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Follow-up: Finding template parameter types

Posted on 2009-07-12
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Last Modified: 2012-05-07
Hi,

This is a follow up from:
http://www.experts-exchange.com/Q_24563512.html

Evilrix posted a solution which gets resolved at compile time, although the if statements appear within the function foo(), how can different function defenitions be created at compile-time based on the parameters put into the template - or perhaps a better question, how does the Boost signals library manage to achieve creating different function defintions for the callbacks?

Thanks,
Uni
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Question by:Unimatrix_001
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11 Comments
 
LVL 40

Expert Comment

by:evilrix
ID: 24834854
hi Uri. Out having food right now. I'll get back to you when I return :)
0
 
LVL 3

Author Comment

by:Unimatrix_001
ID: 24834861
No probs - enjoy yourself. :)
0
 
LVL 40

Expert Comment

by:evilrix
ID: 24835546
Ok, see what you make of the code below...
#include <iostream>
 
struct NoArg {};
 
template<typename A, typename B = NoArg, typename C = NoArg>
class CMyClass
{
public:
	void foo(A, B, C)
	{
		std::cout << "void foo(A, B, C)" << std::endl;
	}
};
 
template<typename A, typename B >
class CMyClass<A, B, NoArg>
{
public:
	void foo(A, B)
	{
		std::cout << "void foo(A, B)" << std::endl;
	}
};
 
template<typename A>
class CMyClass<A, NoArg, NoArg>
{
public:
	void foo(A)
	{
		std::cout << "void foo(A)" << std::endl;
	}
};
 
 
int main()
{
        CMyClass<int, char, float> c1;
        c1.foo(1, 'c', 2.34);
        
        CMyClass<int, char> c2;
        c2.foo(1, 'c');
 
        CMyClass<int> c3;
        c3.foo(1);
}

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LVL 3

Author Comment

by:Unimatrix_001
ID: 24835567
Ah okay - I see... How would things fare if we had something like this for example:
template<typename A, typename B = NoArg, typename C = NoArg, typename D = std::allocator<A> >
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LVL 40

Expert Comment

by:evilrix
ID: 24835635
>> How would things fare if we had something like this for example:
Arrr... brain hurts..... arrrg! :)



#include <iostream>
 
struct NoArg {};
 
template<typename A, typename B = NoArg, typename C = NoArg, typename D = std::allocator<A> >
class CMyClass
{
public:
	void foo(A, B, C)
	{
		std::cout << "void foo(A, B, C)" << std::endl;
	}
};
 
template<typename A, typename B >
class CMyClass<A, B, NoArg>
{
public:
	void foo(A, B)
	{
		std::cout << "void foo(A, B)" << std::endl;
	}
};
 
template<typename A>
class CMyClass<A, NoArg, NoArg>
{
public:
	void foo(A)
	{
		std::cout << "void foo(A)" << std::endl;
	}
};
 
 
int main()
{
        CMyClass<int, char, float, std::allocator<double> > c0;
	
        CMyClass<int, char, float> c1;
        c1.foo(1, 'c', 2.34);
        
        CMyClass<int, char> c2;
        c2.foo(1, 'c');
 
        CMyClass<int> c3;
        c3.foo(1);
}

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LVL 3

Author Comment

by:Unimatrix_001
ID: 24835643
>>Arrr... brain hurts..... arrrg! :)
Hehe, I'm nearly done. ;)

The thing with that is the last CMyClass:

--------------------------------------------------------------------------------------------------------
template<typename A>
class CMyClass<A, NoArg, NoArg>
{
public:
        void foo(A)
        {
                std::cout << "void foo(A)" << std::endl;
        }
};
--------------------------------------------------------------------------------------------------------

doesn't actually know anything about the D typename, so could never use this - correct?
0
 
LVL 40

Accepted Solution

by:
evilrix earned 500 total points
ID: 24835650
>> correct?
Yes. Each class is, in effect, a unique object that knows nothing about the others. Specializing or partial specializing a class is really just defining new types of a class. Without knowing the specific requirements you have all I can do is try and provide you examples that I think may aid your understanding but that are not necessarily the best solutions to specific problems (does that make sense?).
0
 
LVL 3

Author Comment

by:Unimatrix_001
ID: 24835656
Yes, that's perfect! :) No specific requirements just remaining questions on templates. ;) Cheers for the help, hope you enjoyed your evening! :)

Uni.
0
 
LVL 3

Author Closing Comment

by:Unimatrix_001
ID: 31602584
Excellent.
0
 
LVL 40

Expert Comment

by:evilrix
ID: 24835657
>> Cheers for the help, hope you enjoyed your evening! :)
Muahahahaha.... I've just drunk lots of beer, not sure I should even be attempting template questions!

In fact, I am off to bed to sleep myself into a hangover ;)
0
 
LVL 3

Author Comment

by:Unimatrix_001
ID: 24835662
Hahahaha! Enjoy your headache-to-come... ;)
0

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