constexpr Functions: Optimization vs Guarantee -- Andreas Fertig

By Blog Staff | Jul 23, 2025 02:53 PM | Tags: None

Constexpr has been around for a while now, but many don’t fully understand its subtleties. Andreas Fertig explores its use and when a constexpr expression might not be evaluated at compile time.

constexpr Functions: Optimization vs Guarantee

by Andreas Fertig

From the article:

The feature of constant evaluation is nothing new in 2023. You have constexpr available since C++11. Yet, in many of my classes, I see that people still struggle with constexpr functions. Let me shed some light on them.

What you get is not what you see

One thing, which is a feature, is that constexpr functions can be evaluated at compile-time, but they can run at run-time as well. That evaluation at compile-time requires all values known at compile-time is reasonable. But I often see that the assumption is once all values for a constexpr function are known at compile-time, the function will be evaluated at compile-time.

I can say that I find this assumption reasonable, and discovering the truth isn’t easy. Let’s consider an example (Listing 1).

constexpr auto Fun(int v)
{
  return 42 / v; ①
}

int main()
{
  const auto f = Fun(6); ②
  return f;              ③
}

The constexpr function Fun divides 42 by a value provided by the parameter v ①. In ②, I call Fun with the value 6 and assign the result to the variable f.

Three types of name lookups in C++ -- Sandor Dargo

By Blog Staff | Jul 21, 2025 02:50 PM | Tags: None

Let's revisit a core concept in C++: how the compiler finds the names you use in your code. From qualified and unqualified name lookups to the special case of Argument-Dependent Lookup (ADL), understanding these mechanisms is essential for writing clear and correct C++ programs.

Three types of name lookups in C++

by Sandor Dargo

From the article:

Let’s get back to some basics this week and talk about name lookups in C++. In other words: when you refer to a symbol in your code, how does the compiler find it?

Essentially, we can differentiate between three kinds of lookups:

• Qualified name lookup
• Unqualified name lookup
• Argument-Dependent Lookup (ADL)

Let’s explore them in that order.

Qualified Name Lookup

The term qualified refers to symbols that are explicitly scoped using the :: operator. In other words, these are names that appear to the right of a ::, such as x in a::b::x.

Before the compiler can perform a qualified name lookup, it must first resolve the left-hand side of the :: operator. This identifies the namespace or class being referenced.

Qualified name lookup is relatively simple: it only searches the explicitly named scope. It does not search enclosing or outer scopes.

C++ in Embedded Systems - Interview with Author Amar Mahmutbegović

By Meeting C++ | Jul 20, 2025 04:43 AM | Tags: meetingcpp intermediate embedded community c++23 c++20 c++17 basics

This weeks interview with Amar about his book on C++ on embedded systems:

C++ in Embedded Systems Interview with Author Amar Mahmutbegović

by Jens Weller

Watch the video

Why does C++ think my class is copy-constructible when it can’t be copy-constructed? -- Raymond Chen

By Blog Staff | Jul 17, 2025 02:47 PM | Tags: None

In C++, the presence of a user-declared (but not explicitly deleted) copy constructor is enough for the type to be considered copy-constructible by traits like std::is_copy_constructible. However, whether that constructor is instantiable is a separate matter—if it attempts to call a deleted base copy constructor, you'll still get a compile-time error when you actually try to use it.

Why does C++ think my class is copy-constructible when it can’t be copy-constructed?

by Raymond Chen

From the article:

Consider the following scenario:

template<typename T>
struct Base
{
    // Default-constructible
    Base() = default;

    // Not copy-constructible
    Base(Base const &) = delete;
};

template<typename T>
struct Derived : Base<T>
{
    Derived() = default;
    Derived(Derived const& d) : Base<T>(d) {}
};

// This assertion passes?
static_assert(
    std::is_copy_constructible_v<Derived<int>>);

Why does this assertion pass? It is plainly evident that you cannot copy a Derived<int> because doing so will try to copy the Base<int>, which is not copyable.

2025-07 Mailing Available

By Administrator | Jul 15, 2025 05:32 PM | Tags: None

The 2025-07 mailing of new standards papers is now available.

Returning several values from a function in C++ (C++23 edition) -- Daniel Lemire

By Blog Staff | Jul 15, 2025 02:43 PM | Tags: None

While C++ doesn’t have native syntax for returning multiple values like some other languages, modern C++ offers powerful tools to accomplish the same goal. With features like std::tuple, structured bindings, std::expected, and std::optional, handling multiple return values—and even error codes—has become both clean and expressive.

Returning several values from a function in C++ (C++23 edition)

by Daniel Lemire

From the article:

Many programming languages such as the Go programming language are designed to make it easy to return several values at once from a function. In Go, it is often used to return an optional error code. The C++ programming language does not have a built-in support for returning several values. However, several standard types can serve the same purpose. If you need to return two values, you can use an std::pair instance. If you need to return two or more values, an std::tuple instance will do. With recent C++ standards, it works really well!

Suppose we want to compute a division with a string error message when one is trying to divided by zero:

std::tuple<int,std::string> divide(int a, int b) {
 if (b == 0) {
 return {0, "Error: Division by zero"};
 }
 return {a / b, "Success"};
}

This approach works nicely. The code is clear and readable.

You might be concerned that we are fixing the type (int). If you want to write one function for all integer types, you can do so with concepts, like so:

Constructing Containers from Ranges in C++23 -- Sandor Dargo

By Blog Staff | Jul 11, 2025 02:37 PM | Tags: None

Starting from C++23, standard containers support a new set of constructor overloads. These constructors take a std::from_range tag, a range and an optional allocator. These from_range constructors make it easier to construct containers from ranges, helping make C++ code more concise, more expressive, and less error-prone.

