Initializing a std::unique_ptr by passing the address of the pointer - c++

Question

I am creating a class which interops with some Windows API code, now one of the pointers I have to initialize is done by calling a native function which initializes it.
My pointers are of type std::unique_ptr with a custom deleter, which calls the WinAPI deleter function provided, however I cannot pass the unique_ptr with the & address-of operator to the init-function. Why?
I have created a sample that demonstrates my problem:
#include <memory>
struct foo
{
int x;
};
struct custom_deleter {};
void init_foo(foo** init)
{
*init = new foo();
}
int main()
{
std::unique_ptr<foo, custom_deleter> foo_ptr;
init_foo(&foo_ptr);
}
The compiler barks and says:
source.cpp: In function 'int main()':
source.cpp:19:21: error: cannot convert 'std::unique_ptr<foo, custom_deleter>*' to 'foo**' for argument '1' to 'void init_foo(foo**)'

Answer 1

Somewhere under the covers, unique_ptr<foo> has a data member of type foo*.
However, it's not legitimate for a user of the class to directly modify that data member. Doing so would not necessarily preserve the class invariants of unique_ptr, in particular it wouldn't free the old pointer value (if any). In your special case you don't need that to happen, because the previous value is 0, but in general it should happen.
For that reason unique_ptr doesn't provide access to the data member, only to a copy of its value (via get() and operator->). You can't get a foo** out of your unique_ptr.
You could instead write:
foo *tmp;
init_foo(&tmp);
std::unique_ptr<foo, custom_deleter> foo_ptr(tmp);
This is exception-safe for the same reason that std::unique_ptr<foo, custom_deleter> foo_ptr(new foo()); is exception-safe: unique_ptr guarantees that whatever you pass in to its constructor will eventually get deleted using the deleter.
Btw, doesn't custom_deleter need an operator()(foo*)? Or have I missed something?

Answer 2

Steve has already explained what the technical problem is, however, the underlying problem goes much deeper: The code employs an idiom helpful when you deal with naked pointers. Why does this code do two-step initialization (first create the object, then initialize it) in the first place? Since you want to use smart pointers, I'd suggest you carefully adapt the code:
foo* init_foo()
{
return new foo();
}
int main()
{
std::unique_ptr<foo, custom_deleter> foo_ptr( init_foo() );
}
Of course, renaming init_foo() to create_foo() and having it return a std::unique_ptr<foo> directly would be better. Also, when you use two-step initialization, it's often advisable to consider using a class to wrap the data.

Answer 3

You can use the following trick:
template<class T>
class ptr_setter
{
public:
ptr_setter(T& Ptr): m_Ptr{Ptr} {}
~ptr_setter() { m_Ptr.reset(m_RawPtr); }
ptr_setter(const ptr_setter&) = delete;
ptr_setter& operator=(const ptr_setter&) = delete;
auto operator&() { return &m_RawPtr; }
private:
T& m_Ptr;
typename T::pointer m_RawPtr{};
};
// Macro will not be needed with C++17 class template deduction.
// If you dislike macros (as all normal people should)
// it's possible to replace it with a helper function,
// although this would make the code a little more complex.
#define ptr_setter(ptr) ptr_setter<decltype(ptr)>(ptr)
and then:
std::unique_ptr<foo, custom_deleter> foo_ptr;
init_foo(&ptr_setter(foo_ptr));

Answer 4

I eventually came up with an approach that allows to initialise unique_ptr's with a code like this:
struct TOpenSSLDeleter { ... }; // Your custom deleter
std::unique_ptr<EVP_MD_CTX, TOpenSSLDeleter> Ctx;
...
Ctx = MakeUnique(EVP_MD_CTX_create()); // MakeUnique() accepts raw pointer
And here is the solution:
template <class X>
struct TUniquePtrInitHelper {
TUniquePtrInitHelper(X *Raw) noexcept {
m_Raw = Raw;
}
template <class T, class D>
operator std::unique_ptr<T, D>() const noexcept {
return std::unique_ptr<T, D>(m_Raw);
}
private:
X *m_Raw;
};
template <class X>
TUniquePtrInitHelper<X> MakeUnique(X *Raw) noexcept {
return {Raw};
}