How does the custom deleter of std::unique_ptr work?

Question

According to N3290, std::unique_ptr accepts a deleter argument in its constructor.

However, I can\'t get that to work with Visual C++ 10.0 or MinGW g++

Answer 1

This works. The destruction happens properly.

class Base
{
    public:
     Base() { std::cout << "Base::Base\n"; }
     virtual ~Base() { std::cout << "Base::~Base\n"; }
};


class Derived : public Base
{
    public:
     Derived() { std::cout << "Derived::Derived\n"; }
     virtual ~Derived() { std::cout << "Derived::~Derived\n"; }
};

void Delete(const Base* bp)
{
    delete bp;
}

int main()
{
    std::unique_ptr<Base, void(*)(const Base*)> ptr = std::unique_ptr<Derived, void(*)(const Base*)>(new Derived(), Delete);
}

Answer 2

To complement all previous answers, there is a way to have a custom deleter without having to "pollute" the unique_ptr signature by having either a function pointer or something equivalent in it like this:

std::unique_ptr< MyType, myTypeDeleter > // not pretty

This is achievable by providing a specialization to the std::default_delete template class, like this:

namespace std
{
template<>
class default_delete< MyType >
{
public:
  void operator()(MyType *ptr)
  {
    delete ptr;
  }
};
}

And now all std::unique_ptr< MyType > that "sees" this specialization will be deleted with it. Just be aware that it might not be what you want for all std::unique_ptr< MyType >, so chose carefully your solution.

Answer 3

This works for me in MSVC10

int x = 5;
auto del = [](int * p) { std::cout << "Deleting x, value is : " << *p; };
std::unique_ptr<int, decltype(del)> px(&x, del);

And on gcc 4.5, here

I'll skip going to the standard, unless you don't think that example is doing exactly what you'd expect it to do.

Answer 4

My question has been pretty well answered already.

But just in case people wondered, I had the mistaken belief that a unique_ptr<Derived> could be moved to a unique_ptr<Base> and would then remember the deleter for the Derived object, i.e., that Base would not need to have a virtual destructor. That was wrong. I'd select Kerrek SB's comment as "the answer", except one cannot do that for a comment.

@Howard: the code below illustrates one way to achieve what I believed the cost of a dynamically assigned deleter had to mean that unique_ptr supported out of the box:

#include <iostream>
#include <memory>           // std::unique_ptr
#include <functional>       // function
#include <utility>          // move
#include <string>
using namespace std;

class Base
{
public:
    Base() { cout << "Base:<init>" << endl; }
    ~Base() { cout << "Base::<destroy>" << endl; }
    virtual string message() const { return "Message from Base!"; }
};

class Derived
    : public Base
{
public:
    Derived() { cout << "Derived::<init>" << endl; }
    ~Derived() { cout << "Derived::<destroy>" << endl; }
    virtual string message() const { return "Message from Derived!"; }
};

class BoundDeleter
{
private:
    typedef void (*DeleteFunc)( void* p );

    DeleteFunc  deleteFunc_;
    void*       pObject_;

    template< class Type >
    static void deleteFuncImpl( void* p )
    {
        delete static_cast< Type* >( p );
    }

public:
    template< class Type >
    BoundDeleter( Type* pObject )
        : deleteFunc_( &deleteFuncImpl< Type > )
        , pObject_( pObject )
    {}

    BoundDeleter( BoundDeleter&& other )
        : deleteFunc_( move( other.deleteFunc_ ) )
        , pObject_( move( other.pObject_ ) )
    {}

    void operator() (void*) const
    {
        deleteFunc_( pObject_ );
    }
};

template< class Type >
class SafeCleanupUniquePtr
    : protected unique_ptr< Type, BoundDeleter >
{
public:
    typedef unique_ptr< Type, BoundDeleter >    Base;

    using Base::operator->;
    using Base::operator*;

    template< class ActualType >
    SafeCleanupUniquePtr( ActualType* p )
        : Base( p, BoundDeleter( p ) )
    {}

    template< class Other >
    SafeCleanupUniquePtr( SafeCleanupUniquePtr< Other >&& other )
        : Base( move( other ) )
    {}
};

int main()
{
    SafeCleanupUniquePtr< Base >  p( new Derived );
    cout << p->message() << endl;
}