Smart Pointers

[Anmol444]

When i try to compile this code it gives me a error during run-time saying "*program name* has stopped working"

Code:

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

int main()
{
 unique_ptr<double> ptr2;

 double a=30;

 *ptr2 = a;

 return 0;
}

Why is this happening?

Also why do you need the asterisk on smart pointers to assign them a value? Is it just because, or is there a reason.

[cire]

Why is this happening?

Because you're dereferencing a pointer that hasn't been made to point at a valid memory location.

int main()
{
    double * ptr2 = nullptr ;
    double a ;
    *ptr2 = a ;
    delete ptr2 ;
    return 0 ;
}

is essentially the same code with a normal pointer.

[Anmol444]

Oh I got it, I got confused with some code in my book:

#include <iostream>
#include <memory>
using std::cout;
using std::endl;
using std::unique_ptr;
unique_ptr<double> treble(double);
int main(void)
{
double num(5.0);
unique_ptr<double> ptr(nullptr);
ptr = treble(num);
cout << endl << "Three times num = " << 3.0*num;
cout << endl << "Result = " << *ptr;
cout << endl;
return 0;
}
unique_ptr<double> treble(double data)
{
unique_ptr<double> result(new double(0.0));
*result = 3.0*data;
return result;
}

This part:

unique_ptr<double> treble(double data)
{
unique_ptr<double> result(new double(0.0));
*result = 3.0*data;
return result;

Notice how it does *result and assigns the value of result to 3.0*data?
I thought it was being assigned a rvalue because smart pointers support that and I thought you needed the asterisk for smart pointers.


Well thanks again cire, you always point out my stupid mistakes.

[Anmol444]

Wait can Smart pointers only point to dynamically created objects and not normal objects?

[Catfish4]

Seen as the purpose of smart pointers is to automatically delete the memory they "own" once it's no longer needed (or an exception wreaked havoc), I'd say they shouldn't "point" to normal object or they'll try to delete it too. I think. I may be completely wrong.

http://ideone.com/m1zAkO

[Anmol444]

Oh ok thanks, that makes sense. So they can only point to dynamically allocated objects, thanks!