Question

I am slightly confused about how memory management works in C++, I understand when you use pointers (new/delete) but I’m lost when it comes to Frame allocation.

Say I have a simple class (Using QT Classes)

class Demo {
    public:
        Demo();
        ~Demo();
        QString AString() const;
        void setAString(const QString &value);
    private:
        QString aString;
};

And I Allocate it using a pointer

Demo *testInst = new Demo();

Now I understand when i call “delete testInst;” That is freed, but I’m confused about the out of scope part on Frame Allocations. Does that mean when I call delete, all those in the class that are not pointers are automatically freed, or do i have to do specific memory management within the deconstructor of the Demo class? Or does it mean if I leave the class those variables are freed?

I’m new to C++ and came from a .NET background so I’m not 100% understanding of manual memory management.

Answer 1

There are two allocation disciplines in C++. Heap and Stack. I’m suspecting that you mean Stack when you write ‘frame’. Sometimes, especially in old C sources, it’s called ‘auto’.

Demo localDemo;

is how you would use the stack. Before executing the next statement after this, C++ promises to create a temporary object of type Demo and call the no-args constructor. After the last statement in the current {} lexical block that references localDemo, C++ promises to call the destructor and release the storage. The storage is, in fact, part of the stack frame of the procedure or block.

A related question is a data member of class type. If you write:

class Proletariat {
   private:
      Demo localDemo;
};

Then the automatically-generated constructor for Proletariat will call the Demo constructor, and the the destructor will always call the Demo destructor.

If you have a constructor with args, you write something like:

class Proletariat {
   private:
      Demo localDemo;
   public:
      Proletariat() : localDemo("omeD") {}
}

to pass those args.

};