为什么 std::mutex 是标准布局类？

Question

[thread.mutex.class]/3 :

The class mutex meets all of the mutex requirements ([thread.mutex.requirements]). It is a standard-layout class ([class.prop]).

What is the reason for this requirement?

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 ( ie 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.