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From: Michael Kiermaier on 17 Jan 2008 18:09
Hello,

I have this code:

#include <iostream>

template<unsigned int modulus> class modRing {
public:
class element {
public:
element(unsigned int valIn):
val(valIn)
{}
unsigned int val;
};
};

template<unsigned int modulus>
typename modRing<modulus>::element
operator+(typename modRing<modulus>::element const &a, typename
modRing<modulus>::element const &b) {
return typename modRing<modulus>::element((a.val + b.val) % modulus);
}

int main() {
typedef modRing<5> R;
R::element a(3);
R::element b(4);
std::cout << (a + b).val;
}


Compilation with g++ 4.1.2 results in:

In function �int main()�:
24: error: no match for �operator+� in �a + b�


So the compiler does not recognize that it can specialize the operator+
function to the situation in line 24. My questions are:
Why?
How can I fix this problem?

Many thanks in advance,

~Michael


--
[ See http://www.gotw.ca/resources/clcm.htm for info about ]
[ comp.lang.c++.moderated. First time posters: Do this! ]

From: Alberto Ganesh Barbati on 18 Jan 2008 08:21
Michael Kiermaier ha scritto:
> Hello,
>
> I have this code:
>
> #include <iostream>
>
> template<unsigned int modulus> class modRing {
> public:
> class element {
> public:
> element(unsigned int valIn):
> val(valIn)
> {}
> unsigned int val;
> };
> };
>
> template<unsigned int modulus>
> typename modRing<modulus>::element
> operator+(typename modRing<modulus>::element const &a, typename
> modRing<modulus>::element const &b) {
> return typename modRing<modulus>::element((a.val + b.val) % modulus);
> }
>
> int main() {
> typedef modRing<5> R;
> R::element a(3);
> R::element b(4);
> std::cout << (a + b).val;
> }
>
>
> Compilation with g++ 4.1.2 results in:
>
> In function �int main()�:
> 24: error: no match for �operator+� in �a + b�
>
>
> So the compiler does not recognize that it can specialize the operator+
> function to the situation in line 24. My questions are:
> Why?

"typename modRing<modulus>::element" is a so-called "non-deduced"
context that doesn't allow the compiler to deduce modulus.

> How can I fix this problem?
>

template<unsigned int modulus> class modRing {
public:
class element {
public:
element(unsigned int valIn):
val(valIn)
{}
unsigned int val;

friend element operator+(element const &a,
element const &b) {
return element((a.val + b.val) % modulus);
}
};
};


HTH,

Ganesh


--
[ See http://www.gotw.ca/resources/clcm.htm for info about ]
[ comp.lang.c++.moderated. First time posters: Do this! ]

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