Question on explicit template expansion

Question on explicit template expansion

[Garrett Kajmowicz]

I'm working on the embedded C++ library at cxx.uclibc.org

As a part of my work, I've been pushing some of the common template expansions 
out into the binary library to reduce overall expansion.  This works well 
*except* for constructors and destructors.  I would like an explanation as to 
why the following occurs and what I can go to deal with it.


Given (say) class string, I have:

//In the header file

template<class Ch, class Tr, class A> class basic_string {
public:
	explicit basic_string(const A& al = A()) { return; }
	~basic_string() { }

//	<snip>
//	<snip>
//	<snip>

};

#ifndef __UCLIBCXX_COMPILE_STRING__
	template <> string::basic_string(const allocator<char> &);
	template <> string::~basic_string();
#endif

//End of header file


And then inside the source file for the library I have

//Source file

#define __UCLIBCXX_COMPILE_STRING__ 1
#include <string>

template string::basic_string(const allocator<char> &);
template string::~basic_string();

//End of source file

I do this for a large number of functions.  This allows me to use only one 
instance of the code (that I have to type out) while creating the required 
instances.

This seems to work for most code.  However, for constructors and destructors I 
am left with the following error messages when I go to use the library:


../include/string:71: warning: inline function `std::basic_string<Ch, Tr,
   A>::basic_string(const A&) [with Ch = char, Tr = std::char_traits<char>, A 
=
   std::allocator<char>]' used but never defined
../include/string:100: warning: inline function `std::basic_string<Ch, Tr,
   A>::~basic_string() [with Ch = char, Tr = std::char_traits<char>, A =
   std::allocator<char>]' used but never defined


Any thoughts as to why this happens, and why it only happens for constructors 
and destructors.


Thanks.

-	Garrett Kajmowicz

Re: Question on explicit template expansion

[Nathan Sidwell]

Garrett Kajmowicz wrote:
> I'm working on the embedded C++ library at cxx.uclibc.org
> 
> As a part of my work, I've been pushing some of the common template expansions 
> out into the binary library to reduce overall expansion.  This works well 
> *except* for constructors and destructors.  I would like an explanation as to 
> why the following occurs and what I can go to deal with it.
> 
> 
> Given (say) class string, I have:
> 
> //In the header file
> 
> template<class Ch, class Tr, class A> class basic_string {
> public:
> 	explicit basic_string(const A& al = A()) { return; }
> 	~basic_string() { }
> 
> //	<snip>
> //	<snip>
> //	<snip>
> 
> };
> 
> #ifndef __UCLIBCXX_COMPILE_STRING__
> 	template <> string::basic_string(const allocator<char> &);
> 	template <> string::~basic_string();
> #endif

This doesn't compile. Do you have a test case that does compile?

nathan

-- 
Nathan Sidwell    ::   http://www.codesourcery.com   ::     CodeSourcery LLC
nathan@codesourcery.com    ::     http://www.planetfall.pwp.blueyonder.co.uk

Re: Question on explicit template expansion

[Garrett Kajmowicz]

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> This doesn't compile. Do you have a test case that does compile?

I do now.

The problem I've been encountering occurs on both GCC 3.3.5 and GCC 4.0.0.  
This test case was only tested on GCC 3.3.5.

The attached test case comes with a Makefile.  Type make and watch the warning 
message occur.  Note that the warning messages occur only for the constructor 
and destructor, and not for the other method.


Please let me know what's going on.


-	Garrett

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Re: Question on explicit template expansion

[Nathan Sidwell]

Garrett Kajmowicz wrote:

> The attached test case comes with a Makefile.  Type make and watch the warning 
> message occur.  Note that the warning messages occur only for the constructor 
> and destructor, and not for the other method.

header.h contains
	#ifndef __EXPAND_LIBRARY_FOR_COMPILE__
	template <> myclass<char>::myclass();
	template <> myclass<char>::~myclass();
	template <> void myclass<char>::method();
	#endif
these *declare* specializations of the myclass ctor,dtor and method.  There's
no reason to make them NOT visible when compiling the library.

However, it does appear to be a g++ bug in that the inline attribute is
copied from the general template to the specialization.

nathan

-- 
Nathan Sidwell    ::   http://www.codesourcery.com   ::     CodeSourcery LLC
nathan@codesourcery.com    ::     http://www.planetfall.pwp.blueyonder.co.uk