Question

Is this portable or at least safe to use with g++?

#include <iostream>
#include <vector>

struct c {};
std::vector<c*> v;
struct i : c { i () { v.push_back (this); } } a, b, c;

int main () {
  std::cout << v.size () << "\n"; // outputs 3 with g++
}

EDIT:

Ok, what I need turned out to be a bit harder: The same code with templates:

#include <iostream>
#include <vector>

template < typename T > struct c {};
template < typename T > struct cv { static std::vector<c<T>*> v; };
template < typename T > std::vector<c<T>*> cv<T>::v;
template < typename T > struct i : c<T> { i () { cv<T>::v.push_back (this); } };

cv<int> dummy; // even this won't initialize cv<int>::v
i<int> a, b, d;

int main () {
  std::cout << cv<int>::v.size () << "\n"; // outputs 0 :-(
}

How could I fix this to work as above?

EDIT 2:

Here is an ugly fix with macros (I hope there is a better way to do it):

#include <iostream>
#include <vector>

template < typename T > struct c {};
template < typename T > struct cv;
#define INITCV(X) \
  struct v##X { static std::vector<c<X>*> v; }; \
  std::vector<c<X>*> v##X::v; \
  template <> struct cv<X> { typedef v##X V; }
template < typename T > struct i : c<T> { i () { cv<T>::V::v.push_back (this); } };

INITCV(int);
i<int> a, b, d;

int main () {
  std::cout << cv<int>::V::v.size () << "\n"; // outputs 3 again :-)
}

(BTW, should I have posted a new question instead of the edits?)

Answer 1

For your updated question, I haven’t waded through the standard to find out when members of implicitly-instantiated templates are supposed to be initialized, but explicit instantiation does seem to be the solution:

template class cv<int>; // Not a dummy. Declares the template like a class.

Standardese at 14.7.2/7:

The explicit instantiation of a class
template specialization implies the
instantiation of all of its members
not previously explicitly specialized
in the translation unit containing the
explicit instantiation.