C++23 prohibits constructing std::string from nullptr

[StephanTLavavej]

I work on MSVC's C++ Standard Library implementation, and we regularly build MAME (along with many other open-source projects) to identify compiler/library bugs that would break your project, so we can fix them before shipping. This also allows us to provide advance notice of source-breaking changes - which is the case here.

The paper P2166R1 "Prohibiting basic_string And basic_string_view Construction From nullptr" has been accepted for the upcoming C++23 Standard, and we recently implemented it by merging microsoft/STL#1995 from our contributor @sam20908. Our open-source test pass then discovered the following code in MAME that is now disallowed by C++23:

mame/src/lib/formats/cassimg.cpp

Lines 224 to 230 in b0bc45a

	return open_choices(file, procs, nullptr, formats, flags, outcassette);
	}
	cassette_image::error cassette_image::open_choices(void *file, const io_procs *procs, const std::string &extension,
	const Format *const *formats, int flags, ptr &outcassette)

Here, cassette_image::open() is passing nullptr for the const std::string &extension parameter of cassette_image::open_choices(). This has always been undefined behavior in C++98 through C++20 (basic_string's constructor has a precondition that the pointer is non-null); C++23 has added a deleted overload of basic_string's constructor to detect when the argument is known to be nullptr at compile-time. (Runtime values of const char * that happen to be null are still undefined behavior.)

There are several possible ways to fix this code; you could pass an empty string with "" or std::string{} or even {} (depending on whether open_choices() is overloaded).

There may be additional occurrences of this throughout MAME's codebase - this is simply the first one we encountered. Hope this helps!

[cuavas]

Yeah, we shouldn’t be constructing std::string from nullptr. In this case, I think it should be passing std::string() rather than nullptr in the call to open_choices. Could someone (e.g. @Tafoid or @Robbbert) test this change?

[ajrhacker]

Constructing strings from nullptr already causes segmentation faults or runtime assertion errors on many builds. Bugs like this (e.g. 3a04996) have persisted mostly due to obscurity.

[Robbbert]

This bug was created almost 5 years ago and survived a number of refactors since.

I tested std::string(), however a different bug appeared which I can't explain, that being if the tape was preset to PLAY, it instead was STOPPED. So I used {} instead.

See 645aef0

[Robbbert]

No further comments, so closing.

Anyone who has a continuing interest in similar bugs may reopen this issue if they so wish.