Default template parameters

Question

template<class T1, class T2 = int> class A;
template<class T1 = int, class T2> class A;

which is equal to this?

 1)    template<class T1 = int, class T2 = int> class A;   

 2)    template<class T1, class T2> class A;

I know 1st option is correct. according to the standards But ,for me in various compilers 1st option is not accepting giving

  error: redefinition of default argument //for 1st case

and 2nd option is accepting

If any one clarify my doubt..by checking this..Whether i made mistake or else standard is correct...

Answer 1

The second line you have is invalid, you can't have a template parameter with a default value followed by one without a default value (same as regular parameters).

Comeau online gives this error:

"ComeauTest.c", line 1: error: default argument not at end of parameter list
template <class T = int, class Y>

---

Taking into consideration UncleBens' comment I see that the first two lines are equivalent to option 1, but you can't repeat it and must use option 2 in order to define the class.

As a style consideration I don't see why either options is needed, I would go with option 1 without the first two lines.

The following compiles and runs fine (in GCC)

#include <iostream>
template <class T, class Y=int> class A;
template <class T=int, class Y> class A;

int main() { 
    A<>* pa = 0;
    std::cout << pa;
}

Answer 2

template<class T1, class T2 = int> class A;
template<class T1 = int, class T2> class A;

template <class T1, class T2> A {};

A<> var; // == A<int, int> var

The effect of the two declarations is that both arguments have int for default, but the language doesn't allow you to redefine any of the arguments, to avoid

template <class T = int> class A;
template <class T = char> class A;
//etc

Option 1 breaks this rule and the compilers that reject 1) are correct.