digitalmars.D.learn - Templates ~ best way to limit types?

Kris (21/21) Dec 11 2005 class MyTemplate(T)

Don Clugston (19/39) Dec 12 2005 Hmm, sounds like the 'constraints' proposal being discussed for C++0x.

kris (5/54) Dec 12 2005 Thanks Don!
Sean Kelly (12/50) Dec 12 2005 You can also do something like this:

Kris (5/56) Dec 12 2005 Thx Sean;

Sean Kelly (5/9) Dec 12 2005 You probably want "T==char." IIRC, "T:char" requires that T be

Sean Kelly (7/16) Dec 12 2005 Oh, the reason I suggested my method is that it should just not

Oskar Linde (6/10) Dec 12 2005 As far as I have understood,

Kris (3/13) Dec 12 2005 Thx!

"Kris" <fu bar.com> writes:

class MyTemplate(T)
{
        static if (is (T == char)) {}
        else
        static if (is (T == wchar)) {}
        else
        static if (is (T == wchar)) {}
        else
           {
           pragma (msg, "Template type must be char, wchar, or dchar");
           static assert(0);
           }

    // ...
}

what is the best (most succinct?) way to limit the T type? What I'm looking 
for is a nice compile time message saying "type must be x, y, or z" ...

The current approach is noted above, but I imagine/hope it could be a lot 
better?

Also, is there a better way to halt the compiler? So many error messages are 
emitted that it's quite hard to see the pragma message :-)

Thanks;

Dec 11 2005

Don Clugston <dac nospam.com.au> writes:

Kris wrote:
 class MyTemplate(T)
 {
         static if (is (T == char)) {}
         else
         static if (is (T == wchar)) {}
         else
         static if (is (T == wchar)) {}
         else
            {
            pragma (msg, "Template type must be char, wchar, or dchar");
            static assert(0);
            }
 
     // ...
 }
 
 what is the best (most succinct?) way to limit the T type? What I'm looking 
 for is a nice compile time message saying "type must be x, y, or z" ...

Hmm, sounds like the 'constraints' proposal being discussed for C++0x.
Right now, the best I can think of is something like:

template isInList(T, A, B, C) {
static if (is (T==A)) const bool isInList=true;
else static if (is(T==B)) const bool isInList=true;
else static if (is(T==C)) const bool isInList=true;
else const bool isInList=false;
}

class MyTemplate(T)
{
    static assert(isInList(T, char, wchar, dchar));
}

obviously with overloads of isInList() for different numbers of 
arguments. The preprocessor strikes again, until we get varag templates...

 Also, is there a better way to halt the compiler? So many error messages are 
 emitted that it's quite hard to see the pragma message :-)

I think that static assert() should halt compilation, just as assert() 
halts at runtime. Do you see any problems with that?
(I've had terrible problems with a static assert in a recursive 
function, you get the same error two million times).

Dec 12 2005

kris <fu bar.org> writes:

Don Clugston wrote:
 Kris wrote:
 
 class MyTemplate(T)
 {
         static if (is (T == char)) {}
         else
         static if (is (T == wchar)) {}
         else
         static if (is (T == wchar)) {}
         else
            {
            pragma (msg, "Template type must be char, wchar, or dchar");
            static assert(0);
            }

     // ...
 }

 what is the best (most succinct?) way to limit the T type? What I'm 
 looking for is a nice compile time message saying "type must be x, y, 
 or z" ...

 
 
 Hmm, sounds like the 'constraints' proposal being discussed for C++0x.
 Right now, the best I can think of is something like:
 
 template isInList(T, A, B, C) {
 static if (is (T==A)) const bool isInList=true;
 else static if (is(T==B)) const bool isInList=true;
 else static if (is(T==C)) const bool isInList=true;
 else const bool isInList=false;
 }
 
 class MyTemplate(T)
 {
    static assert(isInList(T, char, wchar, dchar));
 }
 
 obviously with overloads of isInList() for different numbers of 
 arguments. The preprocessor strikes again, until we get varag templates...
 
 Also, is there a better way to halt the compiler? So many error 
 messages are emitted that it's quite hard to see the pragma message :-)

 
 
 I think that static assert() should halt compilation, just as assert() 
 halts at runtime. Do you see any problems with that?
 (I've had terrible problems with a static assert in a recursive 
 function, you get the same error two million times).

Thanks Don!

That looks more useful. Static assert() does halt, but not quickly 
enough I'm afraid ~ it just adds another little warning to the sea 
washing over the console :)

Dec 12 2005

Sean Kelly <sean f4.ca> writes:

Don Clugston wrote:
 Kris wrote:
 class MyTemplate(T)
 {
         static if (is (T == char)) {}
         else
         static if (is (T == wchar)) {}
         else
         static if (is (T == wchar)) {}
         else
            {
            pragma (msg, "Template type must be char, wchar, or dchar");
            static assert(0);
            }

     // ...
 }

 what is the best (most succinct?) way to limit the T type? What I'm 
 looking for is a nice compile time message saying "type must be x, y, 
 or z" ...

