Why is std::mutex a standard-layout class?

Question

[thread.mutex.class]/3:

[...] It is a standard-layout class ([class.prop]).

What is the reason for this requirement?

Gibun

Wild guess : it may be for compatibility with preexisting threading libraries like pthread? Just about every threading library before std::mutex had a C API. – Lactic

Another guess: every compiler's implementation of std mutex was a standard layout class, so why not require it? – Allargando

@Yakk-AdamNevraumont, but what benefits? (Except maybe making some standard library implementations jump thru the hoops to keep this property (I'm not kidding)) – Gibun

@FrançoisAndrieux, actually you may be right, the intention might be to allow building pthreads interface on top of C++ mutexes... – Gibun

ABI compatibility maybe? C11 has a mtx_t (well, threading support in C11 is optional, but assuming it is there). And presumably if std::mutex is standard layout, and it has a mtx_t member, then they are pointer-interconvertible. Such things have precedent. std::complex and the C11 _Complex specifier (explicitly binary compatible). Then there's std::atmoic and the C11 _Atomic specifier. – Quaternary

Answer 1

Interoperability with the associated C interface. From N2320 (Multi-threading Library for Standard C++):

The C level interface has been removed from this proposal with the following rationale:

• As long as we specify that the key types in this proposal are standard-layout types (which we have done), WG14 is still free to standardize a C interface which interoperates with this C++ interface.
• WG14 is in a better position to solve the cancellation interoperability problem than WG21 is. [...]
• WG14 asked WG21 to take the lead on this issue. We feel we can best take lead by specifying only a C++ interface which has the minimum hooks in it to support a future C interoperating interface (i.e. types are standard-layout types). We feel we should stop short of actually specifying that C interface in the C++ standard. WG14 can do a better job with the C interface and a future C++ standard can then import it by reference.

Conservatory