Template parameter deduction for constructors?

[Ali Çehreli]

On 04/12/2013 04:00 PM, bearophile wrote:

 > This is one of the few C++14 core language proposals that seem
 > interesting for D:
 >
 > http://www.open-std.org/JTC1/SC22/WG21/docs/papers/2013/n3602.html
 >
 > The purpose of this idea is to avoid the helper functions that are
 > currently used in D to build template structs/classes. Even if this
 > feature is restricted to only a subset of all the templated
 > structs/classes it seems useful to avoid some boilerplate code.
 >
 > Is this idea adaptable to D?
 >
 > Bye,
 > bearophile

This has come up multiple times before, most recently on the D.learn 
forum. My reaction is that what if the constructor is also a template? 
Or what if only the constructor is a template?

To paraphrase Steven Schveighoffer:

- if only the type is a template then the constructor parameters are 
used for deducing the template parameters of the type

- if only the constructor is a template, then the constructor parameters 
are used for deducing the template parameters of the constructor

- if both are templates then it is an ambiguity

I still think that the code will be confusing. If the proposal is 
implemented, the two lines in main below will look the same but will 
have different meanings:

import std.stdio;

// Today:
struct S
{
     this(T)(T param)
     {
         writefln("constructing with %s", T.stringof);
     }
}

// Proposed:
struct S2(T)
{
     this(T param)
     {
         writefln("constructing S2!%s", T.stringof);
     }
}

void main()
{
     /* Can we tell by reading the following code whether S or S2 is a
      * template, or whether the constructor of one of them is a
      * template? Do we care? */
     auto s = S(42);
     auto s2 = S2(43); // This is proposed; not valid yet.
}

Note that the C++ proposal does not touch on this point.

Ali