Plain Old Data (POD) obeys the C standard layout. Therefore, you directly can apply the fast C functions memcopy, memmove, memset, or memcmp.
PODs are in classical C++ fundamental types like booleans, integers of floating-point number. The restriction will not hold for C++11. With C++11 even classes and structs can be PODs. For simplicity reasons I only speak about classes.
Which requirements hold for the C++11 class to be a POD? A class is a POD, if it's trivial, has a standard layout and all of its non-static members are PODs. The definition is quite concise. But what does it mean that class should be trivial and has a standard layout?
Now the standard reads like German legal text.
A class is trivial if it
- has a trivial default constructor.
- is trivial copyable.
A trivially copyable class is a class that
- has no non-trivial copy or move constructor.
- has no non-trivial copy or move assignment operator.
- has a trivial destructor.
Non-trivial means that the mentioned methods are implemented by the developer. If a method is requested from the compiler via the keyword default or automatically generated from the compiler, the method is trivial.
The definition of a POD goes on with the standard layout.
A class has a standard layout if it has no
- virtual functions.
- virtual base classes.
- different access specifier (public, protected, and private).
It's a lot easier to check with the help of the type-traits library if the class is POD.
Checking types with the type-traits library
std::cout << std::boolalpha << std::endl;
std::cout << "std::is_pod<Pod>::value: " << std::is_pod<Pod>::value << std::endl;
std::cout << "std::is_pod<NotPod>::value: " << std::is_pod<NotPod>::value << std::endl;
std::cout << std::endl;
std::cout << "std::is_trivial<NotPod>::value: " << std::is_trivial<NotPod>::value << std::endl;
std::cout << "std::is_standard_layout<NotPod>::value: " << std::is_standard_layout<NotPod>::value << std::endl;
std::cout << std::endl;
The class Pod in lines 6 - 8 is a POD, but not the class NotPod (lines 10 -15). We get the answer quite easy with the help of the function std::is_pod (line 21 - 22) from the type-traits library. But we can do even better with the type-traits library. I analyse in line 26 and 27 the class NotPod even more. The result is: NotPod is trivial, but has no standard layout. NotPod has no standard layout because the variable i is public. On the contrary the variable j is private.
The output of the program depicts the explanation.
This post finishes the series of posts about the features in C++ that are very important from the performance perspective. In the next post, I will continue my blog with posts about the careful handling of resources. Memory management has a high priority in the embedded development. Therefore, it fits very well that C++11 has the new smart pointers std::shared_ptr, std::unique_ptr, and std::weak_ptr and the manual memory management with new becomes almost unnecessary.
Thanks a lot to my Patreon Supporters: Matt Braun, Roman Postanciuc, Tobias Zindl, Marko, G Prvulovic, Reinhold Dröge, Abernitzke, Frank Grimm, Sakib, Broeserl, António Pina, Sergey Agafyin, Андрей Бурмистров, Jake, GS, Lawton Shoemake, Animus24, Jozo Leko, John Breland, espkk, Louis St-Amour, Venkat Nandam, Jose Francisco, Douglas Tinkham, Kuchlong Kuchlong, Robert Blanch, Truels Wissneth, Kris Kafka, Mario Luoni, Neil Wang, Friedrich Huber, lennonli, Pramod Tikare Muralidhara, Peter Ware, Tobi Heideman, Daniel Hufschläger, Red Trip, Alexander Schwarz, Tornike Porchxidze, Alessandro Pezzato, Evangelos Denaxas, Bob Perry, Satish Vangipuram, and Andi Ireland.
Thanks in particular to Jon Hess, Lakshman, Christian Wittenhorst, Sherhy Pyton, Dendi Suhubdy, Sudhakar Belagurusamy, Richard Sargeant, Rusty Fleming, and Said Mert Turkal.
My special thanks to Embarcadero
I'm happy to give online-seminars or face-to-face seminars world-wide. Please call me if you have any questions.
Standard Seminars (English/German)
Here is a compilation of my standard seminars. These seminars are only meant to give you a first orientation.
- Phone: +49 152 31965939
- German Seminar Page: www.ModernesCpp.de
- English Seminar Page: www.ModernesCpp.net