RAII object for releasing pointer with arbitrary function

Question

The following class is a sort of "smart pointer" that can be used for automatically releasing pointers:

template <class Type>
class Releaser {
 public:
 Releaser(std::function<void(Type *)> function = [](Type *value) {
 value->Release();
 });
 ~Releaser();
 Type *operator->();
 Type &operator*();
 Type *get();
 Type **fill();
 private:
 Type *value_ = nullptr;
 std::function<void(Type *)> function_;
};
template <class Type>
Releaser<Type>::Releaser(std::function<void(Type *)> function)
 : function_(function) {}
template <class Type>
Releaser<Type>::~Releaser() {
 if (value_) function_(value_);
}
template <class Type>
Type *Releaser<Type>::operator->() {
 return value_;
}
template <class Type>
Type &Releaser<Type>::operator*() {
 return *value_;
}
template <class Type>
Type *Releaser<Type>::get() {
 return value_;
}
template <class Type>
Type **Releaser<Type>::fill() {
 return &value_;
}

It can be used for example like this:

Releaser<IMFAttributes> attributes;
auto status = MFCreateAttributes(attributes.fill(), 1);

where MFCreateAttributes has the following signature:

MFCreateAttributes(IMFAttributes** ppMFAttributes, unsigned int cInitialSize);

Do you see any way to improve this (e.g by getting rid of the std::function), or make it safer?

• ratchet freak: unique_ptr supports custom deleters. Though the deleter type will leak into the template arguments (which can be alleviated using a typedef).
• Jan Rüegg: unique_ptr doesn't work because of the missing "fill" functionality. See stackoverflow.com/questions/12403750/…

Answer 1

unique_ptr has capability for custom deleters.

Besides that assuming you are doing this to deal with COM objects then you can use ComPtr and its GetAddressOf

---

If you define a destructor you should also define (or disallow in this case) the copy construct and copy assign. Failure to do so will result in dangling pointers and double frees:

Releaser<IMFAttributes> attributes;
auto status = MFCreateAttributes(attributes.fill(), 1);
{
 Releaser<IMFAttributes> att = attributes;
} // att gets cleaned up
//attributes contains a dangling pointer

• Jan Rüegg: Thenks, very useful input! unique_ptr doesn't work because of the missing GetAddressOf function. I didn't know ComPtr, but this works quite well for cases where only a simple Release is needed, however not for custom release functions (for example if value_->Shutdown() has to be called instead of value_->Release()). Preventing a copy constructor is a very good point though. Also, I ended up adding an assert(value != nullptr) in the fill function, to avoid overriding a non-released pointer.