Stuff()
{
mx = mx + " : dead dead dead - you're just lucky if you see this";
}

string mx;
};

void f()
{
Stuff *p;
{

queue<Stuff> q;
string x = "hello";
q.push(x);
p = &q.front(); // p points to "hello" stored in the queue
cout << "p, *p: " << p << ", " << p->mx << endl;
q.pop(); // shouldn't this invalidate p?
cout << "p, *p: " << p << ", " << p->mx << endl;

}
// the queue is dead, why does this work?
cout << "p, *p: " << p << ", " << p->mx << endl;

}

int main()
{
f();
}

 

[Scott Kilpatrick]

It may "work" but it is undefined. Consider yourself "lucky".

Ah, ok. I knew something was screwy there. I spent some time writing a
reference counting smart pointer and noticed this during testing. For a
while I thought I wasted my time!

Thanks,
Scott