220 29387 <668e0a04-c988-4354-ae32-30f626f75403@isocpp.org> article Path: news.gmane.org!.POSTED!not-for-mail From: billconan@gmail.com Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: why not formally support meta programming. Date: Wed, 9 Nov 2016 11:32:54 -0800 (PST) Lines: 15971 Approved: news@gmane.org Message-ID: <668e0a04-c988-4354-ae32-30f626f75403@isocpp.org> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: blaine.gmane.org Mime-Version: 1.0 Content-Type: multipart/mixed; boundary="----=_Part_3475_1461667589.1478719974157" X-Trace: blaine.gmane.org 1478720176 20693 195.159.176.226 (9 Nov 2016 19:36:16 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Wed, 9 Nov 2016 19:36:16 +0000 (UTC) To: ISO C++ Standard - Future Proposals Original-X-From: std-proposals+bncBD7JLV7X6MLRBZ7TRXAQKGQESRJFLHQ@isocpp.org Wed Nov 09 20:33:15 2016 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-yw0-f198.google.com ([209.85.161.198]) by blaine.gmane.org with esmtp (Exim 4.84_2) (envelope-from ) id 1c4YcA-00074s-1U for gclcip-std-proposals@m.gmane.org; Wed, 09 Nov 2016 20:32:55 +0100 Original-Received: by mail-yw0-f198.google.com with SMTP id l124sf193007173ywb.5 for ; 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Wed, 09 Nov 2016 11:32:56 -0800 (PST) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.157.11.242 with SMTP id 105ls8400362oth.30.gmail; Wed, 09 Nov 2016 11:32:55 -0800 (PST) X-Received: by 10.157.15.143 with SMTP id d15mr264767otd.2.1478719975250; Wed, 09 Nov 2016 11:32:55 -0800 (PST) X-Original-Sender: billconan@gmail.com Precedence: list Mailing-list: list std-proposals@isocpp.org; contact std-proposals+owners@isocpp.org List-ID: X-Spam-Checked-In-Group: std-proposals@isocpp.org X-Google-Group-Id: 399137483710 List-Post: , List-Help: , List-Archive: List-Subscribe: , List-Unsubscribe: , Xref: news.gmane.org gmane.comp.lang.c++.isocpp.proposals:29387 Archived-At: ------=_Part_3475_1461667589.1478719974157 Content-Type: multipart/alternative; boundary="----=_Part_3476_746936063.1478719974248" ------=_Part_3476_746936063.1478719974248 Content-Type: text/plain; charset=UTF-8 I'm trying to design a library for different hardware (CPU and GPU) For users, I want the interface looks like this: Blah b(); or Blah c(); ideally inside the class of Blah, I want to do this to avoid duplication and reduce workload: template class Blah { DataType shared_member; #Meta_if(deviceUsed == GPU) //some kind of meta programming syntax to enable code based on template #{ DataType member_only_used_by_gpu; void function_on_gpu(); #} #Meta_if(deviceUsed == CPU) #{ DataType member_only_used_by_cpu; void function_on_cpu(); #} void shared_function() { .... #Meta_if(deviceUsed = GPU) { .... #} } }; This is the actual code I have right now. as you can see, I have to use std::enable_if I really don't like it, because it is a trick, a hacky solution, and only works for functions, not for members or function internals. and to make the following code to work, I had to try many options, some don't compile, some don't do what I want. And It's really hard to understand this syntax: template > template::type > template::type* = 0 > And for class members, since std::enable_if can't be used, I have to put shared stuff into a base class. And this is only manageable to do when I have few template variables. typedef enum { CPU_NAIVE = 0x1, CPU = 0x2, CPU_SIMD = 0x2, GPU = 0x4, GPU_CUDA = 0x4 } DeviceType; #define DEVICE_SPECIFIC(devices) \ template > template class Blob_base { protected: const std::string m_name; const unsigned int m_batchSize; const unsigned int m_depth; const unsigned int m_height; const unsigned int m_width; DataType *m_data; }; template class Blob : public Blob_base { public: Blob(const unsigned int batchSize, const unsigned int depth, const unsigned int height, const unsigned int width, const std::string &name = "no_name"); bool init(); DEVICE_SPECIFIC(CPU | GPU) void test(); }; template class Blob : public Blob_base { public: bool init(); Blob(const unsigned int batchSize, const unsigned int depth, const unsigned int height, const unsigned int width, const std::string &name = "no_name"); }; And because the limitation of partial specification of c++ (you have to partially specify a class first before specifying its member functions), I had to create many versions of the same class, with different template<> variables. I have to duplicate lots of code. Also, I really want the flexibility to do this: template class { #meta_if(device == CPU || device == CPU_SIMD) int a,b; void function() {... }; #meta_end } apparently using the base class still doesn't allow me to do this I think many people share the same pain with me, if you google those questions about std::enable_if and how to specify a template member function. So if meta programming is so useful, why don't you consider adding it formally to C++ by introducing the above syntax, rather than standardizing the partially working ugly hack std::enable_if? PHP for example, can be mixed with html. why can't c++ define something similar: such that what's inside will be executed by the preprocessor and generate code? -- You received this message because you are subscribed to the Google Groups "ISO C++ Standard - Future Proposals" group. To unsubscribe from this group and stop receiving emails from it, send an email to std-proposals+unsubscribe@isocpp.org. To post to this group, send email to std-proposals@isocpp.org. To view this discussion on the web visit https://groups.google.com/a/isocpp.org/d/msgid/std-proposals/668e0a04-c988-4354-ae32-30f626f75403%40isocpp.org. ------=_Part_3476_746936063.1478719974248 Content-Type: text/html; charset=UTF-8 Content-Transfer-Encoding: quoted-printable
I'm trying to design a library for different hardware = (CPU and GPU)

