use method reset of unique_prt:free(): invalid pointer

Question

When I use reset to rebind the unique_ptr, something bad happens but when use std::make_unique, everything is ok.

int main() {
    unique_ptr<string> p= nullptr;
    auto l=[&p]()->void{
        static string x {"1111"};
        cout<<x<<endl;
        //p = std::make_unique<string>(x);// compile ok
        p.reset(&x);//compile error free():invalid pointer; 
        cout<<p.get()<<endl;
        return;
    };
    l();
    cout<<p->length()<<endl;
    cout<<p.get()<<endl;
    return 0;
}

I think that when the reset method is called, the string object is not really handed over to the unique_ptr management. When leaving the scope, the dtor is called twice

Answer 1

Both variants do compile without errors (provided one adds the missing parts). Your code invokes undefined behavior and likely crashes at runtime.

When you use std::make_unique like this:

p = std::make_unique<string>(x);

Then std::make_unique constructs a std::unique_ptr that manages the lifetime of a std::string that is constructed as a copy of x. You now have two strings, x and the one managed by p.

When you use reset like this:

p.reset(&x);

You tell p to take ownership of x. However, x is not dynamically allocated. It makes no sense to transfer its ownership to a smart pointer. When ps destructor is called it will attempt to delete &x but x was not allocated via new, hence there is undefined behavior. When the program terminates the lifetime of the static string ends and x is destroyed (again).

---

I think that when the reset method is called, the string object is not really handed over to the unique_ptr management.

Yes it is. And that is the problem.

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It is not clear what the actual purpose of the code is. To return string from the lambda:

#include <string>
#include <iostream>

int main() {
    auto l=[](){
        std::string x {"1111"};
        std::cout << x << std::endl;        
        return x;
    };
    auto p = l();
    std::cout << p.length() << std::endl;
}