 
 Hmm, sounds like the 'constraints' proposal being discussed for C++0x.
 Right now, the best I can think of is something like:
 
 template isInList(T, A, B, C) {
 static if (is (T==A)) const bool isInList=true;
 else static if (is(T==B)) const bool isInList=true;
 else static if (is(T==C)) const bool isInList=true;
 else const bool isInList=false;
 }
 
 class MyTemplate(T)
 {
    static assert(isInList(T, char, wchar, dchar));
 }
 
 obviously with overloads of isInList() for different numbers of 
 arguments. The preprocessor strikes again, until we get varag templates...

You can also do something like this:

template isValidType( T ) {
     const bit isValidType = is(T:char) || is(T:wchar);
}

template doSomething( T, bit vt : true = isValidType!(T) )
{
	// make sure the user isn't trying to trick us
	static assert( isValidType!(T) );
	...
}


Sean

Dec 12 2005

"Kris" <fu bar.com> writes:

Thx Sean;

What is the difference between "is (T:char)" and "is (T == char)" ? I had 
the impression there was one, but can't find it now ...



"Sean Kelly" <sean f4.ca> wrote in message 
news:dnklve$2cqj$1 digitaldaemon.com...
 Don Clugston wrote:
 Kris wrote:
 class MyTemplate(T)
 {
         static if (is (T == char)) {}
         else
         static if (is (T == wchar)) {}
         else
         static if (is (T == wchar)) {}
         else
            {
            pragma (msg, "Template type must be char, wchar, or dchar");
            static assert(0);
            }

     // ...
 }

 what is the best (most succinct?) way to limit the T type? What I'm 
 looking for is a nice compile time message saying "type must be x, y, or 
 z" ...

 Hmm, sounds like the 'constraints' proposal being discussed for C++0x.
 Right now, the best I can think of is something like:

 template isInList(T, A, B, C) {
 static if (is (T==A)) const bool isInList=true;
 else static if (is(T==B)) const bool isInList=true;
 else static if (is(T==C)) const bool isInList=true;
 else const bool isInList=false;
 }

 class MyTemplate(T)
 {
    static assert(isInList(T, char, wchar, dchar));
 }

 obviously with overloads of isInList() for different numbers of 
 arguments. The preprocessor strikes again, until we get varag 
 templates...

 You can also do something like this:

 template isValidType( T ) {
     const bit isValidType = is(T:char) || is(T:wchar);
 }

 template doSomething( T, bit vt : true = isValidType!(T) )
 {
 // make sure the user isn't trying to trick us
 static assert( isValidType!(T) );
 ...
 }


 Sean

Dec 12 2005

Sean Kelly <sean f4.ca> writes:

Kris wrote:
 Thx Sean;
 
 What is the difference between "is (T:char)" and "is (T == char)" ? I had 
 the impression there was one, but can't find it now ...

You probably want "T==char."  IIRC, "T:char" requires that T be 
convertible to char while "T==char" requires that T be the type 'char'. 
  I really need to fix this for std.atomic as well :-)


Sean

Dec 12 2005

Sean Kelly <sean f4.ca> writes:

Sean Kelly wrote:
 Kris wrote:
 Thx Sean;

 What is the difference between "is (T:char)" and "is (T == char)" ? I 
 had the impression there was one, but can't find it now ...

 
 You probably want "T==char."  IIRC, "T:char" requires that T be 
 convertible to char while "T==char" requires that T be the type 'char'. 
  I really need to fix this for std.atomic as well :-)

Oh, the reason I suggested my method is that it should just not 
instantiate the template at all if the type doesn't match, which should 
provide more useful error messages.  I do like Don's pseudo typelist 
approach for over my isValidType template, so the two could be combined 
as well.


Sean

Dec 12 2005

Oskar Linde <oskar.lindeREM OVEgmail.com> writes:

Kris wrote:
 Thx Sean;
 
 What is the difference between "is (T:char)" and "is (T == char)" ? I had 
 the impression there was one, but can't find it now ...

As far as I have understood,
is (T:char) is true if T is implicitly convertible to char,
is (T == char) is true only for T == char

i.e. is (int:char) is true, while is (int == char) is false.

/Oskar

Dec 12 2005

"Kris" <fu bar.com> writes:

Thx!

"Oskar Linde" <oskar.lindeREM OVEgmail.com> wrote in message 
news:dnknih$2f3j$1 digitaldaemon.com...
 Kris wrote:
 Thx Sean;

 What is the difference between "is (T:char)" and "is (T == char)" ? I had 
 the impression there was one, but can't find it now ...

 As far as I have understood,
 is (T:char) is true if T is implicitly convertible to char,
 is (T == char) is true only for T == char

 i.e. is (int:char) is true, while is (int == char) is false.

 /Oskar

Dec 12 2005

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digitalmars.D.learn - Templates ~ best way to limit types?