On Tuesday, 20 July 2021 at 09:24:07 UTC, Mark Lagodych wrote:
Is there a way to make myvar local to each instance of `X`
without making it a variable of `X`? Just curious.
Yes.
```d
import std.stdio;
class X {
int x(int param) {
static int[typeof(this)] myvar;
if (param == 0) return myvar.get(this, 1234);
else return myvar[this] = param;
}
}
void main()
{
X x1 = new X;
X x2 = new X;
x1.x(0).writeln; //1234
x2.x(0).writeln; //1234
x1.x(17).writeln; //17
x2.x(0).writeln; //1234
x1.x(0).writeln; //17
x2.x(0).writeln; //1234
}
```
However, this is definitely not recommended. When you are calling
a function with any particular object and argument set, you want
it to do the same thing and return the same result, regardless of
what's called before. Otherwise you're making debugging much more
difficult than it needs to be.
This means that `static` variables should generally be used only
for two things:
1: data that is only set at the beginning of the program, or at
first use, and then left to the initial value.
2: caching results of expensive computation. Even this is a bit
controversal, as it's easy to screw up - often it's just better
to split the function in two, and let the caller to cache the
results.
In this case, consider passing `myvar` explicitly:
```d
import std.stdio;
class X {
int x(int param, ref int[typeof(this)] myvar) {
if (param == 0) return myvar.get(this, 1234);
else return myvar[this] = param;
}
}
void main()
{
X x1 = new X;
X x2 = new X;
int[X] myvar;
x1.x(17, myvar).writeln; //17
x2.x(0, myvar).writeln; //1234
x1.x(0, myvar).writeln; //17
x2.x(0, myvar).writeln; //1234
myvar = null; //Forget all calls made so far
x1.x(0, myvar).writeln; //1234
x2.x(0, myvar).writeln; //1234
}
```
