Runtime error trying to call opCall on variant array of objects

[Ali Çehreli via Digitalmars-d-learn]

On 12/24/2016 08:36 AM, aliak wrote:

> "Cannot apply `()' to a value of type `Command!(__lambda1, int)". I
> think it's failing when it checks for "!isFunctionPointer!A &&
> !isDelegate!A".

Storing delegates is a workaround. Three notes in the code:

// (1) Added imports
import std.variant;
import std.stdio;
import std.algorithm;

struct Command(alias fun, Args...) {
     Args args;
     this(Args args) {
         this.args = args;    /* (2) args = args was a common mistake.
                               * dmd, please warn about this one.
                               * (I think there is a bug about that.)
                               */
     }
     auto opCall() {
         writefln("opCall called for %s", args);
         return fun(args);
     }
}

auto command(alias fun, Args...)(Args args) {
     auto c = Command!(fun, Args)(args);
     writefln("Created %s", c);
     // (3) Workaround: Storing delegates, not Command instances.
     return () => c();
}

void main() {
     auto commands = variantArray(
         command!(a => a + 1)(1),
         command!(a => a + 2)(1),
         command!(a => a + 3)(1),
     );

     commands.each!(a => a());

     typeid(commands[0]).writeln; // std.variant.VariantN!32LU.VariantN

     auto cmd = command!(a => a + 1)(1);
     typeid(cmd).writeln; // scratchpad.main.Command!(__lambda4, 
int).Command
}

Alid