www.digitalmars.com         C & C++   DMDScript  

digitalmars.D - Templates, implicit conversions and const

reply Regan Heath <regan netmail.co.nz> writes:
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
Jul 09 2007
parent reply Kirk McDonald <kirklin.mcdonald gmail.com> writes:
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
Jul 09 2007
parent Regan Heath <regan netmail.co.nz> writes:
Kirk McDonald wrote:
 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.

Good point, why didn't I think of that. Regardless I think it would be nice if templates applied implicit conversions to find matches. Regan
Jul 10 2007