There are a few questions already similar to this already on stack overflow, but nothing that seemd to directly answer the question I have. I do apologise if I am reposting.
I'd like to overload a few methods of a templated class (with 2 template parameters) with a partial template specialisation of those methods. I haven't been able to figure out the correct syntax, and am starting to think that it's not possible. I thought I'd post here to see if I can get confirmation.
Example code to follow:
template <typename T, typename U>
class Test
{
public:
void Set( T t, U u );
T m_T;
U m_U;
};
// Fully templated method that should be used most of the time
template <typename T, typename U>
inline void Test<T,U>::Set( T t, U u )
{
m_T=t;
m_U=u;
}
// Partial specialisation that should only be used when U is a float.
// This generates compile errors
template <typename T>
inline void Test<T,float>::Set( T t, float u )
{
m_T=t;
m_U=u+0.5f;
}
int _tmain(int argc, _TCHAR* argv[])
{
Test<int, int> testOne;
int a = 1;
testOne.Set( a, a );
Test<int, float> testTwo;
float f = 1.f;
testTwo.Set( a, f );
}
I know that I could write a partial specialisation of the entire class, but that kinda sucks. Is something like this possible?
(I'm using VS2008)
Edit: Here is the compile error
error C2244: 'Test::Set' : unable to match function definition to an existing declaration
Thanks :)
The particular problem you're sketching is easy:
template< class T >
inline T foo( T const& v ) { return v; }
template<>
float foo( float const& v ) { return v+0.5; }
Then call foo from your Test::Set implementation.
If you want the full generality, then similarly use a helper class with static helper member functions, and partially specialize that helper class.
Cheers & hth.,
You cannot partially specialize a member function without defining partial specialization of the class template itself. Note that partial specialization of a template is STILL a template, hence when the compiler sees Test<T, float>, it expects a partial specialization of the class template.
--
$14.5.4.3/1 from the C++ Standard (2003) says,
The template parameter list of a
member of a class template partial
specialization shall match the
template parameter list of the class
template partial specialization. The
template argument list of a member of
a class template partial
specialization shall match the
template argument list of the class
template partial specialization. A
class template specialization is a
distinct template. The members of the
class template partial specialization
are unrelated to the members of the
primary template. Class template
partial specialization members that
are used in a way that requires a
definition shall be defined; the
definitions of members of the primary
template are never used as definitions
for members of a class template
partial specialization. An explicit
specialization of a member of a class
template partial specialization is
declared in the same way as an
explicit specialization of the primary
template.
Then the Standard itself gives this example,
// primary template
template<class T, int I> struct A {
void f();
};
template<class T, int I> void A<T,I>::f() { }
// class template partial specialization
template<class T> struct A<T,2> {
void f();
void g();
void h();
};
// member of class template partial specialization
template<class T> void A<T,2>::g() { }
I hope the quotation from the Standard along with the example answers your question well.
There's also another solution to the partial specialization problem, if you don't want to introduce additional functions, methods or classes to your code.
#include <type_traits>
template <typename T1, typename T2>
class C
{
void f(T1 t1);
}
template <typename T1, typename T2>
void C<T1, T2>::f(T1 t1)
{
if (std::is_same<T2, float>::value)
// Do sth
else
// Do sth
}
