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By Blog Staff | Feb 3, 2025 02:58 PM | Tags: None
In this episode, you learn about a change in the standard that can give you more performance, thanks to the implicit move.
C++ Insights Episode 57: More performance thanks to more implicit moves in C++20 - Andreas Fertig
By Blog Staff | Feb 1, 2025 02:49 PM | Tags: None
The C++ proposal for indirect and polymorphic introduces two new class templates designed to simplify working with dynamically allocated types while retaining value semantics. This post dives into a curious case with polymorphic, exploring why std::default_initializable<polymorphic<T>> evaluates to true—even when polymorphic<T> can't actually be default-initialized in practice.
When a default-initializable type actually isn't
by Kaashif Hymabaccus
From the article:
I was looking at a proposal for adding value-semantic dynamically-allocated types to C++. You can find it here. This proposal adds two class templates,
indirectandpolymorphic.The primary use case for this is to make it easier to write correct composite classes. A motivating example can be found through existential types.
Suppose you want a type T which owns (ownership is key) some objects implementing some interfaces. You don't know what concrete types the sub-objects have, and you don't care. You're fine with dynamic dispatch, with the overhead that entails at runtime. You want value semantics, meaning (among other things) copying T copies the sub-objects too.
In Rust, this is expressible through
Box<dyn T>, which lets us own objects and perform dynamic dispatch with value semantics. This is whatpolymorphicwill allow in C++, allowing you to avoid hacking aroundunique_ptr. See the proposal above for more detailed motivation.
By Blog Staff | Feb 1, 2025 01:10 PM | Tags: None
Clang provides a few options to generate timing report. Among them, -ftime-report and -ftime-trace can be used to analyze the performance of Clang's internal passes. This post focuses on the traditional -ftime-report, which uses a line-based textual format.
Understanding and Improving Clang -ftime-report
by Fangrui Song
From the article:
The output consists of information about multiple timer groups. The last group spans the largest interval and encompasses timing data from other groups.
Up to Clang 19, the last group is called "Clang front-end time report". You would see something like the following.
The "Clang front-end timer" timer measured the time spent in clang::FrontendAction::Execute, which includes lexing, parsing, semantic analysis, LLVM IR generation, optimization, and machine code generation. However, "Code Generation Time" and "LLVM IR Generation Time" belonged to the default timer group "Miscellaneous Ungrouped Timers". This caused confusion for many users. For example, https://aras-p.info/blog/2019/01/12/Investigating-compile-times-and-Clang-ftime-report/ elaborates on the issues.
By Blog Staff | Jan 29, 2025 11:29 AM | Tags: None
C++26 will introduce a new concurrency feature called std::execution, or senders/receivers. Lucian Radu Teodorescu explains the idea and how to use these in detail.
Senders/Receivers: An Introduction
by Lucian Radu Teodorescu
From the article:
In June 2024, at the WG21 plenary held in St. Louis, the P2300R10:
std::executionpaper [P2300R10], also known as senders/receivers, was formally adopted for inclusion in C++ 26. The content of the paper quickly found its way into the working draft for the C++ standard [WG21]. You can find more about the highlights of the St. Louis meeting in Herb Sutter’s trip report [Sutter24].Senders/receivers represent one of the major additions to C++, as they provide an underlying model for expressing computations, adding support for concurrency, parallelism, and asynchrony. By using senders/receivers, one can write programs that heavily and efficiently exploit concurrency, all while maintaining thread safety (no deadlocks, race conditions, etc.). This is applicable not only to a few classes of concurrent problems but, at least in theory, to all types of concurrency problems. Senders/receivers provide a cost-free way of expressing computations that can run on different hardware with different constraints. They support creating computation chains that execute work on the CPU, GPU, and also enable non-blocking I/O.
Although the proposal has many advantages, there are still people who see the addition of this feature to the C++ standard at this point as a mistake. Some of the cited reasons are the complexity of the feature, compilation times, immaturity, and teachability. The last one caught my attention.
In this article, I plan to provide an introduction to senders/receivers as described in P2300 (and some related papers). The goal is not necessarily to showcase the many advantages of this model or delve into the details of complex topics. Rather, it is to offer a gentle introduction for those who have never read the paper or watched a talk on senders/receivers. We want the reader to understand the basic concepts of using senders/receivers without needing to grasp the intricate details of their implementation.
The hope is that, by the end of the article, the reader will be able to write some programs that use senders/receivers. The examples here are written as if the reader is coding with the feature already included in the standard library. Currently, no standard library provider ships senders/receivers; however, the reader can use the reference implementation of the feature [stdexec].
By Ansel Sermersheim | Jan 26, 2025 11:48 AM | Tags: None
New video on the CopperSpice YouTube Channel:
C++20 and Two's Complement
by Barbara Geller and Ansel Sermersheim
About the video:
Someone mentioned C++20 is unusable because the standard now requires 2' Complement. Is is possible that he has a point and if so, on what basis did he make this claim?
We encourage everyone to watch our video to discover the history of Binary Representation.
