Re: __mul()

[Mark Hamburg <mark@...>]

On Mar 20, 2010, at 10:44 PM, Mark Hamburg wrote:

> See the specification for the __mul operator in the Lua spec (section 2.8).
> 
> The implementation of the * operator looks in the metatable for a __mul entry. It does not perform a message send. It does not index the object. Once it finds the entry, it passes it the two operands. In other words, it does more or less exactly this:
> 
> 	local temp = bot:getLoc()
> 	newpt = getmetatable(temp).__mul(temp, 2)
> 
> On the other hand, the method call approach:
> 
> 	newpt = bot:getLoc():__mul(2)
> 
> Is doing this:
> 
> 	local temp = bot:getLoc()
> 	newpt = temp.__mul(temp, 2)
> 
> This won't look in the metatable. It will look in the index table of the metatable, but that's a different matter unless the metatable points to itself via the __index entry -- i.e., it's use of the metatable if the object doesn't have a __mul entry looks like the following assuming that __index leads to a table:
> 
> 	local temp = bot:getLoc()
> 	newpt = getmetatable(temp).__index.__mul(temp, 2)
> 
> Mark

And yes, this does take some effort to get one's head around if one comes from an object-oriented background where all of this metatable-based behavior would be handled via methods on objects.

Mark