Ben's Corner

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A while back, while watching Handmade Hero, I watched Casey demonstrate one way to produce polymorphism without inheritance (and the vtable pointer dereferencing penalty it carries). He did this by using a tagged union to hold the derived classes, and switching on the tag in the polymorphic method (actually a function in this case). I've never actually seen this, before, so I want to write up how it works, and my thoughts on it.

The Code

#include <cstdio>
#include <cassert>
#include <cstdint>

constexpr double  PI = 3.14159;

enum color
{
color_red,
color_blue
};


Now adding the tag part for the tagged union:

enum shape_type
{
shape_rect,
shape_circle
};


Now we create the unique part of each shape:

struct rect
{
int Width, Height;
};

struct circle
{
};


And create the shared part:

struct shape
{
// shared variables
color Color;

shape_type ShapeType;
union
{
rect Rect;
circle Circle;
};
};


Notice that we have the tag (ShapeType) and a union of each type.

Let's create some constructors- I'm creating one constructor to initialize the shared data, and a two more constructors, one for each shape, that call the shared constructor.

void InitShape(shape* Shape, color Color)
{
Shape->Color = Color;
}

void InitRect(shape* Shape, color Color, int Width, int Height)
{
InitShape(Shape, Color);
Shape->ShapeType = shape_rect;
Shape->Rect.Width = Width;
Shape->Rect.Height = Height;
}

void InitCircle(shape* Shape, color Color, int Radius)
{
InitShape(Shape, Color);
Shape->ShapeType = shape_circle;
}


This is a very C-like approach, but it works and it's easy to understand.

Finally, here's the polymorphic function to compute the area of the shape:

double Area(shape* Shape)
{
switch (Shape->ShapeType)
{
case shape_rect:
{
return Shape->Rect.Width * Shape->Rect.Height;
}
case shape_circle:
{

}
}
assert(0 && "Unknown Shape...");
}


Here's an example main:

int main()
{
shape Shape;

InitRect(&Shape, color_red, 2, 3);
printf("%f\n", Area(&Shape));
// 6.000000

InitCircle(&Shape, color_blue, 4);
printf("%f\n", Area(&Shape));
// 12.566360
}


Conclusions

I think this is an interesting approach. These are some knee-jerk reactions too it that may or may not be well thought out...

Pros

• It's straightforward approach means it's very easy to see how these structs are laid out in memory. No hidden vtables to worry about here...
• Likewise, because you're doing the work of the compiler, it compiles very quickly.
• There's no vtable, so no cache misses- the class's members are always right next to each other.
• No magic

Cons

• No interfaces. grep -ir <class_name> src/ might be your best bet to see what the class does, and that's if the developer names everything to be friendly to that approach.
• Every data member is laid out in memory, so every object of type Shape will be the same size. If one derived Shape has particularly large data members, all Shapes will pay the penalty.
• I think the use of union subverts RAII.
• It's hard to extend. If you want to add a new shape, you must add a new member to the enum, create a new Init function, and modify all polymorphic functions that you care about. It's easy to imagine an undisciplined developer messing up case in that function that worked before.
• No extensibility without source access.

Despite all of those cons, I still think this approach is really readable and I like the potential speed gains. If this is a project where I didn't really need RAII, I was the only developer, and my classes were roughly the same size, I'd probably use it.

Resources

Here are some particularly noteworthy resources on polymorphism in C++ that feel like they belong here: