Templates, implicit conversions and const

[Kirk McDonald]

Regan Heath wrote:
> The 2nd thing which irritated me a little over the weekend was how 
> templates and const can interact.  For example:
> 
> bool contains(T)(T[] array, T value)
> {
>     foreach(v; array) if (v == value) return true;
>     return false;
> }
> 
> void foo(const(char) c) {}
> 
> void main()
> {
>     string tests = "test";
>     char[] testc = tests.dup;
>     
>     tests.contains(cast(const(char))'a');
>     contains!(const(char))(tests, 'a');
>     testc.contains('a');   
>     foo('a');
> }
> 
> 
> 
> The cast in the first contains call is required, without it we get the 
> error:
> 
> C:\D\src\tmp\tplconst.d(14): template tplconst.contains(T) cannot deduce 
> template function from argument types (const(char)[],char)
> 
> The available templates are:
> (const(char)[],const(char))
> (char[],char)
> 
> And looking at the template argument deduction rules I can see why it 
> cannot find a match.  T is found to be both 'const(char)' and 'char'.
> 
> But, I wonder if it should then apply the standard function overloading 
> rule:
> 
> "In D, function overloading is simple. It matches exactly, it matches 
> with implicit conversions, or it does not match. If there is more than 
> one match, it is an error."
> 
> If it performed implicit conversion on 'char' it could get 'const(char)' 
> and would therefore match the template:
> 
> (const(char)[],const(char))
> 
> It cannot implcitly convert 'const(char)[]' to 'char[]' so it will never 
> match the other option.
> 
> Thoughts?
> 
> Regan

Re-write the function as:

bool contains(T, U)(T[] array, U value) {
     foreach(v; array) if (v == value) return true;
     return false;
}

You can still compare a const(char) and a char for equality.

-- 
Kirk McDonald
http://kirkmcdonald.blogspot.com
Pyd: Connecting D and Python
http://pyd.dsource.org