{"id":4733,"date":"2024-09-06T22:18:30","date_gmt":"2024-09-06T20:18:30","guid":{"rendered":"https:\/\/mariusbancila.ro\/blog\/?p=4733"},"modified":"2024-09-06T22:18:33","modified_gmt":"2024-09-06T20:18:33","slug":"whats-new-in-c26-part-1","status":"publish","type":"post","link":"https:\/\/mariusbancila.ro\/blog\/2024\/09\/06\/whats-new-in-c26-part-1\/","title":{"rendered":"What’s new in C++26 (part 1)"},"content":{"rendered":"\n

The C++26 version of the C++ standard is a work in progress, but a series of language and library features have been already added. Furthermore, some of them are already supported by Clang and GCC. One of these new changes was discussed in my previous article, Erroneous behaviour has entered the chat<\/a>. In this post, we will look at several language features added in C++26.<\/p>\n\n\n\n

Specifying a reason for deleting a function<\/h2>\n\n\n\n

Since C++11, we can declare a function as deleted, so that the compiler will prevent its use. This can be used to prevent the use of class special member functions, but also to delete any other function. A function can be deleted as follows (example from the proposal paper):<\/p>\n\n\n\n

class NonCopyable\n{\npublic:\n    \/\/ ...\n    NonCopyable() = default;\n\n    \/\/ copy members\n    NonCopyable(const NonCopyable&) = delete;\n    NonCopyable& operator=(const NonCopyable&) = delete;\n    \/\/ maybe provide move members instead\n};<\/pre>\n\n\n\n
In C++26, you can specify a reason why this function is deleted:<\/p>\n\n\n\n
class NonCopyable\n{\npublic:\n    \/\/ ...\n    NonCopyable() = default;\n\n    \/\/ copy members\n    NonCopyable(const NonCopyable&)\n        = delete(\"Since this class manages unique resources, copy is not supported; use move instead.\");\n    NonCopyable& operator=(const NonCopyable&)\n        = delete(\"Since this class manages unique resources, copy is not supported; use move instead.\");\n    \/\/ provide move members instead\n};<\/pre>\n\n\n\n
The reason for having this feature is to help API authors to provide tailored messages for the removal of a function, instead of just relying on the generic compiler error for using a deleted function.<\/p>\n\n\n\n
For more info see: P2573R2: = delete(“should have a reason”);<\/a><\/p>\n\n\n\n
Placeholder variables with no name<\/h2>\n\n\n\n
There are cases when a variable has to be declared but its name is never used. An example is structure bindings. Another is locks (like lock_guard<\/code>), that are only used for their side-effects. In the future, another example could be pattern matching (for which several proposals exist).<\/p>\n\n\n\n
In C++26, we can use a single underscore (_<\/code>) to define an unnamed variable.<\/p>\n\n\n\n
For instance, in the following example, unused<\/code> is a variable that is not used:<\/p>\n\n\n\n
[[maybe_unused]] auto [data, unused] = get_data();<\/pre>\n\n\n\n
In C++26, the unused<\/code> variable can be named _<\/code> (single underscore):<\/p>\n\n\n\n
auto [data, _] = get_data();<\/pre>\n\n\n\n
When the single underscore identifier is used for the declaration of a variable, non-static class member variable, lambda capture, or structure binding, the [[maybe_unused]]<\/code> attribute is implicitly added, therefore, there is no need to explicitly use it.<\/p>\n\n\n\n
A declaration with the name _<\/code> is said to be name-independent<\/em> if it declares:<\/p>\n\n\n\n