Are C++ Static Variables in Functions Thread Safe?

• Exploration
• Further
• Summary

First conclusion, is thread safe, and has double-checked locking, performance quite good.

Exploration

Usually when writing C++ Singleton pattern, will smoothly write code below:

class Hello {
public:
    static Hello & instance() {
        static Hello obj;
        return obj;
    }
    
    Hello() {
        printf("constructor");
    }
    ~Hello() {
        printf("destructor");
    }
    
public:
    int x = 0;
};

Then when using Hello::instance(), is thread safe? Can Hello’s constructor ensure executing once?

Throw code into Hopper to see

Looks like has lock. Then is thread safe :)

Further

See these methods

__cxa_guard_acquire
__cxa_atexit
__cxa_guard_release

__cxa_atexit can be to call destructor when process ends.

After searching can find this source code cxa_guard.cxx

https://opensource.apple.com/source/libcppabi/libcppabi-14/src/cxa_guard.cxx

Through obj_guard implements a double-checked locking

inUse will through obj_guard point to memory’s first address access 0 and 1, implement first check.

Find this obj_guard in Hopper

This guard variable is a global variable.

Summary

In future can happily use this writing

    static Hello & instance() {
        static Hello obj;
        return obj;
    }

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