Why does the deferenced pointer print the correct value first timne?

[curiousengineer]

Here is the program written and executed via Visual Studio 2012.

#include <iostream>
using std::cout;
using std::endl;

double* tripple(double);

int main()
{
double num(5);
double *p(nullptr);
p = tripple(num);
cout << *p << std::endl;
cout << *p << std::endl;
return 0;
}

double* tripple(double x)
{
double y(0);
y=3*x;
return &y;
}

I am testing the Iron-Cast rule which says:
"Never, ever, return the address of a local automatic variable from a function."

However, in the output from the program, the first cout prints correct value of 15 while the other prints a strange exponential number. I expected both to produce gibberish result. Why does the first cout produce correct result?

If there is any others statement like cout << " "; before the first cout << *p << std::endl; then I get gibberish both times.

[keskiverto]

Memory is allocated from stack for arguments and local variables, when you do call a function. After return, the memory is no longer allocated. Next time something needs stack memory, it will get that available block.

What you call "correct result" -- the 15 -- is as much gibberish as the printed other values. It just a fluke that with your compiler and specific code the bytes, where the tripple() did store value "15", were not reused by anything else before the dereference operation.

[Peter87]

My guess is that the memory where the variable was stored has not yet been used for anything else so it still contains the old value you gave it. When the << operator is called it probably uses its own local variables that overwrites the memory where y in tripple was previously stored.

[cire]

Why does the first cout produce correct result?

It doesn't print a correct result. There is no such thing as a "correct" result when you evoke undefined behavior, or perhaps it would be better to say that every possible result is equally correct.

[curiousengineer]

I was expecting it to be some coincidence as well, anyway. I take note that what the program did is by definition "undefined behaviour" and thus one must not indulge in it.