For users, I want the interface looks like= this:


Blah<GPU, float> b();<= /div>

or=C2=A0

Blah<CPU, dou= ble> c();


ideally inside the cla= ss of Blah, I want to do this to avoid duplication and reduce workload:



template<Devic= eType devi= ceUsed, typename DataType>
class Blah
{
= =C2=A0 =C2=A0 =C2=A0 DataType shared_member;


=C2=A0 =C2=A0 =C2=A0 #Meta_if(deviceUsed =3D=3D GPU) =C2=A0 //some ki= nd of meta programming syntax to enable code based on template
=C2=A0 =C2=A0 =C2= =A0 #{
=C2=A0 =C2= =A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0DataType member_only_used_by_gpu;
=C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0void function_on_gpu();
=C2=A0 =C2=A0 =C2= =A0 #}


= =C2=A0 =C2=A0 =C2=A0 #Meta_if(deviceUsed =3D=3D CPU)
=C2=A0 =C2=A0 =C2=A0 #{
=C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2= =A0 DataTy= pe member_= only_used_by_cpu;=C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 void function_on_cpu();
=C2=A0 =C2=A0 =C2=A0 #}




=C2=A0 =C2=A0 =C2=A0 <= span style=3D"color: #008;" class=3D"styled-by-prettify">void shared_function<= span style=3D"color: #660;" class=3D"styled-by-prettify">()
=C2=A0 =C2=A0 =C2=A0 <= /span>{
=C2=A0 =C2=A0 = =C2=A0 =C2=A0 =C2=A0 =C2=A0 ....
=C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 #Meta_if(deviceUsed =3D GPU= ) {
=C2= =A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0....
=C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 #}
=C2=A0 =C2=A0 =C2= =A0 }
};

=



This is the actual = code I have right now.

as you can see, I have to u= se std::enable_if

I really don't like it, beca= use it is a trick, a hacky solution, and only works for functions, not for = members or function internals.

and to make the fol= lowing code to work, I had to try many options, some don't compile, som= e don't do what I want. And It's really hard to understand this syn= tax:

template<DeviceType=
 T =3D DeviceUsed, typename Enabled =3D typename std::enable_if<(T & devices)=
 !=3D 0&g=
t; >

<DeviceType T =3D DeviceUsed, typename<=
/span> =3D std::enable_if&l=
t;(T & devices) !=3D 0>::type >

template<DeviceType T =3D DeviceUs=
ed, typename std::enable_if<(T & devices) !=3D 0=
>:=
:type* =3D 0 >


And for class members, since std::=
enable_if can't be used, I have to put shared stuff into a base class. =
And this is only manageable to do when I have few template variables=
.. 


=C2=A0
=C2=A0 =C2=A0template<typename DataType =3D float>

=C2=
=A0 =C2=A0 class =
Blob_base

=C2=
=A0 =C2=A0 {

=C2=A0 =C2=A0 protected:

=C2=A0 =C2=A0 =C2=A0 =C2=A0 const std::string m_name;

=C2=A0 =C2=A0 =C2=A0=
 =C2=A0 const unsigned int m_batchSize;

=C2=A0 =C2=A0 =C2=A0 =C2=A0 const=
 unsigned int m_depth<=
/span>;

=C2=A0 =C2=A0 =C2=A0 =C2=A0 const unsig=
ned int m_height;

=C2=A0 =C2=A0 =C2=A0 =C2=A0 const unsigned int m_width;

=C2=A0 =
=C2=A0 =C2=A0 =C2=A0 DataType *m_data;

=C2=A0 =C2=
=A0 };
<= br>
=C2=A0 =C2=A0 =C2=A0 =C2=A0 template<DeviceType DeviceUsed =3D <=
span style=3D" color:#800080;">CPU, typename=
 DataType<=
/span> =3D f=
loat>

