On Wednesday, 11 September 2019 at 08:35:02 UTC, berni wrote:
I'd like to write a template, that takes a different default
value depending on the type of a variable. I tried this, but it
doesn't work:
void main()
{
double a = 1e-8;
double b = 1e-10;
float c = 1e-4;
float d = 1e-6;
assert(!test(a));
assert(test(b));
assert(!test(c));
assert(test(d));
}
auto test(T, U)(T value, U limit=1e-9)
{
return value<limit;
}
auto test(T:float)(T value)
{
return test(value, 1e-5);
}
Although being called with a double in the first two tests, the
second overload is always used and therefore the first test
fails. And without this overload, the last test obviously
doesn't pass.
Is there a way, to provide default values for template
parameters depending on the type of an other parameter?
unittest
{
double a = 1e-8;
double b = 1e-10;
float c = 1e-4;
float d = 1e-6;
assert(!test(a));
assert(test(b));
assert(!test(c));
assert(test(d));
}
auto test(T, U)(T value, U limit = limit!T) {
return value < limit;
}
// Solution 1:
template limit(T) {
static if (is(T == float)) {
enum limit = 1e-5;
} else {
enum limit = 1e-9;
}
}
// Solution 2:
enum limit(T : float) = 1e-5;
enum limit(T : double) = 1e-9;
With some tricks this can also be inlined:
enum If(bool b, T...) = T[b ? 0 : 1];
auto test(T, U)(T value, U limit = If!(is(T == float), 1e-5,
1e-9)) {
return value < limit;
}
--
Simen
