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Template Specialization

Started by _goat Jan 16, 2007 at 10:32 PM 0 replies 1k views

Original Post

Jan 16, 2007 at 10:32 PM

This hurts my head. The following works like a charm:

#include <iostream>

template <typename T> struct A;
template <typename T> struct B;

typedef A<int> AA;
typedef B<int> BB;

template <> struct B<int>
{
	void eat()
	{
		AA::do_nothing();
		std::cout << "EATING" << std::endl;
	}
};

template <> struct A<int>
{
	void do_thing()
	{
		BB b;
		b.eat();
	}
	
	static void do_nothing()
	{
	}
};

struct ESS
{
	ESS()
	{
	}
	
	void d()
	{
		int a = 1, b = 2, c = a + b;
	}
};




int main()
{
	// just to make sure it's not optimised away
	ESS es; 
	es.d();
	
	// do a thing
	AA a; 
	a.do_thing();
}

However, if I move the whole ESS part in between the specialisations, it does not work:

#include <iostream>

template <typename T> struct A;
template <typename T> struct B;

typedef A<int> AA;
typedef B<int> BB;

template <> struct B<int>
{
	void eat()
	{
		AA::do_nothing();
		std::cout << "EATING" << std::endl;
	}
};

// ESS is now in between
struct ESS
{
	ESS()
	{
	}
	
	void d()
	{
		int a = 1, b = 2, c = a + b;
	}
};

template <> struct A<int>
{
	void do_thing()
	{
		BB b;
		b.eat();
	}
	
	static void do_nothing()
	{
	}
};



int main()
{
	// just to make sure it's not optimised away
	ESS es; 
	es.d();
	
	// do a thing
	AA a; 
	a.do_thing();
}

And if I have it anywhere above the specialisation of B, then it compiles again (not going to show this one :D). For the middle case, where it doesn't work, the error is:

main.cpp(19) : error C2027: use of undefined type 'A<T>'
        with
        [
            T=int
        ]
main.cpp(19) : error C3861: 'do_nothing': identifier not found

It is really doing my head in - does anyone know why that could be? I'm on MSVC2k5 if that helps.

joanusdmentia

Jan 17, 2007 at 2:11 AM

Neither version works for me, and I didn't expect them to. You're trying to use the type A<int> before it's been defined, the fact that you're doing template specialisation really has nothing to do with the problem. What you need to do is separate the function definitions from the class definitions:

#include <iostream>template <typename T> struct A;template <typename T> struct B;typedef A<int> AA;typedef B<int> BB;template <> struct B<int>{	void eat();};template <> struct A<int>{	void do_thing();	static void do_nothing() {}};void B<int>::eat(){	AA::do_nothing();	std::cout << "EATING" << std::endl;}void A<int>::do_thing(){	BB b;	b.eat();}struct ESS{	ESS()	{	}		void d()	{		int a = 1, b = 2, c = a + b;	}};int main(){	// just to make sure it's not optimised away	ESS es; 	es.d();		// do a thing	AA a; 	a.do_thing();}

EDIT: I'm using VS2005 as well.

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