I am new to D so I am probably not using the right terminology but here is a piece of C++ code (not complete) that I would like to translate to idiomatic D.
I have defined a function object that I pass to std::sort to std:map as follows:

enum class SortOrder{ ASC, DESC };
typedef std::vector<boost::variant> DataRow; // this is one row of data in a 2D array. Data items are variants but this is not very important
typedef std::vector<DataRow> Data; // this is simply a 2D array
Data the_data;
// the function object is here. I don't want a lamda because I want to be able to call this from multiple places
class MyCompare
{
public:
	explicit MyCompare(int column, SortOrder order) : m_column(column), m_order(order) {}
	bool operator()(const DataRow& lhs, const DataRow& rhs)
	{
		switch (m_order)
		{
		case SortOrder::ASC:
			return lhs[m_column] < rhs[m_column];
		case SortOrder::DESC:
			return rhs[m_column] < lhs[m_column];
		}
	}
private:
	int m_column;
	SortOrder m_order;
};

example 1:
int column = 3;
SortOrder order = DESC;
std::sort(the_data.begin(), the_data.end(), MyCompare(column, order));

example 2:
MyCompare comp(column, order);
std::map<DataRow, Data, MyCompare> mp( comp );

What is the equivalent idiomatic D?

You can pass anything to the sort function that's callable, including an object:

    struct MyCompare {
        SortOrder order;
        int column;
        bool opCall(const ref DataRow lhs, const ref DataRow rhs) {
            return order == SortOrder.ASC ?
                lhs[column] < rhs[column] :
                rhs[column] < lhs[column];
        }
    }

    import std.algorithm;
    MyCompare cmp(SortOrder.ASC, 10);
    my_columns.sort!cmp;

(Untested.)

On Monday, 16 June 2014 at 09:24:22 UTC, Marc Schütz wrote:
> You can pass anything to the sort function that's callable, including an object:
>
>     struct MyCompare {
>         SortOrder order;
>         int column;
>         bool opCall(const ref DataRow lhs, const ref DataRow rhs) {
>             return order == SortOrder.ASC ?
>                 lhs[column] < rhs[column] :
>                 rhs[column] < lhs[column];
>         }
>     }
>
>     import std.algorithm;
>     MyCompare cmp(SortOrder.ASC, 10);
>     my_columns.sort!cmp;
>
> (Untested.)

That works, but it's not very idiomatic. Well, I have never seen a C++ style "functor" used in D template is what I'm saying. I don't know if that's good or bad. A more idiomatic approach would be to simply pass a delegate to your function.

This can either be as a pointer to member function:

struct MyCompare {
    SortOrder order;
    int column;
    bool compare(const ref DataRow lhs, const ref DataRow rhs)
{
    return order == SortOrder.ASC ?
        lhs[column] < rhs[column] :
        rhs[column] < lhs[column];
    }
}

import std.algorithm;
MyCompare cmp(SortOrder.ASC, 10);
auto dg = &cmp.compare;
my_columns.sort!dg;

Or, more generally, via a lambda, or a function with state:

import std.algorithm;
column = 10;
bool compare(const ref DataRow lhs, const ref DataRow rhs)
{
    return order == SortOrder.ASC ?
        lhs[column] < rhs[column] :
        rhs[column] < lhs[column];
    }
}
my_columns.sort!compare;

On Monday, 16 June 2014 at 20:49:29 UTC, monarch_dodra wrote:
> MyCompare cmp(SortOrder.ASC, 10);

This syntax is not valid D.

It should be:

    auto cmp = MyCompare(SortOrder,ASC, 10);

On Tuesday, 17 June 2014 at 04:32:20 UTC, Jakob Ovrum wrote:
> On Monday, 16 June 2014 at 20:49:29 UTC, monarch_dodra wrote:
>> MyCompare cmp(SortOrder.ASC, 10);
>
> This syntax is not valid D.
>
> It should be:
>
>     auto cmp = MyCompare(SortOrder,ASC, 10);

Sorry, that first comma is a typo and should be a dot.

     auto cmp = MyCompare(SortOrder.ASC, 10);

On Tuesday, 17 June 2014 at 04:32:20 UTC, Jakob Ovrum wrote:
> On Monday, 16 June 2014 at 20:49:29 UTC, monarch_dodra wrote:
>> MyCompare cmp(SortOrder.ASC, 10);
>
> This syntax is not valid D.
>
> It should be:
>
>     auto cmp = MyCompare(SortOrder,ASC, 10);

Well, techincally, the *syntax* is valid. If "MyCompare" contains a constructor, it's legit code to boot. It's part of the uniform initialization syntax, and it's what allows things like:

BigInt b = 5;
  or
BigInt b(5);

THAT said, yeah, the MyCompare I posted did not contain a constructor. SO my code was wrong, guilty as charged.

On Tuesday, 17 June 2014 at 07:53:51 UTC, monarch_dodra wrote:
> On Tuesday, 17 June 2014 at 04:32:20 UTC, Jakob Ovrum wrote:
>> On Monday, 16 June 2014 at 20:49:29 UTC, monarch_dodra wrote:
>>> MyCompare cmp(SortOrder.ASC, 10);
>>
>> This syntax is not valid D.
>>
>> It should be:
>>
>>    auto cmp = MyCompare(SortOrder,ASC, 10);
>
> Well, techincally, the *syntax* is valid. If "MyCompare" contains a constructor, it's legit code to boot. It's part of the uniform initialization syntax, and it's what allows things like:
>
> BigInt b = 5;
>   or
> BigInt b(5);
>
> THAT said, yeah, the MyCompare I posted did not contain a constructor. SO my code was wrong, guilty as charged.

Since when is that syntax valid? Is there somewhere it is documented?