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digitalmars.D - detecting classes, structs, arrays in templates

reply "Luke J. West" <luke west.me.uk> writes:
Hi,

I want to specialize a template function - call it print() - for three
cases: classes, structs and arrays. Ideally I'd like something that
looks 'functional' like a proper specialization, but perhaps I need to
use "static if". I'm still at the beginning of my journey with D so I'd
be grateful for any pointers (sorry - that's a terrible pun). Perhaps
there's a corner of the D2 docs I've glossed over.

Thanks,

Luke


// in words you can understand...

void print(T)() {writefln("general");}

// But what does my specialization look like for all (or any)
// of the implementations of print() for the calls below.

int main() {
 C   c;    // some class
 S   s;    // some struct
 int a[4]; // an array
 int p;    // a primitive type
 
 print(c); // writefln("class");
 print(s); // writefln("struct");
 print(a); // writefln("array");
 print(p); // writefln("primitive");

 return 0;
}
Jan 23 2011
next sibling parent "Simen kjaeraas" <simen.kjaras gmail.com> writes:
Luke J. West <luke west.me.uk> wrote:


 Hi,

 I want to specialize a template function - call it print() - for three
 cases: classes, structs and arrays. Ideally I'd like something that
 looks 'functional' like a proper specialization, but perhaps I need to
 use "static if". I'm still at the beginning of my journey with D so I'd
 be grateful for any pointers (sorry - that's a terrible pun). Perhaps
 there's a corner of the D2 docs I've glossed over.

 Thanks,

 Luke


 // in words you can understand...

 void print(T)() {writefln("general");}

 // But what does my specialization look like for all (or any)
 // of the implementations of print() for the calls below.

 int main() {
  C   c;    // some class
  S   s;    // some struct
  int a[4]; // an array
  int p;    // a primitive type
 print(c); // writefln("class");
  print(s); // writefln("struct");
  print(a); // writefln("array");
  print(p); // writefln("primitive");

  return 0;
 }


You want to have a look-see at isExpressions[1]:

void print(T)(T t) if (is(T == class)){
     writeln("class");
}

void print(T)(T t) if (is(T == struct)){
     writeln("struct");
}

void print(T)(T t) if (is(T U == U[])){
     writeln("array");
}

void print(T)(T t) if (isNumeric!T || isSomeChar!T){
     writeln("primitive");
}

Haven't actually tested these, as I don't have D here.

Note also that two specializations are missing - pointer
and associative array:

void print(T)(T t) if (is(T U == U[V], V)){
     writeln("associative array");
}

void print(T)(T t) if (is(T U == U*)){
     writeln("pointer");
}

http://digitalmars.com/d/2.0/expression.html#IsExpression

-- 
Simen
Jan 23 2011
prev sibling parent reply bearophile <bearophileHUGS lycos.com> writes:
Luke J. West:


 Hi,
 
 I want to specialize a template function - call it print() - for three
 cases: classes, structs and arrays. Ideally I'd like something that
 looks 'functional' like a proper specialization, but perhaps I need to
 use "static if". I'm still at the beginning of my journey with D so I'd
 be grateful for any pointers (sorry - that's a terrible pun). Perhaps
 there's a corner of the D2 docs I've glossed over.


I suggest you to ask such questions in the D.learn newsgroup.
Here are two possible implementations:

import std.stdio: writeln;
import std.traits: isArray;

void print(T)(T x) if (!isArray!T && !is(T == class) && !is(T == struct)) {
    writeln("general");
}

void print(T)(T[] x) {
    writeln("Dynamic array or fixed-sized array");
}

void print(T)(T x) if (is(T == class)) {
    writeln("Class instance");
}

void print(T)(T x) if (is(T == struct)) {
    writeln("Struct instance");
}

void print2(T)(T x) {
    static if (isArray!T)
        writeln("Dynamic array or fixed-sized array");
    else static if (is(T == class))
        writeln("Class instance");
    else static if (is(T == struct))
        writeln("Struct instance");
    else
        writeln("general");
}


class C {}
struct S {}

void main() {
    C c;      // some class
    S s;      // some struct
    int[4] a; // a fixed-sized array
    int p;    // a primitive type

    print(a);
    print(p);
    print(c);
    print(s);
    writeln();

    print2(a);
    print2(p);
    print2(c);
    print2(s);
}

Bye,
bearophile
Jan 23 2011
parent "Luke J. West" <luke west.me.uk> writes:
Thanks for that bearophile - I'll get myself subscribed right away.

Bye for now,

Luke

On Sun, 23 Jan 2011 09:17:05 -0500, "bearophile"
<bearophileHUGS lycos.com> said:

 Luke J. West:
 

 Hi,
 
 I want to specialize a template function - call it print() - for three
 cases: classes, structs and arrays. Ideally I'd like something that
 looks 'functional' like a proper specialization, but perhaps I need to
 use "static if". I'm still at the beginning of my journey with D so I'd
 be grateful for any pointers (sorry - that's a terrible pun). Perhaps
 there's a corner of the D2 docs I've glossed over.


 
 I suggest you to ask such questions in the D.learn newsgroup.
 Here are two possible implementations:
 
 import std.stdio: writeln;
 import std.traits: isArray;
 
 void print(T)(T x) if (!isArray!T && !is(T == class) && !is(T == struct))
 {
     writeln("general");
 }
 
 void print(T)(T[] x) {
     writeln("Dynamic array or fixed-sized array");
 }
 
 void print(T)(T x) if (is(T == class)) {
     writeln("Class instance");
 }
 
 void print(T)(T x) if (is(T == struct)) {
     writeln("Struct instance");
 }
 
 void print2(T)(T x) {
     static if (isArray!T)
         writeln("Dynamic array or fixed-sized array");
     else static if (is(T == class))
         writeln("Class instance");
     else static if (is(T == struct))
         writeln("Struct instance");
     else
         writeln("general");
 }
 
 
 class C {}
 struct S {}
 
 void main() {
     C c;      // some class
     S s;      // some struct
     int[4] a; // a fixed-sized array
     int p;    // a primitive type
 
     print(a);
     print(p);
     print(c);
     print(s);
     writeln();
 
     print2(a);
     print2(p);
     print2(c);
     print2(s);
 }
 
 Bye,
 bearophile
Jan 23 2011