partial template specialisation

[dean]

Hi there,

I am trying to partially specialise a template with a fixed param, while keeping the other variables. Right now I am hitting a compiler error when trying to partially specialise the template:

class A{ };
class B{ };

template <typename T1, typename T2>
void foo1(T1 t1, T2 t2){
   cout << "Not specialised" << endl;
}

template <typename T1>
void foo1<B>(T1 t1, B t2){ // ERROR: template-id ‘foo1<B>’ in declaration of primary template
   cout << "Specialised for the 2nd argument" << endl;
}


template <>
void foo1<B,B>(B t1, B t2){
   cout << "Totally specialised" << endl;
}

int main(int argc, char* argv[]) {
   A a;
   B b;
   foo1<A,A>(a, a); // Not specialised
   foo1<A,B>(a, b); // Specialised for the 2nd argument
   foo1<B,B>(b, b); // Totally specialised

return 0;
}

What is the correct syntax in this case?

Kind regards,
Dean

[Cubbi]

clang gives a more explicit diagnostic:

test.cc:13:6: error: function template partial specialization is not allowed
void foo1<B>(T1 t1, B t2){ // ERROR: template-id foo1<B> in declaration...
     ^   ~~~

function templates can be either fully specialized or overloaded, not partially-specialized.

[giblit]

shouldn't line 10 be <B,T1>?

*edit
or <T1,B> rather since t1 is your first template. after thinking about it I dont think you can:

error: function template partial specialization ‘test<b, T1>’ is not allowed
 void test<b,T1>( T1 t1 , b t2 )

**edit looks like cubbi beat me

[JLBorges]

Function templates can't be partially specialized; but they overload, and we can use overloading instead.

Function templates can be fully specialized; but even here, we should prefer overloading over specialization.

See http://www.cplusplus.com/forum/beginner/100227/

[Cubbi]

To give an example of how you could achieve what I think was the goal ("a fixed param, while keeping the other variables."),

template <typename T1, typename T2>
void foo1(T1 t1, T2 t2) {
   std::cout << "Not specialised\n";
}

template <typename T1>
void foo1(T1 t1, B t2) {
   std::cout << "first parameter varies, second parameter fixed\n";
}

void foo1(B t1, B t2) {
   std::cout << "both parameters fixed\n";
}

int main() {
   A a;
   B b;
   foo1(a, a);
   foo1(a, b);
   foo1(b, b);
}

[dean]

ok, thanks everybody for the replies.

Kind regards,
Dean