What are some other languages that support "partial specialization"?

Question

Partial template specialization is one of the most important concepts for generic programming in C++. For example: to implement a generic swap function:

template <typename T>
void swap(T &x, T &y) {
  const T tmp = x;
  y = x;
  x = tmp;
}

To specialize it for a vector to support O(1) swap:

template <typename T, class Alloc>
void swap(vector<T, Alloc> &x, vector<T, Alloc> &y) { x.swap(y); }

So you can always get optimal performance when you call swap(x, y) in a generic function;

Much appreciated, if you can post the equivalent (or the canonical example of partial specialization of the language if the language doesn't support the swap concept) in alternative languages.

EDIT: so it looks like many people who answered/commented really don't known what partial specialization is, and that the generic swap example seems to get in the way of understanding by some people. A more general example would be:

template <typename T>
void foo(T x) { generic_foo(x); }

A partial specialization would be:

template <typename T>
void foo(vector<T> x) { partially_specialized_algo_for_vector(x); }

A complete specialization would be:

void foo(vector<bool> bitmap) { special_algo_for_bitmap(bitmap); }

Why this is important? because you can call foo(anything) in a generic function:

template <typename T>
void bar(T x) {
  // stuff...
  foo(x);
  // more stuff...
}

and get the most appropriate implementation at compile time. This is one way for C++ to achieve abstraction w/ minimal performance penalty.

Hope it helps clearing up the concept of "partial specialization". In a way, this is how C++ do type pattern matching without needing the explicit pattern matching syntax (say the match keyword in Ocaml/F#), which sometimes gets in the way for generic programming.

Answer 1

D supports partial specialization:

• Language overview
• Template feature comparison (with C++ 98 and 0x).

(scan for "partial" in the above links).

The second link in particular will give you a very detailed breakdown of what you can do with template specialization, not only in D but in C++ as well.

Here's a D specific example of swap. It should print out the message for the swap specialized for the Thing class.

import std.stdio;    // for writefln

// Class with swap method

class Thing(T)
{
public:

    this(T thing)
    {
        this.thing = thing;
    }

    // Implementation is the same as generic swap, but it will be called instead.
    void swap(Thing that)
    {
       const T tmp = this.thing;
       this.thing = that.thing;
       that.thing = tmp;
    }

public:

    T thing;
}


// Swap generic function

void swap(T)(ref T lhs, ref T rhs)
{
    writefln("Generic swap.");

    const T tmp = lhs;
    lhs = rhs;
    rhs = tmp;
}

void swap(T : Thing!(U))(ref T lhs, ref T rhs)
{
    writefln("Specialized swap method for Things.");

    lhs.swap(rhs);
}

// Test case

int main()
{
    auto v1 = new Thing!(int)(10);
    auto v2 = new Thing!(int)(20);

    assert (v1.thing == 10);
    assert (v2.thing == 20);
    swap(v1, v2);
    assert (v1.thing == 20);
    assert (v2.thing == 10);

    return 0;
}