Re: C++ Templates book

From:

James Kanze <james.kanze@gmail.com>

Newsgroups:

comp.lang.c++

Date:

Tue, 14 Jul 2009 08:34:38 -0700 (PDT)

Message-ID:

<d45379d6-c9b2-47eb-a8c4-72b72a89fb87@z28g2000vbl.googlegroups.com>

On Jul 12, 9:09 pm, "Fraser Ross" <z...@zzzzzz.com> wrote:

struct C { C(int n) { printf("%d\n", n); } };

    template<int N>
    struct A {
      static C c;
    };

    template<int N>
    C A<N>::c(N);

    A<2> a2;
    A<1> a1;

    int main() {
      A<1>::c; A<2>::c;
    }

I copied this from the moderated group. Why is it necessary
to have the given code in the main function to get an output
from Cs constructor?

Because the compiler is only allowed to instantiate the parts of
a class template which are actually used. (On the other hand,
you don't need the definitions of a1 and a2.)

Is the compiler allowed to forego the creation of a1 and a2
since they are not required for the program?

It is required to forego them.

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