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From: Gerhard Menzl on 11 Apr 2008 02:50
Consider:

#include <algorithm>
#include <vector>

template <typename Element,
typename Member,
Member (Element::*memPtr)()>
bool lessMember(Element const& left, Element const& right)
{
return (left.*memPtr)() < (right.*memPtr)();
}

class C
{
public:
C(int i, long l) : m_i(i), m_l(l) {}

int GetInt() { return m_i; }
long GetLong() { return m_l; }

private:
int m_i;
long m_l;
};

int main()
{
std::vector<C> vc;

static int const elementCount = 3;

for (int elem = 0; elem < elementCount; ++elem)
vc.push_back(C(elem, elementCount - elem));

// linker error without the following line uncommented
// bool b = lessMember<C, long, &C::GetLong>(vc[0], vc[1]);

std::sort(vc.begin(),
vc.end(),
lessMember<C, long, &C::GetLong>);
}

Unless the function template specialization lessMember<C, long,
&C::GetLong> is called explicitly, my compiler (Visual C++ 6.0) does not
instantiate the template and causes a linker error (unresolved
external). Just taking the address of the specialization in the call to
std::sort does not seem to suffice. Is this one of the many limitations
of this outdated compiler, or does the Standard really not require an
instantiation here?

According to 14.7.1/2, a "function template specialization is implicitly
instantiated when the specialization is referenced in a context that
requires a function definition to exist". I would have assumed that
taking the address of a function (template specialization) requires its
definition.

In case instantiation is not required: is there any way to force
instantiation without an explicit specialization (in the actual code,
the function template is called from with in another function template),
and without inserting a dummy call for every specialization?

--
Gerhard Menzl

Non-spammers may respond to my email address, which is composed of my
full name, separated by a dot, followed by at, followed by "fwz",
followed by a dot, followed by "aero".



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