Empty Base Class Optimisation, [[no_unique_address]] and other C++20 Attributes
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The document discusses optimization techniques in C++20, including empty base class optimization and the new [[no_unique_address]] attribute. It explains how these features contribute to improving memory efficiency and simplifies code. The author also highlights the evolution of attribute syntax since C++11 and introduces additional attributes in C++20.
![EMPTY BASE CLASS OPTIMISATION,
[[NO_UNIQUE_ADDRESS]]
And Other C++20 Attributes…
Bartłomiej Filipek, bfilipek.com21st July 2020](https://image.slidesharecdn.com/emptybaseattributespublic-200727061030/85/Empty-Base-Class-Optimisation-no_unique_address-and-other-C-20-Attributes-1-320.jpg)
![The plan
Unique_ptr and a custom deleter
Digging into the STL implementation
Empty Base Class Optimisation
[[no_unique_address]]
Other C++20 Attributes and Features](https://image.slidesharecdn.com/emptybaseattributespublic-200727061030/85/Empty-Base-Class-Optimisation-no_unique_address-and-other-C-20-Attributes-3-320.jpg)
![[[no_unique_address]]
Let’s rewrite the compressed pair into something really simple thanks
to the new attribute from C++20
http://coliru.stacked-crooked.com/a/7ccae3a3168e73b9](https://image.slidesharecdn.com/emptybaseattributespublic-200727061030/85/Empty-Base-Class-Optimisation-no_unique_address-and-other-C-20-Attributes-7-320.jpg)
![Attributes in C++17
[[noreturn]] indicates that the function does not return
[[carries_dependency]]
indicates that dependency chain in release-consume
std::memory_order propagates in and out of the function
[[deprecated]]
[[deprecated("reason")]]
indicates that the use of the name or entity declared with this
attribute is allowed, but discouraged for some reason
[[fallthrough]]
indicates that the fall through from the previous case label is
intentional and should not be diagnosed by a compiler that warns
on fall-through
[[nodiscard]]
encourages the compiler to issue a warning if the return value is
discarded
[[maybe_unused]] suppresses compiler warnings on unused entities, if any
https://www.bfilipek.com/2017/07/cpp17-in-details-attributes.html
https://www.bfilipek.com/2017/11/nodiscard.html](https://image.slidesharecdn.com/emptybaseattributespublic-200727061030/85/Empty-Base-Class-Optimisation-no_unique_address-and-other-C-20-Attributes-8-320.jpg)
![Attributes in C++20
[[nodiscard("reason")]]
encourages the compiler to issue a warning if the
return value is discarded
[[likely]]
[[unlikely]]
indicates that the compiler should optimize for the
case where a path of execution through a statement
is more or less likely than any other path of execution
[[no_unique_address]]
indicates that a non-static data member need not
have an address distinct from all other non-static
data members of its class
• Apply [[nodiscard]] to the standard library - P0600
• [[nodiscard]] for constructors - P1771](https://image.slidesharecdn.com/emptybaseattributespublic-200727061030/85/Empty-Base-Class-Optimisation-no_unique_address-and-other-C-20-Attributes-9-320.jpg)
![Summary
Empty Base Class Optimisation relies on the fact that if you inherit
from a empty class then you don’t need more memory, but you get
access to member functions.
The technique is however a bit complicated
Thanks to the new C++20 attribute – [[no_unique_address]] the code can
be much simpler
Since C++11 the Standard has been taking vendor specific annotation
syntax into a common form of [[attrib_name]].
C++20 adds new attributes: [[no_unique_address]], [[likely]] and
[[unlikely]]](https://image.slidesharecdn.com/emptybaseattributespublic-200727061030/85/Empty-Base-Class-Optimisation-no_unique_address-and-other-C-20-Attributes-10-320.jpg)
Empty Base Class Optimisation, [[no_unique_address]] and other C++20 Attributes
- 1. EMPTY BASE CLASS OPTIMISATION, [[NO_UNIQUE_ADDRESS]] And Other C++20 Attributes… Bartłomiej Filipek, bfilipek.com21st July 2020
- 2. About me See my coding blog at: www.bfilipek.com Slowly moving to cppstories.com ~13y coding experience Microsoft MVP, since 2018 C++ ISO Member @Xara.com Mostly text related features for advanced document editors Somehow addicted to C++ ☺ Xara Cloud DemoC++17 In Detail
- 3. The plan Unique_ptr and a custom deleter Digging into the STL implementation Empty Base Class Optimisation [[no_unique_address]] Other C++20 Attributes and Features
- 4. Custom deleter for unique_ptr Background, the article from 2016: https://www.bfilipek.com/2016/04/custom-deleters-for-c-smart- pointers.html#custom-deleter-for-uniqueptr Why unique_ptr is just 8 bytes (one pointer) with default deleters? Or stateless deleters? The working example with a custom deleter: http://coliru.stacked-crooked.com/a/e2638649daffd406
- 5. Digging into the STL implementation Let’s try to understand the internal implementation of unique_ptr We can go to: https://github.com/microsoft/STL https://github.com/microsoft/STL/blob/master/stl/inc/memory#L2435 Unique_ptr seems to hold the pointer and the deleter in something called compressed pair It has two specialisation: the first one if both types are non empty and the second one if the first type is empty ◼ In that case it just declares one member field and derive from the empty type ◼ By inheritance we get access to all of the type member functions
- 6. Empty Base Class Optimisation When a class is empty it costs at least 1 byte on stack, but if you inherit from such a class then in costs you nothing https://en.cppreference.com/w/cpp/language/ebo See an example: http://coliru.stacked-crooked.com/a/affe60d81ac52163 Some example: https://github.com/microsoft/STL/blob/master/stl/inc/xmemory#L1319 GCC tuple: https://github.com/gcc-mirror/gcc/blob/master/libstdc%2B%2B- v3/include/bits/unique_ptr.h#L201
- 7. [[no_unique_address]] Let’s rewrite the compressed pair into something really simple thanks to the new attribute from C++20 http://coliru.stacked-crooked.com/a/7ccae3a3168e73b9
- 8. Attributes in C++17 [[noreturn]] indicates that the function does not return [[carries_dependency]] indicates that dependency chain in release-consume std::memory_order propagates in and out of the function [[deprecated]] [[deprecated("reason")]] indicates that the use of the name or entity declared with this attribute is allowed, but discouraged for some reason [[fallthrough]] indicates that the fall through from the previous case label is intentional and should not be diagnosed by a compiler that warns on fall-through [[nodiscard]] encourages the compiler to issue a warning if the return value is discarded [[maybe_unused]] suppresses compiler warnings on unused entities, if any https://www.bfilipek.com/2017/07/cpp17-in-details-attributes.html https://www.bfilipek.com/2017/11/nodiscard.html
- 9. Attributes in C++20 [[nodiscard("reason")]] encourages the compiler to issue a warning if the return value is discarded [[likely]] [[unlikely]] indicates that the compiler should optimize for the case where a path of execution through a statement is more or less likely than any other path of execution [[no_unique_address]] indicates that a non-static data member need not have an address distinct from all other non-static data members of its class • Apply [[nodiscard]] to the standard library - P0600 • [[nodiscard]] for constructors - P1771
- 10. Summary Empty Base Class Optimisation relies on the fact that if you inherit from a empty class then you don’t need more memory, but you get access to member functions. The technique is however a bit complicated Thanks to the new C++20 attribute – [[no_unique_address]] the code can be much simpler Since C++11 the Standard has been taking vendor specific annotation syntax into a common form of [[attrib_name]]. C++20 adds new attributes: [[no_unique_address]], [[likely]] and [[unlikely]]