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Date: Fri, 01 Mar 2013 08:52:00 -0000
From: Florian Weimer <fweimer@redhat.com>
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To: Ulrich Drepper <drepper@gmail.com>
CC: Benjamin De Kosnik <bkoz@redhat.com>,        =?UTF-8?B?RGFuaWVsIEtyw7xnbGVy?= <daniel.kruegler@gmail.com>,        "libstdc++@gcc.gnu.org" <libstdc++@gcc.gnu.org>,        GCC Patches <gcc-patches@gcc.gnu.org>
Subject: Re: [PATCH] C++ math constants
References: <CAOPLpQdxQkFcUb6-us3gdzesTwx8MyzxccF_TTZvO3=E0A1xsw@mail.gmail.com> <CAGNvRgAf8oNHAcORU-RapgTWep9fYWcf=Z7d5OvP2zbYstu3dg@mail.gmail.com> <20130221093859.33b4d2fe@oakwood> <CAOPLpQfyDy1xmiR3E7z1_o0UsNbf_45CXT7-Fbx_FvsCiQ=HfA@mail.gmail.com>
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On 03/01/2013 05:15 AM, Ulrich Drepper wrote:
> On Thu, Feb 21, 2013 at 12:38 PM, Benjamin De Kosnik <bkoz@redhat.com> wrote:
>> Not seeing it.
>>
>> Say for:
>>
>> #include <iostream>
>>
>>    // A class for math constants.
>>    template<typename _RealType>
>>      struct __math_constants
>>      {
>>        // Constant @f$ \pi @f$.
>>        static constexpr _RealType __pie =
>>    3.1415926535897932384626433832795029L; };
>>
>> template<class T>
>> void print(const T& t) { std::cout << t; }
>>
>> int main()
>> {
>>    print(__math_constants<double>::__pie);
>>    return 0;
>> }
>>
>> I'm not getting any definition, even at -O0.
>
> Even more so: how would an explicit instantiation even work?

You don't need an explicit instantiation, just a regular templated 
definition.  This example shows that a definition is needed, and how one 
is provided.

template <class T>
struct a {
   static constexpr T m = T(1);
};

template <class T>
constexpr T a<T>::m;

const int *
f()
{
   return &a<int>::m;
}

int
main()
{
   return f() == 0;
}

-- 
Florian Weimer / Red Hat Product Security Team