[LLVMbugs] [Bug 20825] "extern template" seems to invoke explicit instanciation but it should prohibit it.

bugzilla-daemon at llvm.org bugzilla-daemon at llvm.org
Tue Sep 2 13:17:35 PDT 2014


Reid Kleckner <rnk at google.com> changed:

           What    |Removed                     |Added
             Status|NEW                         |RESOLVED
                 CC|                            |rnk at google.com
         Resolution|---                         |INVALID

--- Comment #1 from Reid Kleckner <rnk at google.com> ---
I think you can forward declare your specialization of the static data member
like so:

// Start .h
template <typename T> class A {
  static const char *info;

template <> const char *A<int>::info;
template <> const char *A<char>::info;

extern template class A<int>;
extern template class A<char>;

// Start .cpp
#include <iostream>

template <> const char *A<int>::info = "This is int";
template <> const char *A<char>::info = "This is char";

int main(int, char **) {
  std::cout << "A<int>:" << A<int>::info << "\n";
  std::cout << "A<char>:" << A<char>::info << "\n";
  return 0;

template class A<int>;
template class A<char>;


Otherwise, the compiler can do things like inline the generic, unspecialized
version of an inline function into your program.  Consider:

template <typename T>
struct A { static int f() { return sizeof(int); } };
extern template struct A<int>;
int main() {
  return A<int>::f();

After optimizations, main will return 4, even if some other TU contains:

template <> int A<int>::f() { return 5; };

This specialization can be similarly forward declared as:

template <> int A<int>::f();

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