• Bill Baxter (30/30) Dec 24 2006 I have a need to define methods in a templated class that only
• Kirk McDonald (12/51) Dec 24 2006 Pyd's automatic wrapping of operator overloads uses this trick. In that

Bill Baxter <dnewsgroup billbaxter.com> writes:

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

Kirk McDonald <kirklin.mcdonald gmail.com> writes:

Bill Baxter wrote:
 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

Pyd's automatic wrapping of operator overloads uses this trick. In that 
case, I know the type in question is always a struct or class, and so I 
check for the existence of the operator overload method directly:

static if (is(typeof(T.opAddAssign)))

This wouldn't work for the built-in types, but, on the other hand, the 
set of built-in types is finite, and you could fairly easily check for 
those some other way.

-- 
Kirk McDonald
Pyd: Wrapping Python with D
http://pyd.dsource.org