WEBVTT 00:00.090 --> 00:00.630 Hello again! 00:00.930 --> 00:04.410 In this video, we are going to look at the so-called "pImpl" idiom. 00:05.880 --> 00:11.120 There are various ways you can implements the Handle-Body pattern: the pImpl idiom is a very 00:11.130 --> 00:12.060 popular way of doing it. 00:12.690 --> 00:19.110 The handle class, which has the interface, has a private member, which is a pointer to a body object, 00:19.110 --> 00:20.760 which has the actual implementation. 00:21.540 --> 00:24.870 So that is a "pointer to implementation" or "pImpl". 00:25.320 --> 00:29.760 And pronouncing it as "pimple" is easier and slightly more amusing. 00:30.810 --> 00:38.340 It's also known as a compiler firewall, because it introduces a rigid separation between implementation 00:38.340 --> 00:39.780 code and client code. 00:40.230 --> 00:45.000 So that reduces the amount of recompilation that is needed if the implementation gets changed. 00:51.470 --> 00:52.640 In the Handle class, 00:52.640 --> 00:54.590 there is a pointer to the Body. 00:55.340 --> 01:00.710 We do not need the full definition of the Body class for this, just a forward declaration. 01:01.070 --> 01:03.170 So the compiler knows that Body is a class. 01:05.320 --> 01:09.680 So this means we do not need to include the Body header in this Handle 01:09.700 --> 01:10.030 header. 01:13.050 --> 01:16.770 The interface is just going to forward to the Body interface. 01:17.160 --> 01:22.680 So when we call a member function here, this will call the corresponding member function on the Body. 01:25.840 --> 01:29.110 The Handle class is implemented using RAII. 01:29.680 --> 01:35.650 The constructor allocates the Body object and the destructor releases it. And we may need to think about 01:35.650 --> 01:38.800 what happens if objects of this class are copied or moved. 01:41.040 --> 01:45.240 So I have written a very simple example, this is a very basic Date class. 01:45.870 --> 01:47.100 This is going to be the Body. 01:47.370 --> 01:49.740 So it is going to be the implementation of the Date. 01:51.770 --> 01:54.740 The Handle class will be the interface to Date. 01:55.160 --> 01:58.370 So this is the Date class, as seen by the rest of the code. 01:59.240 --> 02:01.430 It has our pointer to the implementation. 02:02.180 --> 02:07.070 We have the forward declaration of the implementation class, but no other information about it. 02:09.280 --> 02:15.520 In the constructor, we call new, to allocate the implementation pointer, and then we call delete in the 02:15.530 --> 02:18.640 destructor. And then the member functions 02:18.850 --> 02:23.350 just call the corresponding member functions in the Date implementation. 02:25.550 --> 02:29.180 And finally, we have a very simple main() program, to test this out. 02:30.770 --> 02:31.310 And there we are. 02:32.060 --> 02:37.370 So if the client sees this Date_impl in the header and tries to use it, what do you think would 02:37.370 --> 02:37.670 happen? 02:40.810 --> 02:45.910 Undefined class! So the client only knows the name, but it cannot compile against it, because it does not have 02:45.910 --> 02:47.560 the full class definition. 02:48.040 --> 02:50.890 So we have a pointer, and a call to new. 02:51.400 --> 02:53.140 Could we use a unique_ptr here? 02:54.130 --> 02:55.480 And yes, we can. 02:55.840 --> 02:58.320 But there are a couple of things we need to bear in mind. 03:00.700 --> 03:05.590 So we can put a unique_ptr to Body, instead of having a traditional pointer to a Body. 03:06.070 --> 03:08.770 And this will manage the memory allocation for us. 03:09.760 --> 03:12.640 However, this code is not going to compile as it stands. 03:13.240 --> 03:19.180 We need to add a destructor to the Handle class and that will make it a so-called "complete type". 03:20.890 --> 03:25.690 Then if we do have a constructor, that means the compiler is not going to synthesize move operators. 03:26.290 --> 03:28.870 So if we want those, we need to provide them ourselves. 03:29.620 --> 03:34.570 And also, we do not get copy operators, because the class has a member which cannot be copied. 03:35.110 --> 03:36.520 unique_ptr is move-only. 03:40.360 --> 03:47.410 So let's try that out. So we can use the same Date implementation and also the same client. In the Date 03:47.410 --> 03:47.650 header, 03:47.650 --> 03:50.440 we now have a unique_ptr to the implementation. 03:51.430 --> 03:55.810 We also have the destructor. And we have added some move operators. 03:58.490 --> 04:04.340 In the constructor, we call make_unique() to initialize the implementation pointer. And the arguments 04:04.340 --> 04:07.760 for the constructor are going to be forwarded to the implementation. 04:09.030 --> 04:14.040 And then for the destructor and move operators, we can just default those, unless we want to do something 04:14.040 --> 04:14.490 special. 04:15.150 --> 04:17.160 And then we have the same member functions again. 04:19.680 --> 04:20.190 And then we are. 04:25.240 --> 04:27.490 We have already looked at the advantages of this. 04:27.490 --> 04:30.750 We do not need to have the clients including Body.h. 04:31.630 --> 04:35.590 If the implementation changes, we do not need to modify or recompile the clients. 04:36.100 --> 04:42.700 And if we are shipping the implementation as a DLL, then users do not need to install a new program 04:42.700 --> 04:43.330 executable. 04:44.560 --> 04:47.950 It is also useful for hiding the implementation from clients. 04:48.640 --> 04:53.680 So if you have some code which is super secret or commercially sensitive, then no one will be able 04:53.680 --> 04:54.910 to find out what it is. 04:57.120 --> 04:58.200 The disadvantages. 04:58.200 --> 05:01.230 This requires an extra memory allocation for the Body object. 05:01.650 --> 05:04.110 So your program is going to use slightly more memory. 05:05.040 --> 05:10.020 If you call a member function, then it is going to dereference a pointer 05:10.170 --> 05:15.600 to call the Body member function. So that will make your program run slightly more slowly. 05:16.620 --> 05:21.930 Loading a shared library will increase the time that it takes your program to start up. And this approach 05:21.930 --> 05:23.760 makes the codebase more complex. 05:25.510 --> 05:27.310 So where can you use pImpl? 05:27.730 --> 05:35.440 pImpl is often used in large projects, to reduce completion time and simplify product updates. It is also 05:35.470 --> 05:37.810 used a lot by QT, or "cute", 05:37.840 --> 05:41.350 I think it is pronounced. Which is a popular graphics library. 05:45.090 --> 05:50.220 Another possibility is that you could use pImpl to implement the container classes in the standard 05:50.550 --> 05:53.640 template library. For example, with the string class. 05:54.000 --> 06:00.270 This could have a body which stores the character data, and does the actual manipulation of the data. 06:00.840 --> 06:06.360 And then the Handle class, which provides the interface of the string class to the rest of the code. 06:07.500 --> 06:12.930 And if you are very careful, you could arrange for two string objects which share the same data to 06:12.930 --> 06:14.330 share the same Body pointer. 06:14.910 --> 06:18.450 So you could use that for reducing the amount of memory that your program consumes, 06:19.020 --> 06:23.910 if you have a lot of duplicated string objects. Obviously you have to be careful with that, especially 06:23.910 --> 06:24.840 if threads are involved. 06:25.380 --> 06:27.090 But that is something to think about. 06:28.460 --> 06:28.760 Okay. 06:28.760 --> 06:29.840 So that is it for this video. 06:30.290 --> 06:31.110 I will see you next time. 06:31.110 --> 06:33.440 But until then, keep coding!