Injection Rules

The dependency injection mechanism relies heavily on template metaprogramming and it has some limitations.

General

• Only one constructor should be defined for each instance implementation

• If the service is registered with interface and implementation, the interface should have a virtual destructor

• If multiple services are registered by the same interface, all should have the same lifetime (the build method will throw an exception)

• Only one service implementation can be registered (the build method will throw an exception)

Injecting Services

• Singleton/scoped services can be injected using one of:

  • References: (T&)

  • Pointers: (T*)

• Transient services can be injected using one of:

  • std::unique_ptr: (unique_ptr<T>)

  • In-place object if type is movable or copyable: T

• Multiple services implementing the same interface can be injected using std::vector:

  • Transient (std::vector<std::unique_ptr<T>>)

  • Singleton/scoped (std::vector<T*>)

Injection table

Constructor param type	ServiceProvider method used
T - if movable or copyable	provider.createServiceInPlace<T>()
std::unique_ptr<T>	provider.createService<T>()
T&	provider.getService<T>()
T*	provider.tryGetService<T>()
std::vector<T*>	provider.getServices<T>()
std::vector<std::unique_ptr<T>>	provider.createServices<T>()

Examples/Guides/InjectionRules

#include <SevenBit/DI.hpp>
#include <cassert>
#include <memory>

using namespace sb::di;

struct NotReqisteredService
{
};
struct SingletonService
{
};
struct ScopedService
{
};
struct TransientService
{
};

class ServiceExecutor
{
  public:
    ServiceExecutor(NotReqisteredService *notRegistered, SingletonService &singleton, ScopedService *scoped,
                    std::unique_ptr<TransientService> transient, TransientService transientInPlace,
                    std::vector<ScopedService *> scopedList,
                    std::vector<std::unique_ptr<TransientService>> trasientList)
    {
        assert(notRegistered == nullptr);
        assert(scoped != nullptr);
        assert(transient != nullptr);
        assert(scopedList.size() == 1);
        assert(trasientList.size() == 1);
    }
};
int main()
{
    ServiceProvider provider = ServiceCollection{}
                                   .addSingleton<SingletonService>()
                                   .addScoped<ScopedService>()
                                   .addTransient<TransientService>()
                                   .addScoped<ServiceExecutor>()
                                   .buildServiceProvider();

    auto &consumer = provider.getService<ServiceExecutor>();

    return 0;
}