lazy compilation of templated class members

Bill Baxter dnewsgroup at billbaxter.com
Sun Dec 24 23:19:10 PST 2006


I have a need to define methods in a templated class that only 
conditionally exist.  For instance with a 'class Foo(T)', I'd like to 
give it an opNeg method as long as T is a type that can be negated.

So far so good, I can get that with:

class Foo(T)
{
     static if(is(typeof(-T.init))) {
     Foo neg() {
        auto ret = new Foo;
        ret.m_value = -m_value;
        return ret;
     }
     }

     T m_value;
}

1)
Is this something useful enough that it might be worth special syntax? 
Something like 'lazy' in front of the method declaration:

     lazy Foo neg() {
        auto ret = new Foo;
        ret.m_value = -m_value;
        return ret;
     }

The lazy meaning basically "leave this method out if it contains 
semantic errors".

2)
I can't figure out how to do the static if for opXXXAssign type methods.
static if(is(typeof(T.init+=T.init))) is always false, I guess because 
T.init isn't assignable.  Is there some way to do that check?


--bb



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