Shared Ptr vs Normal Ptr: Object Creation After Declaration

Question

With normal pointers, I can declare a pointer and then set it equal to a new object, however with shared pointers I am unable to do that. Why?

#include <memory>
struct node{
    int num;
    node* next;
};
int main()
{
    std::shared_ptr<node> new_node1 = NULL; // WORKS
    new_node1 = new node;   // ERROR, why?
    node* new_node2 = NULL; //WORKS
    new_node2 = new node;   //WORKS

    return 0;
}

Why can't we create a new object for a shared pointer? Is there a way to do it?

new_node1.reset (new node); or new_node1 = std::make_shared <node> (); — David Schwartz
– David Schwartz, Commented May 19, 2015 at 23:41
The short answer to your first question is that if you could do so easily, terrible things would happen. Consider Foo *f = new Foo(); bar(f); bar(f);. Now, what happens if bar takes a std::shared_ptr<Foo>? (Think about the destruction of the temporary shared pointer.) You want to know for sure where and when you are constructing a shared_ptr because that takes over control of the lifetime of the underlying object. — David Schwartz
– David Schwartz, Commented May 20, 2015 at 0:00

Tas · Accepted Answer · 2015-05-19 23:44:29Z

4

std::shared_ptr<node> n(new node);
n.reset(new node);
n = std::make_shared<node>();

You should prefer make_shared

answered May 19, 2015 at 23:44

Tas

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Bill Lynch · Accepted Answer · 2015-05-19 23:48:17Z

2

This is because the constructor that would be called during the operator=() call is marked as explicit.

To work around this:

new_node1 = std::shared_ptr<node>(new node);

Or:

new_node1 = std::make_shared<node>();

answered May 19, 2015 at 23:48

Bill Lynch

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