xvalue and std::move in D

[Ali Çehreli]

On 03/06/2014 03:21 AM, Edwin van Leeuwen wrote:

 > Lately in C++ I have become a fan of the type of functional programming
 > discussed here:
 > 
http://blog.knatten.org/2012/11/02/efficient-pure-functional-programming-in-c-using-move-semantics/ 


I haven't read that yet but I have always returned a vector from a 
function that produced it instead of adding to a reference parameter.

 > and was wondering if something similar is possible in D.

Most of the time it is automatic and a non-issue for arrays.

 > Basically the idea is to define functions as follows:
 >
 > std::vector<double> add_to_vector( double x, std::vector<double> &&v ) {
 >   v.push_back(x);
 >   return v;
 > }

Actually, when the name of the function is add_to_vector() anyway, there 
is an obvious side-effect. So, I would not return the result in C++.

However, when the name is like make_vector() then I always return the 
result. The alternatives have many hard questions to answer:

void make_vector(vector<double> & v)
{
     // Should I clear v first?
     // Should I simply start appending to v?
     // etc.
}

All of the efficiency of doing that goes out the window when one 
considers exception safety.

Anyway... Too much off topic... :)

 > Is this possible in D?

T[] append(T)(T[] arr, T value)
{
     arr ~= value;
     return arr;
}

Done. :) Slices consist of two members that are cheap to copy: The 
number of elements and the pointer to the first element. It is already 
as efficient as move in C++.

You should also read the following article:

   http://dlang.org/d-array-article.html

Ali