Question about explicit template instantiation

[Jesse Phillips]

On Sun, 10 Feb 2008 17:31:36 -0500, Edward Diener wrote:

> Janice Caron wrote:
>> On 10/02/2008, Edward Diener <eddielee_no_spam_here at tropicsoft.com>
>> wrote:
>>>>     template MyNamespace(T)
>>>>     {
>>>>         int x;
>>>>     }
>> 
>>> This is confusing to me coming from C++. In C++ instantiating a class
>>> template produces a type. Is that not the way D works ?
>> 
>> That's /exactly/ the way that D works. With CLASS templates, D works
>> just like C++. No problem. Compare: c++
>> 
>>     template<class T> class A
>>     {
>>         int x;
>>     };
>> 
>>     A<int> a1;
>>     A<double> a2;
>> 
>> and the D equivalent:
>> 
>>     class A(T)
>>     {
>>         int x;
>>     }
>> 
>>     auto a1 = new A!(int);
>>     auto a2 = new A!(double);
>> 
>> They're exactly the same, except that D has nicer syntax. I don't think
>> /class/ templates are confusing you at all! I think what's confusing
>> you is that in D you can have /namespace/ templates, which don't exist
>> in C++.
> 
> The explanation for Class Templates in the D1 doc does not explain
> anything at all. That is why I assumed the template A(T) notation
> referred to the equivalent of the C++ class template.
> 
> 
>> Let me make an approximate C++ translation. Here's the D again:
>> 
>>     template A(T)
>>     {
>>         int x;
>>     }
>> 
>> That is roughly equivalent to, in C++
>> 
>>     namespace A_int
>>     {
>>         int x;
>>     }
>> 
>>     namespace A_float
>>     {
>>         int x;
>>     }
>> 
>>     namespace A_double
>>     {
>>         int x;
>>     }
>> 
>> ...and so on for every imaginable type. Now it should be clear to you
>> that A_int::x is a different variable from A_float::x, yes?
> 
> Now I understand. I was surely fooled by the doc.
> 
> 
>> For a namespaces to be "instantiated" just means that the corresponding
>> chunk of code is there. To be /not/ instantiated would mean that it
>> isn't there - which is just as well really, because there are in
>> infinite number of possible types!
>> 
>> To instantiate a namespace template, you only have to refer to it. I
>> doesn't matter how.
>> 
>> Clear?
> 
> I think so. Instantiating a namespace template merely puts the
> equivalent code, with the types you instantiate it with substituted each
> time those types are referred to in the namespace, directly into your
> source.
> 
> I still do not understand what the object used for the instantiation is.
> In your example above what is:
> 
> A!(int)
> 
> ? Is it some kind of namespace ? Can I say:
> 
> A!(int) x;
> 
> ? If so, what is x ?

Well, you can't write A!(int) x; unless you precede it with alias.

If we have:

template A(T) { T f; }

I'm new to templates so I don't really know how to say what A!(int) is, 
but I think a good way to think of it as declaring an instance of a 
struct.

struct B { int f };

where B = A!(int);