Re: How to check for instantiation of specific template?
On 2013-10-10 19:23, H. S. Teoh wrote:
I have a template used for storing compile-time values:
template Def(int x, string y) {
alias impl = TypeTuple!(x,y);
}
How do I define a template isDef that, given some template alias A,
evaluates to true if A is some instantiation of Def?
template isDef(alias A) {
enum A = ... /* what to put here? */
}
The intent is to be able to write signature constraints like this:
auto myFunc(alias def)(...)
if (isDef!def)
{
...
}
...
// Pass an instantiation of Def to myFunc
auto x = myFunc!(Def!(1, abc))(args);
I tried using std.traits.isInstanceOf but apparently it expects the
second argument to be an actual type, which doesn't work in this case
because Def is a typetuple of compile-time values, not an actual type.
I found this old thread
http://forum.dlang.org/thread/mailman.1382.1371938670.13711.digitalmar...@puremagic.com
With this code:
template getTemplate(alias T : TI!TP, alias TI, TP...)
{
alias getTemplate = TI;
}
But I haven't figured out how to do the comparison.
--
/Jacob Carlborg
Re: How to check for instantiation of specific template?
This is actually a very interesting question. Usual approach is to use pattern matching on types but template instance symbol is not a type on its own, it is just a symbol. (with typeof == void) My second guess was to check __traits(identifier, AliasParam) but it looks like for aliased instance (contrary to aliased template symbol) identifier is its mangled name. I can't imagine really clean way to do it but comparing __traits(identifier) to that specific mangled name can be a reliable enough hack (better than stringof which has undefined format).
How to check for instantiation of specific template?
I have a template used for storing compile-time values:
template Def(int x, string y) {
alias impl = TypeTuple!(x,y);
}
How do I define a template isDef that, given some template alias A,
evaluates to true if A is some instantiation of Def?
template isDef(alias A) {
enum A = ... /* what to put here? */
}
The intent is to be able to write signature constraints like this:
auto myFunc(alias def)(...)
if (isDef!def)
{
...
}
...
// Pass an instantiation of Def to myFunc
auto x = myFunc!(Def!(1, abc))(args);
I tried using std.traits.isInstanceOf but apparently it expects the
second argument to be an actual type, which doesn't work in this case
because Def is a typetuple of compile-time values, not an actual type.
T
--
Help a man when he is in trouble and he will remember you when he is in trouble
again.
Re: How to check for instantiation of specific template?
On 2013-10-10, 19:23, H. S. Teoh wrote:
I have a template used for storing compile-time values:
template Def(int x, string y) {
alias impl = TypeTuple!(x,y);
}
How do I define a template isDef that, given some template alias A,
evaluates to true if A is some instantiation of Def?
template isDef(alias A) {
enum A = ... /* what to put here? */
}
This seems to be impossible. The standard way of doing this would be
with an isExpression[0], but that only works for types.
A solution would be to beef up your Def template by turning it into
a struct:
struct Def(int x, string y) {
alias impl = TypeTuple!(x,y);
}
With this definition, the following works:
template isDef(alias A) {
enum isDef = is(A == Def!T, T...);
// Or enum isDef = is(A == Def!(x, y), int x, string y);
}
This is a shortcoming of the language, and I have filed an enhancement
request for it[1].
The problems of using an actual type for this, is that someone might
instantiate that type (unlikely), and that the compiler will generate
TypeInfo for it, which will bloat your executable. A Sufficienctly
Smart Compiler[2] would see that the type is never used and thus not
include the TypeInfo. Something which the compiler could do without
a lot of smarts, is optimize away final abstract classes, which you
cannot derive from and cannot instantiate. That oughta be enough.
[0]: http://dlang.org/expression#IsExpression
[1]: http://d.puremagic.com/issues/show_bug.cgi?id=11219
[2]: http://c2.com/cgi/wiki?SufficientlySmartCompiler
--
Simen
Re: How to check for instantiation of specific template?
On Thursday, 10 October 2013 at 17:24:37 UTC, H. S. Teoh wrote:
I have a template used for storing compile-time values:
template Def(int x, string y) {
alias impl = TypeTuple!(x,y);
}
How do I define a template isDef that, given some template
alias A,
evaluates to true if A is some instantiation of Def?
template isDef(alias A) {
enum A = ... /* what to put here? */
}
The intent is to be able to write signature constraints like
this:
auto myFunc(alias def)(...)
if (isDef!def)
{
...
}
...
// Pass an instantiation of Def to myFunc
auto x = myFunc!(Def!(1, abc))(args);
I tried using std.traits.isInstanceOf but apparently it expects
the
second argument to be an actual type, which doesn't work in
this case
because Def is a typetuple of compile-time values, not an
actual type.
T
I've used this horrible hack to solve this. Not very generic as
you can see:
else static if(T.stringof.length 13 T.stringof[0..13] ==
TupleWrapper!)