=C2=A0 =C2=A0 =
class Blob=
 : public Blob_base<DataType>

=C2=A0=
 =C2=A0 =C2=A0 =C2=A0 {



=C2=A0 =C2=A0 =
public:

=C2=A0 =C2=A0 =C2=A0 =C2=A0 =
=C2=A0 =C2=A0 =C2=A0 =C2=A0 <=
span style=3D"color: #606;" class=3D"styled-by-prettify">Blob=
(const unsig=
ned int batchSize, 

=
=C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=
=A0 =C2=A0 =C2=A0const unsigned <=
/span>int =
depth,

=C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0=
 =C2=A0 =C2=A0 =C2=A0const unsigned int height<=
span style=3D"color: #660;" class=3D"styled-by-prettify">,

=C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =
=C2=A0 =C2=A0 =C2=A0 =C2=A0const=
 unsigned int width<=
/span>,
=C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =
=C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0const=
 std::string &name=
 =3D "no_name");



=C2=A0 =C2=A0 =C2=A0 =C2=A0 bool init();



=C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0=
 =C2=A0 =C2=A0 =C2=A0 DEVICE_SPECIFIC(CPU | GPU)

=C2=A0 =C2=A0 =C2=A0 =C2=A0 =
=C2=A0 =C2=A0 =C2=A0 =C2=A0 <=
span style=3D"color: #008;" class=3D"styled-by-prettify">void=
 test();

=C2=A0 =C2=A0 =C2=A0 =C2=A0 };


=
=C2=A0 =C2=A0 =
template<<=
span style=3D" color:#808000;">typename DataType>

=C2=A0 =C2=A0 class B=
lob=
<GPU, DataType> : public Blob_base<DataType>

=C2=A0 =C2=A0 {

=C2=A0 =C2=A0 public:

=C2=A0 =C2=A0 =C2=A0 =C2=A0 bool=
 init();



=C2=A0 =C2=A0 =C2=A0 =C2=A0 Blob(const unsigned <=
/span>int =
batchSize<=
span style=3D"color: #660;" class=3D"styled-by-prettify">,

=C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0const unsigned int<=
/span> depth,

=C2=A0 =C2=A0 =C2=A0 =C2=
=A0 =C2=A0 =C2=A0 =C2=A0const unsigned int <=
/span>height,
<=
span style=3D"color: #000;" class=3D"styled-by-prettify">
=C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0=
const unsigned int<=
/span> width,

=C2=A0 =C2=A0 =C2=A0 =C2=
=A0 =C2=A0 =C2=A0 =C2=A0const std::string &name =3D "no_name");

=C2=
=A0 =C2=A0 };



And because the limitation of partial specification of c++ (you have to pa= rtially specify a class first before specifying its member functions), I ha= d to create many versions of the same class, with different template<>= ; variables.

I have to duplicate lots of code.

Also, I really want the flexibility to do this:
=

template<Devic= eType devi= ce>
class
{
=C2=A0 =C2=A0 =C2=A0 #meta_if(device =3D=3D CP= U || device =3D=3D CPU_SIMD)
=C2=A0 =C2=A0 =C2=A0 =C2=A0int a,b;=C2=A0 =C2=A0 =C2=A0 =C2=A0void function()
=C2= =A0 =C2=A0 =C2=A0 =C2=A0{... }= ;
=C2= =A0 =C2=A0 =C2=A0 =C2=A0#meta_end
}


appare= ntly using the base class still doesn't allow me to do this=C2=A0
=


I think many people share the same pain = with me, if you google those questions about std::enable_if and how to spec= ify a template member function.

So if meta program= ming is so useful, why don't you consider adding it formally to C++ by = introducing the above syntax, rather than standardizing the partially worki= ng ugly hack std::enable_if?


PHP fo= r example, can be mixed with html.

why can't c= ++ define something similar:

<?c++=C2=A0echo=C2=A0'While=C2=A0this=C2=A0is=C2=A0goi= ng=C2=A0to=C2=A0be=C2=A0parsed.';=C2=A0?>

such that what's inside=C2=A0<= span style=3D"font-family: "Fira Mono", "Source Code Pro&quo= t;, monospace; font-size: 14px; color: rgb(0, 0, 187);"><?c++ ...= =C2=A0?> will be e= xecuted by the preprocessor and generate code?


--
You received this message because you are subscribed to the Google Groups &= quot;ISO C++ Standard - Future Proposals" group.
To unsubscribe from this group and stop receiving emails from it, send an e= mail to std-proposa= ls+unsubscribe@isocpp.org.
To post to this group, send email to std-proposals@isocpp.org.
To view this discussion on the web visit https://groups.google.= com/a/isocpp.org/d/msgid/std-proposals/668e0a04-c988-4354-ae32-30f626f75403= %40isocpp.org.
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