[Bug c++/28017] New: lack of guard variables for explicitly instantiated template static data

[hhinnant at apple dot com]

I was passed a test case consisting of two translation units:

// test.h

#include <iostream>

class A
{
public:

    A()
    {
        mString = new char[2];
        std::cout << "new: mString has value " << (void*) mString << std::endl;
    }

    ~A()
    {
        std::cout << "delete: mString has value " << (void*) mString <<
std::endl;
        delete [] mString;
    }

    void f()
    {
    }

private:

    char* mString;
};


template<typename T>
class Test
{
public:

    Test()
    {
        sA.f();
    }

private:
    static A sA;
};

template<typename T>
A Test<T>::sA;

// test.cpp

#include "test.h"

template class Test<int>;

int foo()
{
    return 0;
}

// main.cpp

#include "test.h"

int foo();

int main()
{
    Test<int> theTest;
    foo();
    return 0;
}

When compiled with something like:

g++ -o test test.cpp main.cpp

the test shows evidence of the static Test<int>::sA being
constructed/destructed twice:

new: mString has value 0x5002d0
new: mString has value 0x500300
delete: mString has value 0x500300
delete: mString has value 0x500300

The desired output should be more like:

new: mString has value 0x5002d0
delete: mString has value 0x5002d0

Further inspection reveals that the guard variable for this static is not being
generated in test.cpp because of the explicit instantiation there (tested on
Apple's gcc 4.0.1).

I am wondering if an appropriate fix might be in:

gcc/cp/decl2.c, in function start_static_initialization_or_destruction

Change the if statement that looks like:

  if (TREE_PUBLIC (decl) && (DECL_COMMON (decl)
                             || DECL_ONE_ONLY (decl)
                             || DECL_WEAK (decl)))
    {
      tree guard_cond;

to:

  if (TREE_PUBLIC (decl) && (DECL_COMMON (decl)
                             || DECL_ONE_ONLY (decl)
                             || DECL_WEAK (decl)
                             || DECL_EXPLICIT_INSTANTIATION (decl)))
    {
      tree guard_cond;

I looked in the mainline decl2.c and found the same logic in the macro
NEEDS_GUARD_P.  Here I'm suggesting changing:

#define NEEDS_GUARD_P(decl) (TREE_PUBLIC (decl) && (DECL_COMMON (decl)      \
                                                    || DECL_ONE_ONLY (decl) \
                                                    || DECL_WEAK (decl)))

to:

#define NEEDS_GUARD_P(decl) (TREE_PUBLIC (decl) && (DECL_COMMON (decl)      \
                                                    || DECL_ONE_ONLY (decl) \
                                                    || DECL_WEAK (decl) \
                                                    ||
DECL_EXPLICIT_INSTANTIATION (decl)))


-- 
           Summary: lack of guard variables for explicitly instantiated
                    template static data
           Product: gcc
           Version: 4.0.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: hhinnant at apple dot com
  GCC host triplet: darwin ppc


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=28017