Implied template parameters.

[jason.cipriani]

In this code:

===

template <int N> class Base { };

template <int N> class Other {
public:
explicit Other (Base<N> &) { }
};

int main () {
Base<3> ok;
Other<3> fine(ok);
}

===

Maybe this is a dumb question, but why do I have to specify the
template parameter to "Other<3> fine(ok)"? Can't it deduce that it
should be a 3 on it's own (since a Base<3> is being passed to the
constructor, and Base<3> has no casting operators that could confuse
it, there's no other type it could be)? I want to just say "Other
fine(ok);".

Jason

 

[James Kanze]

In this code:

template <int N> class Base { };

template <int N> class Other {
public:
explicit Other (Base<N> &) { }
};

int main () {
Base<3> ok;
Other<3> fine(ok);
}

Maybe this is a dumb question, but why do I have to specify
the template parameter to "Other<3> fine(ok)"? Can't it deduce
that it should be a 3 on it's own (since a Base<3> is being
passed to the constructor, and Base<3> has no casting
operators that could confuse it, there's no other type it
could be)? I want to just say "Other fine(ok);".

The simple answer is: because the standard says so. Template
type deduction only works for function templates. Other is a
class template. Basically, in this case, the compiler must
first know the type of Other, in order to know where it should
look for the constructors. It can't do it in the reverse order.

 

[jason.cipriani]

The simple answer is: because the standard says so. Template
type deduction only works for function templates. Other is a
class template. Basically, in this case, the compiler must
first know the type of Other, in order to know where it should
look for the constructors. It can't do it in the reverse order.

That makes sense.

Thanks for clearing that up,
Jason