Constructing Containers from Ranges in C++23

by Sandor Dargo

From the article:

I’ve written plenty on this blog about standard algorithms, but far less about ranges. That’s mostly because, although I’ve had production-ready compilers with C++20 ranges since late 2021, the original ranges library lacked a few key capabilities.

The biggest gap was at the end of a pipeline: you could transform data lazily, but you couldn’t drop the result straight into a brand-new container. What you got back was a view; turning that view into, say, a std::vector still required the old iterator-pair constructor.

C++23 fixes that in two complementary ways:

• std::to (an adaptor that finishes a pipeline by converting to a container), and
• from_range constructors on every standard container.

Today we’ll focus on the second improvement, because it’s the one you can implement in your own types, too.

The from_range constructor

Every standard container now supports a new set of constructors that make integration with ranges smoother — the so-called from_range constructors.

These constructors allow you to build a container directly from a range, rather than from a pair of iterators.

C++ Insights now uses Clang 20 -- Andreas Fertig

By Blog Staff | Jul 9, 2025 02:34 PM | Tags: None

Time flies—C++ Insights just turned 7! To celebrate, I’ve upgraded the tool to Clang 20, unlocking even more C++23 and C++26 features for you to explore.

C++ Insights now uses Clang 20

by Andreas Fertig

From the article:

size_t

For a long time now, when you used size_t or std::size_t the resulting transformation kept the name. It did not expand to the underlying machine-specific date type. To be honest, that was more like a happy accident. Clang 20 came with two changes to libc++

The first https://github.com/llvm/llvm-project/commit/d6832a611a7c4ec36f08b1cfe9af850dad32da2e modularized <cstddef> for better internal structuring, avoiding too much content to be included. This patch was followed by a second one: https://github.com/llvm/llvm-project/commit/5acc4a3dc0e2145d2bfef47f1543bb291c2b866a. This one now made an interesting change.

Previously, libc++ defined std::size_t as

1	using ::size_t _LIBCPP_USING_IF_EXISTS;

As the second patch highlighted, this required including the operating systems <stddef.h>. In the spirit of reducing unnecessary includes the line was changed to:

1	using size_t = decltype(sizeof(int));

This is an easy C++ solution to get the proper data type for size_t. Which is great. Yet, the AST nodes of the two versions look different. Previously, the operating system (macOS in this case) defined in its header:

1	typedef unsigned long size_t;

Well, with the new version, the transformation no longer stops at size_t but expands it all down to unsigned long. This probably should have been the case from the beginning, but I liked that tests and transformations did not change across platforms in this specific case. However, there are other instances where the transformation did yield different output on different platforms, so I accept this one.

 

What’s New for C++ Developers in Visual Studio 2022 17.14 -- Sy Brand

By Blog Staff | Jul 7, 2025 02:31 PM | Tags: None

Visual Studio 2022 version 17.14 is now generally available! This post summarizes the new features you can find in this release for C++. You can download Visual Studio 2022 from the Visual Studio downloads page or upgrade your existing installation by following the Update Visual Studio Learn page.

What’s New for C++ Developers in Visual Studio 2022 17.14

by Sy Brand

From the article:

We’ve made a myriad of fixes and improvements to the MSVC compiler and standard library. See C++ Language Updates in MSVC in Visual Studio 2022 17.14 for a full list of changes on the compiler side, and the STL Changelog for all the standard library updates.

Compiler

We’ve added support for several C++23 features, which are available under the /std:c++latest and /std:c++23preview flags.

You can now omit () in some forms of lambdas that previously required them, thanks to P1102R2:

auto lambda = [] constexpr { }; //no '()' needed after the capture list

We implemented if consteval, with which you can run different code depending on whether the statement is executed at compile time or run time. This is useful for cases where your run time version can be heavily optimized with compiler intrinsics or inline assembly that are not available at compile time:

constexpr size_t strlen(char const* s) {
    if consteval {
        // if executed at compile time, use a constexpr-friendly algorithm
        for (const char *p = s; ; ++p) {
            if (*p == '\0') {
                return static_cast<std::size_t>(p - s);
            }
        }
    } else {
        // if executed at run time, use inline assembly
        __asm__("SSE 4.2 magic");
    }
}

C++26: Erroneous Behaviour -- Sandor Dargo

By Blog Staff | Jul 3, 2025 02:29 PM | Tags: None

C++’s undefined behaviour impacts safety. Sandor Dargo explains how and why uninitialised reads will become erroneous behaviour in C++26, rather than being undefined behaviour.

C++26: Erroneous Behaviour

by Sandor Dargo

From the article:

If you pick a random talk at a C++ conference these days, there is a fair chance that the speaker will mention safety at least a couple of times. It’s probably fine like that. The committee and the community must think about improving both the safety situation and the reputation of C++.

If you follow what’s going on in this space, you are probably aware that people have different perspectives on safety. I think almost everybody finds it important, but they would solve the problem in their own way.

A big source of issues is certain manifestations of undefined behaviour. It affects both the safety and the stability of software. I remember that a few years ago when I was working on some services which had to support a 10× growth, one of the important points was to eliminate undefined behaviour as much as possible. One main point for us was to remove uninitialized variables which often lead to crashing services.

Thanks to P2795R5 by Thomas Köppe, uninitialized reads won’t be undefined behaviour anymore – starting from C++26. Instead, they will get a new behaviour called ‘erroneous behaviour’.

The great advantage of erroneous behaviour is that it will work just by recompiling existing code. It will diagnose where you forgot to initialize variables. You don’t have to systematically go through your code and let’s say declare everything as auto to make sure that every variable has an initialized value. Which you probably wouldn’t do anyway.