In my code I wanted to use std.typecons.Nullable to get
advantages in web application when working with database values.
I'm writing some struct that can get Nullable!int as input value.
But instead of getting struct object itself (in case of
overloading) *get* method called and programme fails. This
happens when Nullable is null. I'll give an example:
//------
import std.stdio;
import std.typecons;
template isStdNullable(N)
{
static if( is( N == NullableRef!(TL1), TL1... ) )
enum bool isStdNullable = true;
else static if( is( N == Nullable!(TL2), TL2... ) )
enum bool isStdNullable = true;
else
enum bool isStdNullable = false;
}
template getStdNullableType(N)
{
static if( is( N == NullableRef!(TL2), TL2... ) )
alias TL2[0] getStdNullableType;
else static if( is( N == Nullable!(TL2), TL2... ) )
alias TL2[0] getStdNullableType;
else
static assert (0, `Type ` ~ fullyQualifiedName!(N) ~ ` can't be
used as Nullable type!!!` );
}
struct Test
{
//Uncommenting these lines makes programme fail with assertion
//string opIndex(int key) const
//{ return "Not null"; }
//inout(bool) opBinaryRight(string op)(int key) inout
//{ return true; }
string opIndex(N)(N key) const
if( isStdNullable!(N) && is( getStdNullableType!(N) == int ) )
{ return ( key.isNull() ? "null" : "Not null" );
}
inout(bool) opBinaryRight(string op, N)(N key) inout
if(op == "in" && isStdNullable!(N) && is(
getStdNullableType!(N) == int ))
{ return ( key.isNull() ? true : false ); }
}
void main()
{
Test test;
Nullable!(int, 10) val5;
val5 = 5;
Nullable!(int, 15) valNull;
writeln(test[val5]);
writeln(test[valNull]);
writeln(val5 in test);
writeln(valNull in test);
}
//--------------------
In this code I want get ability to pass int values and
Nullable!int values using overloaded operators. Template
"functions" isStdNullable, getStdNullableType help to get
information about type of *N*. I think that there is something
that I don't understand about order of instantiation for
overloaded functions.
How could I solve this problem? Or I need to write my own
Nullable variant without "alias get this;"?
The example above is also available at:
http://dpaste.dzfl.pl/6790f60e
