C++ moving a unique_ptr to a struct member - c++

Question

I have the following program -
#include <iostream>
#include <memory>
class Person
{
public:
Person(const std::string& name):
name(name) { }
~Person() { std::cout << "Destroyed" << std::endl; }
std::string name;
};
typedef struct _container
{
std::unique_ptr<Person> ptr;
}CONTAINER;
void func()
{
CONTAINER* c = static_cast<CONTAINER*>(malloc(sizeof(CONTAINER)));
std::unique_ptr<Person> p(new Person("FooBar"));
c->ptr = std::move(p);
std::cout << c->ptr->name << std::endl;
}
int main()
{
func();
getchar();
return 0;
}
The program prints "FooBar". I expect the program to print "Destroyed" when func() return but it does not. Can someone help me with why that would not be happening in this case?

Answer 1

You've actually got undefined behaviour here.
You cant just cast a malloc'd buffer to an object type. The constructors are never called and your member variables are in an invalid state.
You need to do either:
void func()
{
CONTAINER c;
std::unique_ptr<Person> p(new Person("FooBar"));
c.ptr = std::move(p);
std::cout << c.ptr->name << std::endl;
}
Or
void func()
{
CONTAINER * c = new CONTAINER();
std::unique_ptr<Person> p(new Person("FooBar"));
c->ptr = std::move(p);
std::cout << c->ptr->name << std::endl;
delete c;
}
or if you really want to use malloc - you'll need to use placement new to get the right behaviour - but usually you dont want that, so I wont elaborate for now...

Answer 2

You forget to add this line at the end of func().
delete c;
Here is the test (ideone).
c is a raw pointer. It is not a smart pointer.
Thus, you have to delete it manually.
Deleting c will automatically delete CONTAINER::ptr because CONTAINER::ptr is a unique pointer.
However, you have malloc yourself, the more proper code might be :-
c->~_container();
Then free(), but I don't think it is needed in this case, because CONTAINER is not on a heap.
( I have never used malloc, so I am not sure about this part. )
Edit:
My solution is a quick patch to solve a single problem. (not print "Destroyed")
Please also read Michael Anderson's solution.
It addresses another underlying issue of the OP's code. (malloc)
Edit2:
Here is a good link about placement new that Michael Anderson mentioned.
The code below is copied from the link (with little modification):-
int main(int argc, char* argv[]){
const int NUMELEMENTS=20;
char *pBuffer = new char[NUMELEMENTS*sizeof(A)];
//^^^ difference : your "CONTAINER" could be char[xxxx] (without new)
A *pA = (A*)pBuffer;
for(int i = 0; i < NUMELEMENTS; ++i) {
pA[i] = new (pA + i) A();
}
printf("Buffer address: %x, Array address: %x\n", pBuffer, pA);
// dont forget to destroy!
for(int i = 0; i < NUMELEMENTS; ++i){
pA[i].~A();
}
delete[] pBuffer;//<--- no need to delete char[] if it is a stack variable
return 0;
}
For further detail, please see that above link (because I don't want to copy more of it to here).
Here is another useful link : Using malloc in C++ is generally not recommended.