调用方堆栈无效后，回调func引用的调用方本地变量如何？

如何解决调用方堆栈无效后，回调func引用的调用方本地变量如何？

我认为以下代码由于本地变量“不可用”而可能存在潜在问题，行为是否应定义？

class foo {
public:
    foo(){
        std::cout << "foo constructed" << std::endl;
    }
    ~foo(){
        std::cout << "foo destructed" << std::endl;
    }
    int val;
};
void do(std::function<void()> func) {
    // let other thread to invoke func
    dispatch_async(conqueue,^{
        sleep(5); // wait 5s for testLocalRef return,so all local variable should be invalid due to stack popup?
        func();
    });
}
void testLocalRef() {
    auto res = true; // local variable
    foo fo; // local variable
    std::function<void()> func = [&res,&fo] () {
        res = false; // invalid? the behavior is undefined?
        fo.val = 10; // invalid? the behavior is undefined?
        std::cout << "callback"<<std::endl;
    };
    do(func); // return immediately
}
int main(int argc,const char * argv[]) {
    conqueue = dispatch_queue_create("MyConcurrentDiapatchQueue",disPATCH_QUEUE_CONCURRENT);
    testLocalRef();
    // other function calls in order to use the stack
    // ...
    CFRunLoopRun(); // no return
}

能请你帮我吗？预先感谢！