I've been trying to implement the Service Locator pattern using a templated base class and inheriting from it:
// Header File
namespace one {
template <class I, class N>
class Locator {
public:
static void initialize() {
service = &nullService;
}
static void provide(I* newService) {
if (newService == 0) {
initialize();
} else {
service = newService;
}
static I& get() { return *service; }
virtual ~Locator() {
if (service != &nullService) {
delete service;
}
private:
Locator();
static I* service;
static N nullService;
};
}
// Source File
#include "Locator.hpp"
namespace one {
template<class I, class N>
I* Locator<I, N>::service;
template<class I, class N>
N Locator<I, N>::nullService;
}
class I is the instance class, and class N is the null service class. And for the derived class:
// Header File
namespace two {
class ExampleLocator : public one::Locator<Service, NullService> {}
}
// Source File
namespace one {
template class Locator<Service, NullService>;
}
When I try and use this implementation,
int main(int argc, char const *argv[]) {
two::ExampleLocator::initialize();
two::ExampleLocator::get().doSomething();
}
gcc fails compiling with the following errors:
- undefined reference to one::Locator::initialize()
- in function one::Locator::get(): undefined reference to one::Locator::service
What am I doing wrong?
NOTE: I could get the project to compile by re-declaring Locator's static properties in the Derived class's cpp:
// Alternative Derived class Source file
namespace one {
template<class I, class N>
I* Locator<I, N>::service;
template<class I, class N>
N Locator<I, N>::nullService;
template class Locator<Service, NullService>;
}
NOTE2: the Derived class is in a different namespace than the Locator class.
