Question

I had been doing dependency injection using raw pointers and I decided to convert my code to use shared_ptr. This works but I’m wondering if I could use unique_ptr instead? In my example below, MyClass would manage the lifetime of the credit card service.

class PaymentProcessor
{
    PaymentProcessor(?? creditCardService):
      :creditCardService_(creditCardService)
      {

      }

private:
   CreditCardService *creditCardService_;     
}

class MyClass
{ 
public:
   void DoIt()
   {
     creditCardService_.reset(new VisaCardService());
     PaymentProcessor pp(creditCardService_);
     pp.ProcessPayment();
   }

private:   
   std::unique_ptr<CreditCardService> creditCardService_;
}

Can you pass a unique_ptr to another class where the other class is just “using” the pointer (without owning it??)? If so is this a good idea and what should the type of the parameter be in the constructor for PaymentProcessor?

UPDATE

In the example as shown above I can alternatively create a VisaCardService variable on the stack and have the PaymentProcessor constructor take this as a reference parameter. This seems to be the recommended C++ practice. However, in the case where the concrete type of creditCardService_ is not known until runtime (e.g., the user chooses the particular Credit Card Service to use at runtime), is using std::unique_ptr with references the best solution?

Answer 1

Can you pass a unique_ptr to another class where the other class is
just “using” the pointer (without owning it??)?

In that case, change the pointer to reference :

class PaymentProcessor
{
public:
    PaymentProcessor(CreditCardService & creditCardService_):
      :creditCardService_(creditCardService_)
      {
      }

private:
   CreditCardService &creditCardService_;     
};

   void DoIt()
   {
     creditCardService_.reset(new VisaCardService());
     PaymentProcessor pp(*creditCardService_);
     pp.ProcessPayment();
   }

If you still want to use a pointer, then you need to use get method :

class PaymentProcessor
{
public:
    PaymentProcessor(CreditCardService * creditCardService_):
      :creditCardService_(creditCardService_)
      {
      }
private:
   CreditCardService *creditCardService_;     
};

   void DoIt()
   {
     creditCardService_.reset(new VisaCardService());
     PaymentProcessor pp(creditCardService_.get());
     pp.ProcessPayment();
   }