静态初始化的哪一部分是线程安全的？

Question

如果我有一个像这样的全球价值获取者

bool get_global_bool() {
    static const bool b{get_the_value()};
    return b;
}

我知道

b

将以线程安全的方式初始化，但我是否也能保证同时调用

get_global_bool

的多个线程永远不会多次执行

get_the_value

？

Answer 1

是的，根据cppref

If multiple threads attempt to initialize the same static local variable concurrently, the initialization occurs exactly once (similar behavior can be obtained for arbitrary functions with std::call_once).

Note: usual implementations of this feature use variants of the double-checked locking pattern, which reduces runtime overhead for already-initialized local statics to a single non-atomic boolean comparison.

好吧，您还可以通过检查 gen asm 输出来验证它，输出将类似于 x86-64 gcc trunk 上的以下内容

get_the_value():
        movzx   eax, BYTE PTR v[rip]
        ret
get_global_bool():
        movzx   eax, BYTE PTR guard variable for get_global_bool()::b[rip]
        test    al, al
        je      .L14
        movzx   eax, BYTE PTR get_global_bool()::b[rip]
        ret
.L14:
        sub     rsp, 8
        mov     edi, OFFSET FLAT:guard variable for get_global_bool()::b
        call    __cxa_guard_acquire
        test    eax, eax
        jne     .L15
        movzx   eax, BYTE PTR get_global_bool()::b[rip]
        add     rsp, 8
        ret
.L15:
        movzx   eax, BYTE PTR v[rip]
        mov     edi, OFFSET FLAT:guard variable for get_global_bool()::b
        mov     BYTE PTR get_global_bool()::b[rip], al
        call    __cxa_guard_release
        movzx   eax, BYTE PTR get_global_bool()::b[rip]
        add     rsp, 8
        ret

您可以看到 get_the_value 受 cxa_guard 保护并且仅调用一次