Confused using unique_ptr and a custom deleter

Question

I'm trying to use unique_ptr with a custom deleter for SDL_Surface type. This is only an example using int type, but I hope you get the idea.

#include <iostream>
#include <functional>
#include <memory>

typedef int SDL_Surface;


SDL_Surface * CreateSurface()
{
    SDL_Surface * p = new SDL_Surface;
    return p;
}

void FreeSurface(SDL_Surface *p)
{
    delete p;
}

int main() {
    std::unique_ptr<SDL_Surface, std::function< void (SDL_Surface *) > > uptr_1; 

    //how to assign a value to uptr_1 and the deleter? 

    return 0;
}

Is uptr_1 correctly declared and initialized to nullptr ? If so, how can I assign the pointer and the deleter function?

And how can I encapsulate this: std::unique_ptr< SDL_Surface, std::function< void (SDL_Surface *) > > with the deleter to not always write that line on every SDL_Surface I want, another typedef?

I'm just starting to learn C++11 features and this is a hard one for me.

Answer 1

You can initialise the unique_ptr with a pointer and deleter, or use = normally if re-assigning later:

std::unique_ptr<SDL_Surface, std::function<void (SDL_Surface *)>> uptr_1(CreateSurface(), &FreeSurface);

uptr_1 = std::unique_ptr<SDL_Surface, std::function<void (SDL_Surface *)>>(CreateSurface(), &FreeSurface);

Refer to suitable docs for details.

To shorten the long type, you can indeed use a type alias ( typedef or using ):

typedef std::unique_ptr<SDL_Surface, void (*)(SDL_Surface*)> Surface_ptr;

//or

using Surface_ptr = std::unique_ptr<SDL_Surface, void (*)(SDL_Surface*)>;

Notice I've actually used void (*)(SDL_Surface*) for the deleter type. If you know you'll always pass an actual function (or stateless lambda) in, there's no reason to drag in std::function , which has some overhead due to type erasure.

Also, you can shorten it even further by creating a default-constructible functor for the deleter:

struct FreeSurface_Functor
{
  void operator() (SDL_Surface *s) const
  {
    FreeSurface(s);
  }
};

That way, you can make the type of your pointer std::unique_ptr<SDL_Surface, FreeSurface_Functor> (possibly aliased) and you don't have to provide the deleter; it will be default-constructed:

std::unique_ptr<SDL_Surface, FreeSurface_Functor> uptr_1(CreateSurface());

Answer 2

I would go with decltype :

std::unique_ptr<SDL_Surface, decltype(&FreeSurface)> uptr_1(
          CreateSurface(),
          FreeSurface
);

Answer 3

Is uptr_1 correctly declared and initialized to nullptr

Yes, a default constructed unique_ptr will refer to null.

if so, how can I assign the pointer and the deleter function?

You should be constructing the unique_ptr with arguments

 std::unique_ptr<SDL_Surface, std::function< void (SDL_Surface *) > > uptr_1{CreateSurface(), FreeSurface};

Alternatively, after the default construction you could use move assignment with a temporary

uptr_1 = std::unique_ptr<SDL_Surface, std::function< void (SDL_Surface *) > >{CreateSurface(), FreeSurface};

As you've suggested yourself, a type alias can help

using SDL_Uptr = std::unique_ptr<SDL_Surface, std::function< void (SDL_Surface *)>>;
SDL_Uptr  uptr_1;
uptr_1 = SDL_Uptr{CreateSurface(), FreeSurface};

An intermediate function could help simplify this if it becomes repetitive (which it probably will if you make a lot of them).

std::unique_ptr<SDL_Surface, void (*)(SDL_Surface *)>
make_sdl_ptr() {
    return std::unique_ptr<SDL_Surface, void (*)(SDL_Surface *)>{CreateSurface(), FreeSurface};
}

You could then call this with auto uptr = make_sdl_ptr();

Angew's answer with a DefaultConstructible deleter calling your function is also a really nice solution.