Take a look and remember to subscribe.
By Blog Staff | Jan 24, 2025 11:26 AM | Tags: None
Static reflection is under consideration for C++26. Wu Yongwei demonstrates how to achieve reflection now and shows some examples of what C++26 might make possible.
Static reflection will be an important part of C++ compile-time programming, as I discussed in the October issue of Overload [Wu24]. This time I will discuss static reflection in detail, including how to emulate it right now, before it’s been added to the standard.
Static Reflection in C++
by Wu Yongwei
From the article:
Background
Many programming languages support reflection (Python and Java, for example). C++ is lagging behind.
While this is the case, things are probably going to change in C++26. Also, what will be available in C++ will be very different from what is available in languages like Java or Python. The keyword is ‘static’.
Andrew Sutton defined ‘static reflection’ as follows [Sutton21]:
Static reflection is the integral ability for a metaprogram to observe its own code and, to a limited extent, generate new code at compile time.
‘Compile-time’ is the special sauce in C++, and it allows us to do things impossible in other languages:
- Zero-overhead abstraction. As Bjarne Stroustrup famously put it, ‘What you don’t use, you don’t pay for. What you do use, you couldn’t hand-code any better.’ If you do not need static reflection, it will not make your program fatter or slower. But it will be at your hand when you do need it.
- High performance. Due to the nature of compile-time reflection, it is possible to achieve unparalleled performance, when compared with languages like Java or Python.
- Versatility at both compile time and run time. The information available at compile time can be used at run time, but not vice versa. C++ static reflection can do things that are possible in languages like Java, but there are things that C++ can do but are simply impossible in other languages.
What we want from reflection
When we talk about static reflection, what do we really want? We really want to see what a compiler can see, and we want to be able to use the relevant information in the code. The most prominent cases are
enumandstruct. We want to be able to iterate over all the enumerators, and know their names and values. We want to be able to iterate over all the data members of a struct, and know their names and types. Obviously, when a data member is an aggregate, we also want to be able to recurse into it during reflection. And so on.Regretfully, we cannot do all these things today with ‘standard’ definitions. Yes, in some implementations it is possible to hack out some of the information with various tricks. I would prefer to use macros and template techniques to achieve the same purpose, as the code is somewhat neater, more portable, and more maintainable – at the cost of using non-standard definition syntaxes. Of course, nothing beats direct support from the future C++ standard.
By Blog Staff | Jan 21, 2025 11:21 AM | Tags: None
The trick to understanding C++ compiler error messages is to focus on two things. First, look at the beginning of the error message, which tells you what went wrong at a very low level. Then skip over the intermediate errors that follow the chain of calls until you end up at the line of code that you wrote. That original line of code is the one that is leading the compiler to a bad place. After that, you sometimes get supplemental information that helps you understand the low-level error better.
Learning to read C++ Compiler Errors: Failing to Create a shared_ptr
by Raymond Chen
From the article:
Consider the following erroneous code:
#include <memory> #include <string> struct WidgetOptions { // imagine there is interesting stuff here }; struct Widget { Widget(WidgetOptions const* options); // imagine there is other interesting stuff here }; void oops() { WidgetOptions options; // The next line fails to compile std::shared_ptr<Widget> widget = std::make_shared<Widget>(options); }Here comes the error explosion.
// gcc In file included from bits/stl_tempbuf.h:61, from memory:66, from sample.cpp:1: bits/stl_construct.h: In instantiation of 'void std::_Construct(_Tp*, _Args&& ...) [with _Tp = Widget; _Args = {WidgetOptions&}]': bits/alloc_traits.h:657:19: required from 'static void std::allocator_traits<std::allocator<void> >::construct(allocator_type&, _Up*, _Args&& ...) [with _Up = Widget; _Args = {WidgetOptions&}; allocator_type = std::allocator<void>]' 657 | { std::_Construct(__p, std::forward<_Args>(__args)...); } | ~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ bits/shared_ptr_base.h:607:39: required from 'std::_Sp_counted_ptr_inplace<_Tp, _Alloc, _Lp>::_Sp_counted_ptr_inplace(_Alloc, _Args&& ...) [with _Args = {WidgetOptions&}; _Tp = Widget; _Alloc = std::allocator<void>; __gnu_cxx::_Lock_policy _Lp = __gnu_cxx::_S_atomic]' 607 | allocator_traits<_Alloc>::construct(__a, _M_ptr(), | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~ 608 | std::forward<_Args>(__args)...); // might throw | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ bits/shared_ptr_base.h:969:16: required from 