Re: Initializing defaults based on type.

[Ali Çehreli via Digitalmars-d-learn]

On 03/06/2015 08:04 AM, Benjamin Thaut wrote:

On Friday, 6 March 2015 at 15:36:47 UTC, anon wrote:

Hi,

I can't figure this out.

struct Pair(T)
{
   T x;
   T y;

   alias x c;
   alias y r;
}

What would like is that the x and y to be initialized to different
values depending on type eg:

struct Container
{
  Pair!double sample1; // This will initialize sample1 with 0 for both
x and y
  Pair!int    sample2; // This will initialize sample2 with 1 for both
x and y
}

currently I'm using two different struct one with doubles and the
other with ints and initialized with default value but was wondering
if its possible to do the above.

anon

struct Pair(T)
{
  static if(is(T == int))
    enum int initValue = 1;
  else
    enum T initValue = 0;

    T x = initValue;
    T y = initValue;

    alias x c;
    alias y r;
}

Perhaps less cluttered:

enum PairInitValue(T : int) = 1;
enum PairInitValue(T : double) = 0;

struct Pair(T)
{
   T x = PairInitValue!T;
   T y = PairInitValue!T;

   alias x c;
   alias y r;
}

And an overengineered solution: :p

import std.typetuple;

alias PairInitValues = TypeTuple!(
    int, 1,
    double, 0,
);

string makePairInitValueDefinitions()
{
    import std.string;

    string result;

    foreach (i, e; PairInitValues) {
        static if (i % 2 == 0) {
            // This is a type
            result ~= format(`enum PairInitValue(T : %s) = `, e.stringof);

        } else {
            // this is a value
            result ~= format(`%s;`, e);
        }
    }

    return result;
}

mixin (makePairInitValueDefinitions());

struct Pair(T)
{
   T x = PairInitValue!T;
   T y = PairInitValue!T;

   alias x c;
   alias y r;
}

unittest
{
    auto p = Pair!int();
    assert(p.x == 1);
    assert(p.y == 1);
}

unittest
{
    auto p = Pair!double();
    assert(p.x == 0);
    assert(p.y == 0);
}

void main()
{}

Ali