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bigfaceworm
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 Posted: Wed Jul 28, 2004 3:09 am Post subject: template class with template member function, want to specia |
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All,
I'm trying what I would have expected to be a relatively
straight-forward problem.
I've got a class, I want to add a new member function that's a
templatized member function. So far so good.
Now I want to specialize that template member function. But I can't
get past the compile time errors.
Is this possible? Or am I just dreaming.
I'm building on Linux with gcc 3.2.2
#include <iostream>
using namespace std;
template <class T>
struct Mine
{
Mine(const T&t) : t_(t) {} ;
T t_;
template <bool>
void func(ostream &o);
};
template <class T>
template <bool b>
void Mine<T>::func(ostream &o)
{
o << "Func for type T " << t_ << endl;
}
template
template <bool>
void Mine<T>::func<false>(ostream &o) ///// COMPILE ERROR HERE
{
o << "FALSE FALSE FALSE FALSE Func for type T " << t_ << endl;
}
int main(int, char *[])
{
Mine
t.func<true>(cout);
t.func<false>(cout);
return 0;
}
//////////////////////
here are the errors:
~/tmp/template_specialize.cpp:25: template-id `func<false>' in
declaration of primary template
~/tmp/template_specialize.cpp:25: redefinition of `void
Mine<T>::func(_STL::ostream&)'
~/tmp/template_specialize.cpp:18: `void Mine<T>::func(_STL::ostream&)'
previously declared here
~/tmp/template_specialize.cpp:25: no `void
Mine<T>::func(_STL::ostream&)' member function declared in class
`Mine<T>'
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Victor Bazarov
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| Posted: Wed Jul 28, 2004 3:21 pm Post subject: Re: template class with template member function, want to sp |
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bigfaceworm wrote:
| Quote: | I'm trying what I would have expected to be a relatively
straight-forward problem.
I've got a class, I want to add a new member function that's a
templatized member function. So far so good.
Now I want to specialize that template member function. But I can't
get past the compile time errors.
Is this possible? Or am I just dreaming.
|
It is not possible. You're not allowed to specialise a member template
without specialising the class. See Standard 14.7.3/18.
[ See http://www.gotw.ca/resources/clcm.htm for info about ]
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Gareth Stockwell
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| Posted: Thu Jul 29, 2004 10:30 am Post subject: Re: template class with template member function, want to sp |
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[email]bigfaceworm (AT) hotmail (DOT) com[/email] (bigfaceworm) wrote in message news:<2bd747c7.0407271541.702d8eb7 (AT) posting (DOT) google.com>...
| Quote: | I've got a class, I want to add a new member function that's a
templatized member function. So far so good.
Now I want to specialize that template member function. But I can't
get past the compile time errors.
Is this possible? Or am I just dreaming.
|
Well, not directly, but you can *almost* achieve this with a
work-around. The idea is that you declare a structure outside of your
class, which is templated on all the parameters involved (i.e. all
your class parameters, plus the parameters for the member function).
Inside this new structure, define a static member function which takes
all the arguments of the original member function, plus a reference to
the original object. This function constitutes the implementation for
your original templated member function.
An example should make this clear:
//============================================
#include <iostream>
// Forward declaration of our class
template<typename T> struct MyClass;
// Implementation for the non-specialised version of MyClass::func
template<typename T, bool B>
struct FuncImplementation {
static void execute(const MyClass<T>& c, std::ostream& out)
{ out << "non-specialised " << c.t() << std::endl; }
};
// Specialised version
template
struct FuncImplementation<T,false> {
static void execute(const MyClass<T>& c, std::ostream& out)
{ out << "specialised " << c.t() << std::endl; }
};
// The class itself
template
class MyClass {
T t_;
public:
MyClass(const T& t) : t_(t) { }
// Member function defers to the static method of
// the implementation class defined above
template<bool B>
void func(std::ostream& out) const
{ FuncImplementation<T,B>::execute(*this, out); }
// Accessor for the data member
const T& t() const { return t_; }
};
int main() {
MyClass<int> C(10);
C.func<true>(std::cout);
C.func<false>(std::cout);
}
//============================================
The reason it's only *almost* a solution is that in the general case,
FuncImplementation::execute cannot truly emulate the original member
function, because it does not have access to the private members of
MyClass. Friendship? Unless I'm missing a trick, you can't declare a
partial specialisation to be a friend...
HTH
Gareth
[ See http://www.gotw.ca/resources/clcm.htm for info about ]
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