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calculate number of template instantiations

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Hi folks,

  I got an interesting problem to play with templates. It's not 
abstract and come from real life. I'm not sure wether it has solution 
within C++, so I need advise and you opinions.

  In brief: I need to calculate number of template instantiation, 
reasoning below.

  What I need. Assume I have some template Templ<T>. In the code I 
instantiated it several times like Templ<A>, Templ<B>, ... Next I need 
to get number of instantiation _prior_ to creating any of instances of 
instantiated types. The code, I believe, clarify. Note, it's not 
compiling.

#include <iostream>

template<typename T>
struct Templ
{
	T some_dummy_value;
};

class A{};
class B{};

struct InstanceA
{
	Templ<A> inst;
	void foo(){}
};

struct InstanceB
{
	Templ<B> inst;
	void foo(){}
};


//here the question
struct Counter
{
	// ???????????
	enum
	{
		INSTANCES_NUM = // ????????
	};
}

int main()
{
	// I need this value _before_ creating any objects of
	// instantiated types.
	std::cout << Counter::INSTANCES_NUM << std::endl;

	// And only now (!) create instances of InstanceA, InstanceB
	InstanceA a; a.foo();
	InstanceB b; b.foo();
}

  As you see I need number of instantiation before creating any objects 
of instantiated types, so it's almost (at least i see so) the same as 
knowing at compile time, so i wrote

enum { INSTANCE_NUM };


  The reasoning. What I wrote above is extraction of what I need for 
simpleness and clearness. The real problem.
  I recently wrote a wrapper for plain C written simulation system (SS) 
designed with performance issues in mind, E.G. data locality. So i 
chose to use template-based approach for wrapping. This discrete 
event-driven SS managed memory itself and asks me for maximum size of 
messages _IT_ will create for me. So i wrote wrapers, but now I need to 
feed it with maximum message size of templates (==wraper for messages) 
i created.

The flow.
---------------

#include <ss.h>

struct MsgA{};
struct MsgB{};

void fill_a( MsgA * msg ){}
void fill_b( MsgB * msg ){}

// callback for SS
void event_handler( void * msg ){}

// callback for SS
void init()
{
	void * event = ss_create_event(time, recepient);
	MsgA * msg = (MsgA*) ss_message_data(event);
	fill_a( msg );
	ss_send_msg( event );
	
   // the same with msg B
	...
};

SS_AgentType agent_type = {
	&init,
	&event_handler
};

int main()
{
	agents_num = 10;
	max_msg_size = max( sizeof( MsgA ) , sizeof( MsgB ) );

	ss_init( agents_num, max_msg_size, agent_type );
	ss_run();
	// process results.
	ss_stop();
}
----------------

  I wrote wrapper that (almost) statically dispatch agents and message  
types and i avoided cast's from void *. But I still need to calculate 
message sizes. I want to avoid it to. It's quite the same as calculate 
number of instances.

BR, Ruslan Mullakhmetov

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Thread

calculate number of template instantiations Ruslan Mullakhmetov <tiabaldu@gmail.com> - 2011-10-25 17:04 +0400
  Re: calculate number of template instantiations Victor Bazarov <v.bazarov@comcast.invalid> - 2011-10-25 09:32 -0400
    Re: calculate number of template instantiations Noah Roberts <roberts.noah@gmail.com> - 2011-10-25 08:30 -0700
      Re: calculate number of template instantiations Ruslan Mullakhmetov <tiabaldu@gmail.com> - 2011-10-25 20:05 +0400
      Re: calculate number of template instantiations Arne Mertz <arnemertz@googlemail.com> - 2011-10-26 02:10 -0700
    Re: calculate number of template instantiations Ruslan Mullakhmetov <tiabaldu@gmail.com> - 2011-10-25 19:42 +0400
      Re: calculate number of template instantiations Noah Roberts <roberts.noah@gmail.com> - 2011-10-25 09:43 -0700
        Re: calculate number of template instantiations Ruslan Mullakhmetov <tiabaldu@gmail.com> - 2011-10-25 20:46 +0400

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