Check Instance of Template for Parameter Type/Value

[stew via Digitalmars-d-learn]

On Monday, 19 October 2015 at 14:51:29 UTC, Stewart Moth wrote:
> I'm working with a library that has template structs of 
> mathematical vectors that can sometimes be the type of an array 
> I'm passing to a function.
>
> The definition of the struct is like this:
>
> struct Vector(type, int dimension_){ ... }
>
> Where type is going to be an int/float/etc and dimension_ is 
> 2/3/4.
>
> I could write a bunch of functions for each case, but I'd 
> rather not... I'd like to use something like the following:
>
> void foo(T)(Array!T array){
>     if(isInstanceOf!(Vector, T)){
>         //get type of T or check it
>         //test if dimension_ is 2 or 3 or 4
>         ...
>     } else {
>         //Non-vector stuff
>         ...
>     }
> }
>
> But to do that I need to check that parameters of T as if it 
> were an instantiated instance of Vector and I'm not sure how to 
> accomplish that... Can anyone help me out with what I need to 
> do?

I'm not sure if this is exactly what you're after, but with the 
traits shown below you should be able to perform the following 
compile time checks.

---
void foo(T)(T v){

     static if(isVec!(T)){ } // Vec of any type and size
     static if(isVec!(T, int)){ } // Vec of type 'int' but any size
     static if(isVec!(T, 3)){ } // Vec of size 3 but any type
     static if(isVec!(T, 3, int)){ } // Vec of size 3 and type 
'int'

}
---


Traits impl:
(this is from memory with no D compiler around so excuse any 
typos)

---

struct Vec(size_t Dim, Type) {} // Vector template parms 
dimension and type.

/**
  * Is V a Vec of any element type and any size
  */
template isVec(V)
{
     enum isVec = is(Unqual!V == Vec!(n, U), size_t n, U);
}
///
unittest {
     mixin(UT_start);
     static assert(isVec!(Vec!(2,float)));
     static assert(isVec!(Vec!(4,int)));
     mixin(UT_scopeExit);
}

/**
  *  Is V a Vec of a specific size but any type.
  */
template isVec(V, size_t n)
{
     static if(is(Unqual!V == Vec!(n2, U), size_t n2, U))
         enum isVec = n == n2;
     else
         enum isVec = false;
}

/**
  *  Is V a Vec of any size and specific type T.
  */
template isVec(V, T)
{
     static if(is(Unqual!V == Vec!(n, U), size_t n, U))
         enum isVec = is(T == U);
     else
         enum isVec = false;
}
///
unittest {
     mixin(UT_start);
     static assert(isVec!(Vec!(2,int), int));
     static assert(isVec!(Vec!(3,int), int));
     static assert(!isVec!(Vec!(2,int), float));
     static assert(!isVec!(Vec!(3,int), float));
     mixin(UT_scopeExit);
}

/**
  *  Is V a Vec of specific size N and type T
  */
template isVec(V,size_t N,T)
{
     enum isVec = is(Unqual!V == Vec!(N,T));
}
///
unittest {
     mixin(UT_start);
     static assert(isVec!(Vec!(2,int), 2, int));
     static assert(!isVec!(Vec!(2,int), 3, int));
     static assert(!isVec!(Vec!(2,int), 2, float));
     static assert(!isVec!(Vec!(2,int), 3, float));
     mixin(UT_scopeExit);
}
---

Cheers,
Stew