Ben Deane's global dependency injection pattern

bean_deane_global_api_injection.cpp

// Ben Deane's global dependency injection pattern.
// https://www.youtube.com/watch?v=BYpcAEfG3mo

#include <utility>

//
// Header providing service and its default implementation.
//

struct default_logger {
    template<class ...Args>
    void log(Args&&...);
};

// Template so it can be overriden by specialization.
template<class...>
inline auto injectable_logger = default_logger();

// API
template<class ...DummyArgs, class ...Args>
void log(Args&&... args) {
    // DummArgs are just there to allow specialization.
    static_assert(sizeof...(DummyArgs) == 0);

    auto interface = injectable_logger<DummyArgs...>;
    interface.log(std::forward<Args>(args)...);
}

//
// Header providing alternative implementation
//

struct test_logger {
    template<class ...Args>
    void log(Args&&...);
};

// If the following line is commented out, main() will call the
// default implementation.
template<> inline auto injectable_logger<> = test_logger();

//
// Client
// 

int main() {
    log(1, "2", 3);
}