By disabling copy constructor and copy assignment, we can be sure that the ownership of B is “moved” from one owner to another.

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#include <iostream>
#include <memory>
using namespace std;

struct B {
~B() {
puts("~B");
}
};


struct A {
unique_ptr<B> o;

A() {}

explicit A(B* o_) : o{o_} {}

A(const A&) = delete;
A& operator=(const A&) = delete;

A(A&& other) : o{move(other.o)} {
puts("move ctor");
}

A& operator=(A&& other) {
puts("move =");
o = move(other.o);
return *this;
}
};

int main()
{
{
A a1{new B};
printf("a1.o is %p\n", a1.o.get());
// A a2 = a1; // error; deleted copy ctor
}

puts("");

{
A a1{new B};
printf("a1.o is %p\n", a1.o.get());
// A copied = a1; // error; deleted copy ctor
A a2 = move(a1);
printf("a1.o is %p\n", a1.o.get());
}

puts("");

{
A a1{new B};
printf("a1.o is %p\n", a1.o.get());
A a2{move(a1)};
printf("a1.o is %p\n", a1.o.get());
}

puts("");

{
A a1{new B};
printf("a1.o is %p\n", a1.o.get());
A a2;
a2 = move(a1);
printf("a1.o is %p\n", a1.o.get());
}

return 0;
}