Introduction to traits (and __traits)

[Joseph Rushton Wakeling]

Hello all,

I find myself wanting to write for the first time one of these isSomething(T) or 
hasSomething(T) templates that perform compile-time checks, so I was hoping 
people could give me some good general advice on traits.

The goal here is to be able to confirm (i) type T has certain members and (ii) 
they have certain properties.  (This is for my graph library, Dgraph.)

So, to start off with, I thought I'd try starting with,

     template isGraph(G)
     {
         isGraph = __traits(hasMember, G, "directed") && 
isBoolean!(typeof(G.directed));
     }

... which I'd assumed would cover the case where G does not have a member 
"directed".  But in fact if I pass it a struct that does not have the entity 
.directed defined therein, it will return an error:  "no property 'directed' for 
type ..."

So, can someone give me a good idea of how to go about writing such a 
compile-time template that checks (i) for the existence of certain 
methods/functions/members and (ii) confirms their characteristics, such as their 
return values or arguments?

Thanks & best wishes,

     -- Joe