I'm trying to specialize a member function template for two different types of classes as follows:
#include <iostream>
#include <boost/utility/enable_if.hpp>
struct Wibble
{
static const bool CAN_WIBBLE = true;
};
struct Wobble
{
static const bool CAN_WIBBLE = false;
};
struct Foo
{
//template<typename T> // Why isn't this declaration sufficient?
//void doStuff();
template<typename T>
typename boost::enable_if_c<T::CAN_WIBBLE,void>::type
doStuff();
template<typename T>
typename boost::enable_if_c<!T::CAN_WIBBLE,void>::type
doStuff();
};
template<typename T>
typename boost::enable_if_c<T::CAN_WIBBLE,void>::type
Foo::doStuff()
{
std::cout << "wibble ..." << std::endl;
}
template<typename T>
typename boost::enable_if_c<!T::CAN_WIBBLE,void>::type
Foo::doStuff()
{
std::cout << "I can't wibble ..." << std::endl;
}
int main()
{
Foo f;
f.doStuff<Wibble>();
f.doStuff<Wobble>();
}
Whereas GCC 4.8.2 compiles the code, VS .NET 2008 spits out the error message:
error C2244: 'Foo::doStuff' : unable to match function definition to an existing declaration
definition
'boost::enable_if_c<!T::CAN_WIBBLE,void>::type Foo::doStuff(void)'
existing declarations
'boost::enable_if_c<!T::CAN_WIBBLE,void>::type Foo::doStuff(void)'
'boost::enable_if_c<T::CAN_WIBBLE,void>::type Foo::doStuff(void)'
I suggest to use tag dispatching: https://ideone.com/PA5PTg
struct Foo
{
template<bool wibble>
void _doStuff();
public:
template<typename T>
void doStuff()
{
_doStuff<T::CAN_WIBBLE>();
}
};
template<>
void Foo::_doStuff<true>() { std::cout << "wibble ..." << std::endl; }
template<>
void Foo::_doStuff<false>() { std::cout << "I can't wibble ..." << std::endl; }
You can't partially specialize (member) function templates. End of story.
Even if you could, you should have had a SFINAE-friendly primary template. In pseudo code:
template<typename T, typename Enable> void doStuff();
template<typename T> void doStuff<T, typename boost::enable_if_c<T::CAN_WIBBLE,void>::type>()
{ std::cout << "wibble ..." << std::endl; }
template<typename T> void doStuff<T, typename boost::enable_if_c<!T::CAN_WIBBLE,void>::type>()
{ std::cout << "I can't wibble ..." << std::endl; }
You could still use this technique if you are ready class templates (as functors or just types defining non-template methods...).
As a rule of thumb, for function templates , overload resolution provides static polymorphism that removes the need for partial specialization. See
Both by Herb Sutter
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