'std::__shared_count<_Lp>::__shared_count(_Tp*&, std::_Sp_alloc_shared_tag<_Alloc>, _Args&& ...) [with _Tp = Widget; _Alloc = std::allocator<void>; _Args = {WidgetOptions&}; __gnu_cxx::_Lock_policy _Lp = __gnu_cxx::_S_atomic]' 969 | auto __pi = ::new (__mem) | ^~~~~~~~~~~~~ 970 | _Sp_cp_type(__a._M_a, std::forward<_Args>(__args)...); | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ bits/shared_ptr_base.h:1713:14: required from 'std::__shared_ptr<_Tp, _Lp>::__shared_ptr(std::_Sp_alloc_shared_tag<_Tp>, _Args&& ...) [with _Alloc = std::allocator<void>; _Args = {WidgetOptions&}; _Tp = Widget; __gnu_cxx::_Lock_policy _Lp = __gnu_cxx::_S_atomic]' 1713 | : _M_ptr(), _M_refcount(_M_ptr, __tag, std::forward<_Args>(__args)...) | ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ bits/shared_ptr.h:463:59: required from 'std::shared_ptr<_Tp>::shared_ptr(std::_Sp_alloc_shared_tag<_Tp>, _Args&& ...) [with _Alloc = std::allocator<void>; _Args = {WidgetOptions&}; _Tp = Widget]' 463 | : __shared_ptr<_Tp>(__tag, std::forward<_Args>(__args)...) | ^ bits/shared_ptr.h:1007:14: required from 'std::shared_ptr<typename std::enable_if<(! std::is_array<_Tp>::value), _Tp>::type> std::make_shared(_Args&& ...) [with _Tp = Widget; _Args = {WidgetOptions&}; typename enable_if<(! is_array<_Tp>::value), _Tp>::type = Widget]' 1007 | return shared_ptr<_Tp>(_Sp_alloc_shared_tag<_Alloc>{__a}, | ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 1008 | std::forward<_Args>(__args)...); | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ sample.cpp:22:33: required from here 22 | std::make_shared<Widget>(options); | ~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~ bits/stl_construct.h:119:7: error: no matching function for call to 'Widget::Widget(WidgetOptions&)' 119 | ::new((void*)__p) _Tp(std::forward<_Args>(__args)...); | ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ sample.cpp:11:5: note: candidate: 'Widget::Widget(const WidgetOptions*)' 11 | Widget(WidgetOptions const* options); | ^~~~~~ sample.cpp:11:33: note: no known conversion for argument 1 from 'WidgetOptions' to 'const WidgetOptions*' 11 | Widget(WidgetOptions const* options); | ~~~~~~~~~~~~~~~~~~~~~^~~~~~~ sample.cpp:9:8: note: candidate: 'constexpr Widget::Widget(const Widget&)' 9 | struct Widget | ^~~~~~ sample.cpp:9:8: note: no known conversion for argument 1 from 'WidgetOptions' to 'const Widget&' sample.cpp:9:8: note: candidate: 'constexpr Widget::Widget(Widget&&)' sample.cpp:9:8: note: no known conversion for argument 1 from 'WidgetOptions' to 'Widget&&' Compiler returned: 1
By Meeting C++ | Jan 19, 2025 09:18 AM | Tags: meetingcpp community c++20 basics advanced
Meeting C++ online did kick off the year with a sponsored event by NVIDIA with Bryce Adelstein Lelbach presenting his talk Think Parallel:
Think Parallel - Bryce Adelstein Lelbach
By Blog Staff | Jan 15, 2025 11:15 AM | Tags: None
This article explores how C++20 concepts can replace CRTP when implementing static interfaces for a family of classes. By leveraging concepts, we achieve cleaner, more readable, and less error-prone code—provided you have access to C++20.
Replace CRTP with Concepts
by Sandor Dargo
From the article:
In my Meeting C++ 2024 trip report, among my favourite ideas I mentioned Klaus Iglberger’s talk where he mentioned the possibility of replacing the curiously returning template pattern with the help of class tagging and concepts.
Class tagging might mean different things in different contexts, or at least might be implemented in different ways. The end goal is to mark, in other words, tag classes or functions to be used in certain contexts, with certain algorithms. As you’ll see, in our case it’ll also be a tool to prevent duck typing.
We are going to see an example implementation of a static interface with CRTP with a couple of different derived classes, then we’ll see the implementation without CRTP.
The CRTP solution
With the static interface, we are creating a static family of types. There is no need for dynamic polymorphism to share the same interface. It’s still granted through a base class, which is a template taking the deriving class as a parameter.
By Blog Staff | Jan 10, 2025 11:06 AM | Tags: None
Last time, we investigated the puzzle of why the compiler wouldn’t let us put an object into a std::optional. It came down to the fact that the object is not copy-constructible, move-constructible, copy-assignable, or move-assignable, so there’s no way to put the temporary object into the std::optional.
Solving the Puzzle of Trying to Put an Object into a std::optional
by Raymond Chen
From the article:
What we have to do is construct the object in place inside the
std::optional. And the C++ standard library term for “construct an object inside a container” is “emplace”.struct Doodad { Doodad(); ~Doodad(); std::unique_ptr<DoodadStuff> m_stuff; }; struct Widget { std::optional<Doodad> m_doodad; Widget() { if (doodads_enabled()) { m_doodad.emplace(); } } };The parameters to
emplaceare whatever parameters you would have passed to theDoodadconstructor. In our case, we wanted the default constructor, so that means that we pass nothing toemplace().
