On Tuesday, 7 September 2021 at 17:24:34 UTC, james.p.leblanc
wrote:
// this fails with: "Error: variable 'i' cannot be read at
compile time
//
// foreach( i ; 0 .. 3 ){
// ptr = u.tupleof[i].x.ptr;
tuples only exist at compile time, so you'd have to make sure the
indexing is itself compile time. Consider that unlike an array,
each index might give a different type, so like what would
struct A { int a; string b; }
A a;
int idx;
some_type c = a.tupleof[idx];
Which type is some_type? Is it int or string? Impossible to tell
since it doesn't know what idx is. So idx needs to be known at
compile time so it knows which type you get there.
The reason why it works here:
foreach( i, val ; u.tupleof ){
is because the compiler knows you're specifically looping over
the tupleof, so it expands it and knows what i is going to be at
compile time. If you assigned that i to an intermediate variable
then it would break this direct knowledge and it doesn't compile
again.
If you want to do a runtime lookup, you need to separate the two
pieces. This pattern works:
switch(runtime_index) {
foreach(i, val; item.tupleof)
case i:
// use val
}
So the switch is at runtime but the loop and cases are all known
at compile time.
