On Monday, 1 September 2025 at 22:45:58 UTC, Steven Schveighoffer
wrote:
...
And also note that structs already support encapsulation. They
just don't do inheritance/polymorphism.
True, but 'structs + templates' is a usable pattern for 'static
polymorphism' - the effect of which, is to allow interchangeable
objects, but without having to use inheritance or virtual method
tables.
In this example below, any type passed to the templated utility
function 'calculateTotalArea', must have a callable area() method.
The templated utility function is actually very fast, because the
compiler hardcodes the exact function call (rect.area() or
sq.area()). That is, it creates a completely new function
optimized specifically for the data type T.
However, there is a limitation. You can only use the templated
utility function to process an array of Rectangles, and then
again, to process an array of Squares.
Of course, with runtime polymorphism you could make a single,
unified call - but then you have some overhead associated with
the need to make complex decision-making during runtime execution.
If you're process a large collection of simple shape objects in a
tight loop, then the performance of static polymorphism will
likey win - primarly because you reduce the likelihoood of branch
prediction failures, and having the data (value types) laid out
contiguously in memory is ideal to fully utilize the various CPU
cache levels.
// -------------------------------------
module mymodule;
@safe:
private:
import std.stdio;
struct Rectangle
{
int width;
int height;
int area() const { return width * height; }
}
struct Square
{
int side;
int area() const { return side * side; }
}
int calculateTotalArea(T)(T[] shapes)
{
int total = 0;
foreach (s; shapes)
{
total += s.area;
}
return total;
}
void main()
{
writeln("--- Shape Calculations (D) - Templates (Final) ---");
Rectangle[] rects = [
Rectangle(10, 5),
Rectangle(20, 3)
];
int rectsArea = calculateTotalArea(rects);
writeln("\nTotal Area of Rectangles: ", rectsArea);
Square[] squares = [
Square(7),
Square(12)
];
int squaresArea = calculateTotalArea(squares);
writeln("Total Area of Squares: ", squaresArea);
}
// -------------------------------------
