Is it possible to prevent stack allocation of an object and only allow it to be instantiated with 'new'?

Question

Is it possible to prevent stack allocation of an object and only allow it to be instiated with \'new\' on the heap?

Answer 1

You could make the constructor private, then provide a public static factory method to create the objects.

Answer 2

You could create a header file that provides an abstract interface for the object, and factory functions that return pointers to objects created on the heap.

// Header file

class IAbstract
{
    virtual void AbstractMethod() = 0;

public:
    virtual ~IAbstract();
};

IAbstract* CreateSubClassA();
IAbstract* CreateSubClassB();

// Source file

class SubClassA : public IAbstract
{
    void AbstractMethod() {}
};

class SubClassB : public IAbstract
{
    void AbstractMethod() {}
};

IAbstract* CreateSubClassA()
{
    return new SubClassA;
}

IAbstract* CreateSubClassB()
{
    return new SubClassB;
}

Answer 3

The following allows public constructors and will stop stack allocations by throwing at runtime. Note thread_local is a C++11 keyword.

class NoStackBase {
    static thread_local bool _heap;
protected:
    NoStackBase() {
        bool _stack = _heap;
        _heap = false;
        if (_stack)
            throw std::logic_error("heap allocations only");
    }
public:
    void* operator new(size_t size) throw (std::bad_alloc) { 
        _heap = true;
        return ::operator new(size);
    }
    void* operator new(size_t size, const std::nothrow_t& nothrow_value) throw () {
        _heap = true;
        return ::operator new(size, nothrow_value);
    }
    void* operator new(size_t size, void* ptr) throw () {
        _heap = true;
        return ::operator new(size, ptr);
    }
    void* operator new[](size_t size) throw (std::bad_alloc) {
        _heap = true;
        return ::operator new[](size);
    }
    void* operator new[](size_t size, const std::nothrow_t& nothrow_value) throw () {
        _heap = true;
        return ::operator new[](size, nothrow_value);
    }
    void* operator new[](size_t size, void* ptr) throw () {
        _heap = true;
        return ::operator new[](size, ptr);
    }
};

bool thread_local NoStackBase::_heap = false;

Answer 4

One way you could do this would be to make the constructors private and only allow construction through a static method that returns a pointer. For example:

class Foo
{
public:
    ~Foo();
    static Foo* createFoo()
    {
        return new Foo();
    }
private:
    Foo();
    Foo(const Foo&);
    Foo& operator=(const Foo&);
};

Answer 5

This should be possible in C++20 using a destroying operator delete, see p0722r3.

#include <new>

class C
{
private:
  ~C() = default;
public:
  void operator delete(C *c, std::destroying_delete_t)
  {
    c->~C();
    ::operator delete(c);
  }
};

Note that the private destructor prevents it from being used for anything else than dynamic storage duration. But the destroying operator delete allows it to be destroyed via a delete expression (as the delete expression does not implicitly call the destructor in this case).

Answer 6

In the case of C++11

class Foo
{
  public:
    ~Foo();
    static Foo* createFoo()
    {
        return new Foo();
    }

    Foo(const Foo &) = delete; // if needed, put as private
    Foo & operator=(const Foo &) = delete; // if needed, put as private
    Foo(Foo &&) = delete; // if needed, put as private
    Foo & operator=(Foo &&) = delete; // if needed, put as private

  private:
    Foo();
};