Add a variable to a class instance at runtime: Difference between revisions

=={{header|D}}==

D is a statically compiled language, so there are some limits.

This adds a new 'attribute' to a struct/class instance, of type T:

<lang d>import std.stdio: writeln;

import std.traits: isImplicitlyConvertible;

struct S(T) {

T[string] data;

template opDispatch(string name) {

T opDispatch(Types...)(Types args)

if (!args.length || (args.length == 1 && isImplicitlyConvertible!(Types[0],T))) {

static if (args.length) {

data[name] = args[0];

return args[0];

} else

return data[name];

}

}

}

void main() {

S!long s;

s.foo = 1;

writeln(s.foo());

}</lang>

If the attribute name is not known at compile-time, you have to use a more normal syntax:

<lang d>import std.stdio: writeln;

struct S(T) {

T[string] data;

alias data this;

}

void main() {

S!long s;

string name = "bar";

s[name] = 2;

writeln(s[name]);

}</lang>

If the type is variable it can be used an associative array of variants, but things become more complex. Adding a name to all instances of a class can be possible.