Don't know how to construct class type 'S' with argument types '()'

[timotheecour]

Seems like msgpack-d requires empty constructor for classes.
Can we relax this?
Classes could be allocated without calling 'new' constructor to enable this.

[repeatedly]

You mean Unpacker?
Unpacker's class deserialization requires arguments of constructor, not empty constructor.

msgpack-d/src/msgpack.d

Line 2649 in b068112

object = new T(args);

Could you show me your code example?

[timotheecour]

Yes, Unpacker.
Actually I'm not sure anymore.
Here's an example adapted from http://wiki.dlang.org/Memory_Management but without calling emplace (which has same restrictions as new, as does Object.factory apparently).

class A{
int x=2;
this(int x){
this.x=x;
}
}

void test(){
A a=allocClass!A;
// works but not sure how safe this is
}

T allocClass(T, Args...) (Args args) {
import core.stdc.stdlib : malloc;
import core.memory : GC;
auto size = __traits(classInstanceSize, T);
auto memory = malloc(size)[0..size];
assert(memory);

// instead of 0-fill, would fill up the buffer from deserialization
(cast(byte[])memory)[]=0;

GC.addRange(memory.ptr, size);
return cast(T)cast(void*)memory;
}

[timotheecour]

Started a thread here: http://forum.dlang.org/thread/mailman.105.1432199217.7663.digitalmars-d-learn@puremagic.com [deserialization: creating a class instance without calling constructor]

Some proposed:

CT construct(CT, A...)(A a) if (is(CT == class)){
auto memory = malloc(typeid(CT).init.length);
memory[0 .. typeid(CT).init.length] = typeid(CT).init[];
return cast(CT) memory;
}

Although I think he meant this:

CT construct(CT)() if (is(CT == class)){
auto memory = malloc(typeid(CT).init.length);
memory[0 .. typeid(CT).init.length] = typeid(CT).init[];
return cast(CT) memory;
}

[repeatedly]

I know we can use manual construction without new for classes.

Could you show me your code example?

This means I want to know what the code causes Don't know how to construct class type 'S' with argument types '()'.
But I noticed you call unpack like below:

class A
{
    this(int a) { }
}

A a;
unpacker.unpack(a, 10);  // ok
unpacker.unpack(a);  // error

Latter case is error prone but it seems acceptable. I will support it.

[timotheecour]

Thanks, the patch works for DMD, however see my comments in the PR; it may not work for LDC compiler.