Where are the template members?

[Gor Gyolchanyan]

But why aren't they non-static members? If they were non-static members the
structure's size would still be compile-time, computed by the sum of sizes
of all template instances. the .sizeof would work perfectly well.
This would be an incredibly useful feature. Among other use cases this
would allow having a fully type-safe and very fast Variant type:

struct Variant
{
public:
TypeInfo type() @property
{
return _type;
}
 ref Type_ as(Type_)() @property
{
if(typeid(Type_) != _type)
throw new Exception("The requested static type doesn't match the available
dynamic type.");
 return _value!Type_;
}
private:
TypeInfo _type;

union
{
template _value(Type_)
{
Type_ _value;
}
}
}

Is there any good reason why this feature wouldn't be a good idea?


On Wed, Dec 5, 2012 at 12:51 PM, Simen Kjaeraas <simen.kjaras at gmail.com>wrote:

> On 2012-39-05 09:12, Gor Gyolchanyan <gor.f.gyolchanyan at gmail.com> wrote:
>
>  Consider this piece of code:
>>
>> struct Test
>> {
>> template member(Type)
>> {
>> Type member;
>> }
>> }
>>
>> unittest
>> {
>> Test test;
>> test.member!int = 0;
>> test.member!long = 0;
>> test.member!short = 0;
>> import std.stdio; writeln(test.sizeof);
>> assert(test.sizeof == int.sizeof + long.sizeof + short.sizeof); // fails
>> assert(test.sizeof == 1); // succeeds
>> }
>>
>> I don't get why the structure's size remains unchanged even after
>> instantiating 3 members inside it.
>> How can I get the real size of the structure, including all its members
>> (template or not)?
>>
>
> You have. Its real size is 1, and the template instantiations are not
> member
> fields.
>
> A template (not a templated function [actually, sorta, but that's a
> different
> discussion]) is basically static - if you try instead Test.member!int, you
> will see that this works perfectly.
>
> What we see here is another example of the confusion that can be caused by
> static names being accessible through an instance:
>
> struct Foo {
>     static int n;
> }
>
> Foo f;
>
> f.n = 4;
>
> --
> Simen
>



-- 
Bye,
Gor Gyolchanyan.
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