确保我们初始化每个变量一次且仅一次

Question

Below is a test from LLVM's exception handling library libcxxabi (which by the way uses LLVM's stack unwind library libunwind):

// libcxxabi\test\test_guard.pass.cpp
...
// Ensure that we initialize each variable once and only once.
namespace test1 {
    static int run_count = 0;
    int increment() {
        ++run_count;
        return 0;
    }
    void helper() {
        static int a = increment();
        ((void)a);
    }
    void test() {
        static int a = increment(); ((void)a);
        assert(run_count == 1);
        static int b = increment(); ((void)b);
        assert(run_count == 2);
        helper();
        assert(run_count == 3);
        helper();
        assert(run_count == 3);
    }
}

...

int main()
{
    test1::test();
}

Maybe I'm missing something obvious but I'm not sure what's the idea behind this test (what does it test and how). Do you have any ideas?

Why are these three variables

static int run_count
static int a (in test(), not in helper())
static int b 

declared to be static?

Answer 1

It's a test to ensure that the compiler works correctly. It should pass on any confirming c++ compiler.

I'd guess it's there as a quick sanity check, there will be more in-depth tests which might be harder to understand why by they are failing on a buggy compiler.

The variables are declared static to ensure the various forms of static variables are initialised correctly and that the initialisers are only called once.