C++23 language changes

Breaking changes

Many transitive includes have been removed

In C++23, libc++ dropped a bunch of transitive includes, so a lot of includes you were getting by accident now need to be explicitly included. Most popular ones seem to be <algorithm>, <memory>, <mutex>, and <vector>.

Fix: Add missing includes. You can reference cppreference.com for the includes of any function, type, or symbol.

std::unique_ptr's destructor became constexpr

In C++23, std::unique_ptr's destructor became constexpr. As a result, std::unique_ptr<T> has started to complain in more cases that sizeof(T) is invalid when T is an incomplete type.

Fix: Ensure a type (T) is completely defined before it is used in unique_ptr, typically by moving the definition of the type (T) before its use. In certain cases, it might be hard to move the definition so you can move the destructor of the class containing the unique_ptr<T> after the definition of type (T).

std::string does not allow nullptr anymore

In C++23, std::string removed the constructor that accepted nullptr. You can no longer assign nullptr to a std::string or return nullptr for a fuction with return type of std::string. These actions now trigger a compile-time error, preventing the runtime crashes that occurred in previous versions.

Fix: Change your nullptr to "".

-Wfortify-source with C++23 is more precise

Fortify performs safety checks at compile-time whenever possible, falling back to runtime only when necessary. With C++23's increased use of constexpr, Fortify can verify more conditions during compilation. This shifts some errors from runtime to compile-time, potentially causing build failures in code now provably known to be unsafe.

Fix: Look at the compile-time warnings and make the fix such that the size fits the destination.

Libc++ removed __resize_default_init for resize_and_overwrite

Libc++ removed __resize_default_init recently because it was an internal API used only in a specific case. They decided to move to the public one, resize_and_overwrite, that became available since C++23.

Fix: Replace all your uses of __resize_default_init with resize_and_overwrite for C++23.

Non-Breaking changes

Range-based For Loop issue was fixed

Since C++11, the range-based for loop had undefined behavior for specific cases because they would result in dangling references. C++23 fixed this issue by extending the lifetime of all temporaries in the range expression to the end of the loop. You can find more information and examples in the design paper P2644R1.

Other Changes

You can find the other new C++23 language features at https://en.cppreference.com/w/cpp/23.html