template instantiation and anonymous namespaces

[Stefan Lampe]

Hi,

Here's some code that produced a surprising result with GCC 4.3.3 on linux 64. 
I'd have expected all addresses output to be the same.

I guess the reason for this is that when merging the templates instantiated in 
one.cpp and two.cpp, the linker doesn't perform a complete merge, but only 
merges for those methods that were instantiated in each translation unit. When 
using the zero argument constructor one type is used, when using the one 
argument constructor, another type is used.

As I understand it, the anonymous namespace has internal linkage, so Dummy 
should be considered a different type in each unit, and the template 
instantiations shouldn't really be merged. Under -O3 the addresses output are 
the same. Whether this is due to separate templates or a single merged 
template, I'm not sure.

I really just wanted to check that my understanding is correct. For the program 
I was working on I use named namespaces to provide the right semantics.

Thanks for your time,
Stefan

-- nonsingleton.hpp
 

#ifndef NON_SINGLETON
#define NON_SINGLETON

#include 

namespace {

class Dummy {};

}

template 
class Singleton {
    static ObjectT* object;
public:
    static ObjectT& Instance() {
        if (!object)
            object = new ObjectT();
        return *object; } };

template 
ObjectT* Singleton::object = 0;

class NonSingleton {
    typedef Singleton<> SingletonType;
public:
    NonSingleton() {
        std::cout << "0 arg constructor. Singleton: " << 
(&SingletonType::Instance()) << std::endl; }
    NonSingleton(int i) {
        std::cout << "1 arg constructor. Singleton: " << 
(&SingletonType::Instance()) << std::endl; }
    ~NonSingleton() {
        std::cout << "Destructor. Singleton: " << (&SingletonType::Instance()) 
<< std::endl; } };

#endif

-- one.cpp

#include "nonsingleton.hpp"

void f() {
    NonSingleton a; }

-- two.cpp

#include "nonsingleton.hpp"

int main() {
    NonSingleton a;
    NonSingleton b(1);
    return 0; }

-- output

0 arg constructor. Singleton: 0x502010
1 arg constructor. Singleton: 0x502030
Destructor. Singleton: 0x502010
Destructor. Singleton: 0x502010