Re: A link error related variadic-template

[Jonathan Wakely <jwakely dot gcc at gmail dot com>]

Bah, I hope it was obvious that when I said "this function overloads
args()" I meant it overloads add() ... sorry for any confusion.



On 30 March 2012 10:51, Jonathan Wakely <jwakely.gcc@gmail.com> wrote:
> On Mar 30, 2012 7:38 AM, <chmodexplorer@sina.com> wrote:
>>
>> I used g++ 4.7.0 to compile the piece of code:
>>
>> #include <stdio.h>
>> #include <boost/call_traits.hpp>
>>
>> using namespace boost;
>>
>> template <typename... Args> long add(Args...);
>
> This function is not defined.
>
>> template <typename T> long add( typename call_traits<T>::param_type t) { return t; }
>
> This function overloads args() but it can never deduce the parameter
> T, because T appears in a non-deduced context (to the left of the ::
> scope operator)
>
>> template <typename A, typename... Args>
>> long add( typename call_traits<A>::param_type a, typename call_traits<Args>::param_type... args )
>
> Same here, this adds a third overload of args (it does not provide a
> definition for the first overload above) and because they appear in
> non-deduced contexts A and Args cannot be deduced.
>
>> {
>> ? ? ? ?return a + add( args... );
>> }
>>
>> int main()
>> {
>> ? ? ? ?char c = 0;
>> ? ? ? ?short s = 1;
>> ? ? ? ?int i = 2;
>> ? ? ? ?long l = 3;
>> ? ? ? ?long n = add( c, s, i, l );
>
> This cannot call the third overload, because argument deduction cannot
> succeed, so it calls the first overload of add() which you haven't
> defined so you get a linker error.
>
>>
>> ? ? ? ?return 0;
>> }
>>
>> And I encountered a link error:
>> /tmp/cc8rJ4xe.o: In function `main':
>> variadic-templates.cpp:(.text+0x33): undefined reference to `long add<char, short, int, long>(char, short, int, long)'
>> collect2: error: ld returned 1 exit status
>>
>> Is there anyone who can help me to resolve the problem? Thanks very much!