Re: Introduction to traits (and __traits)

[Benjamin Thaut]

Am 30.08.2013 21:36, schrieb 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

You need to put it into a static if, otherwise the compiler will 
continue semantic checks on the second part of the expression. E.g.

template isGraph(G)
{
  static if(__traits(hasMember, G, "directed"))
    enum bool isGraph = isBoolean!(typeof(G.directed));
  else
    enum bool isGraph = false;
}